FIELD OF THE INVENTION

The present invention relates to a detection circuit. More particularly, the present invention relates to a circuit for detecting electrical shock hazard on a metal enclosure of an alternating current (AC) equipment, an AC equipment and a communication system.

BACKGROUND

In an Alternate Current (AC) equipment, in case of a fault condition the metal enclosure may become electrical. In such a situation there might be a risk of an electrical shock. Specifically, if the metal enclosure is earthed well, there will be no electrical shock hazard, while a not well earthed metal enclosure represents, in case of a fault, an electrical shock hazard for people who touch it.

Whenever the AC equipment is operated, all exposed metal parts must be securely earthed to avoid any electrical shock for the operator. In buildings this is accomplished by earthing the AC equipment, for instance by connecting the metal parts of the AC equipment via a wire to the earthing point in the normal three pin mains socket (including neutral, line connections, and a protective earthing connection for exposed metal parts of an equipment). However, when the AC equipment is installed outdoors, the earthing may not be stable. Therefore, it is not sure whether or not an electrical installation is earthed properly. In this case, electrical shock hazard detection becomes important.

A conventional solution to detect electrical shock hazard on a metal enclosure of an alternating current (AC) equipment is to sample a voltage between a neutral conductor N and an earth pin of the socket or the device enclosure, to compare the sample voltage with a threshold voltage and to make a judgment. A judging criterion is that, normally the voltage between the neutral conductor N and the device enclosure is low. Hence, if the sample voltage is lower than the threshold voltage, the device enclosure is considered to be no electrical shock hazard. If the sample voltage is higher than the threshold voltage, the device enclosure is considered to be electrical shock hazard. However, the judge criterion of the conventional solution relies on a hypothesis, i.e. the earth pin of the socket or the enclosure is earthed well, and thus the threshold voltage is set relatively low. According to this solution, when the earth pin of the socket or the enclosure is not earthed well, even in the absence of a fault condition and the metal enclosure does not represent an electrical hazard, it is possible that the sample voltage between the neutral conductor N and the earth pin of the socket or the device enclosure is higher than the threshold voltage, thereby the device enclosure is wrongly considered to be electrical shock hazard. Therefore the detection accuracy of the conventional solution is relatively low, and cannot meet the operators' safety requirement.

SUMMARY

Accordingly, object of the present application is to improve detection accuracy of electrical shock hazard on an electrical equipment, in particular a metal enclosure of an alternating current, AC, equipment.

The above-mentioned object of the present invention is achieved by the solution provided in the independent claims. Further, implementations are defined in the dependent claims.

A first aspect of the present invention provides a circuit for detecting electrical shock hazard on a metal enclosure of an alternating current, AC, equipment, the circuit comprising a voltage sample and rectifying module and a voltage comparison module, wherein

the voltage sample and rectifying module is connected between a line conductor (L) and the metal enclosure, and is configured to:

sample a line-to-enclosure sample voltage (VS.LE) between the line conductor and the metal enclosure and rectify the line-to-enclosure sample voltage (VS.LE), and

output the rectified line-to-enclosure sample voltage (VR.LE) to the voltage comparison module; and

the voltage comparison module is configured to compare the rectified line-to-enclosure sample voltage (VR.LE) with a first threshold voltage, and to detect an electrical shock hazard on the metal enclosure when the rectified line-to-enclosure sample voltage (VR.LE) is lower than the first threshold voltage. Since the line-to-enclosure sample voltage is not affected by the earthing, regardless of whether the metal enclosure of the AC equipment is earthed well or not, the correct line-to- enclosure sample voltage (VS.LE) is sampled, and an electrical shock hazard on the metal enclosure can be detected when the rectified line-to-enclosure sample voltage (VR.LE) is lower than the first threshold voltage, thus the detection accuracy can be improved or ensured, and the operators' safety requirement can be met.

In a first implementation form of the apparatus according to the first aspect, the first threshold voltage (V _re f) is chosen from a range which is higher than 0 and lower than the predefined voltage value, the predefined voltage value being proportional to a calculated line-to-enclosure voltage (VLE.C) value between the line conductor L and the metal enclosure when a minimum line-to-neutral voltage value (Vi.N.min) among a set of predefined main voltages between the line conductor L and a neutral conductor N is used. Therefore, the limit scenario is considered for defining the predefined voltage value, and the first threshold voltage (V _re f) is chosen from a range which is higher than 0 and lower than the predefined voltage value. Hence, regardless of whether the metal enclosure is earthed well or not, as long as the rectified L2E sample voltage (VR.LE) is lower than the first threshold voltage, an actual electrical shock hazard will be present on the metal enclosure, thus the detection accuracy can be further improved or ensured, and the operators' safety requirement can be met.

In a second implementation form of the apparatus according to the first aspect or according to the first implementation form of the first aspect, the circuit further includes a voltage divider circuit, connected between a neutral conductor N and the metal enclosure and being configured to divide a current main voltage (VL-N) between the line conductor L and the neutral conductor N, so that a line-to-enclosure voltage (VL-E) between the line conductor L and the metal enclosure and a neutral-to-enclosure voltage (VN-E) between the neutral conductor N and the metal enclosure are a fraction of the main voltage (VL-N).

Therefore, by means of the voltage divider circuit, the line-to-enclosure voltage VL-E and the neutral-to-enclosure voltage VN-E can be definite. Once the line-to-enclosure voltage VL-E and the neutral-to-enclosure voltage VN-E are known, these can be used to define the predefined voltage value, thereby allowing a more efficient choice of the first threshold voltage.

In a third implementation form of the apparatus according to the first aspect or according to the first or the second implementation form of the first aspect, the voltage sample and rectifying module comprises a voltage sample module and a voltage rectification module; the voltage sample module includes a first string of resistors comprising a first sample resistor and a second sample resistor connected in series with each other, a first end of the first string of resistors being connected to the line conductor L and a second end of the first string of resistors being connected to the metal enclosure,

wherein the line-to-enclosure sample voltage (VS.LE) is a voltage across the second sample resistor, and the voltage sample module being configured to output the line-to-enclosure sample voltage (VS.LE) to the voltage rectification module;

a first end of the voltage rectification module is connected to the common end between the first sample resistor and the second sample resistor, and a second end of the voltage rectification module is connected to the voltage comparison module, the voltage rectification module being configured to rectify the line-to-enclosure sample voltage (VS.LE); and the voltage comparison module is configured to compare the rectified line-to-enclosure sample voltage (VR.LE) with the first threshold voltage and output an alarm signal when the rectified line-to-enclosure sample voltage (VR.LE) is lower than the first threshold voltage.

Therefore, the first string of resistors has two functions: firstly it is used to sample a line-to- enclosure sample voltage across the second sample resistor and secondly it can be used to ensure an insulation between the line conductor L and the enclosure. In this manner even through the operators touch one or more parts of the AC equipment, the electric current passing through the body of the operators will be reduced compared with the used main voltage, thereby further increasing the safety of the circuit.

In a fourth implementation form of the apparatus according to the second implementation form of the first aspect, the voltage divider circuit is further configured to output a neutral-to- enclosure sample voltage (VS.NE) to the voltage sample and rectifying module, the neutral-to- enclosure sample voltage (V S.NE) being a fraction of the neutral-to-enclosure voltage (VN-E) between the neutral conductor N and the metal enclosure;

the voltage sample and rectifying module is further configured to rectify the neutral-to- enclosure sample voltage (VS.NE) and output the rectified neutral-to-enclosure sample voltage (VR.NE) to the voltage comparison module; and

the voltage comparison module is further configured to compare the rectified neutral-to- enclosure sample voltage (VR.NE) with a second threshold voltage, and to detect an electrical shock hazard on the metal enclosure when the rectified line-to-enclosure sample voltage (V R.LE) is lower than the first threshold voltage and the rectified neutral-to-enclosure sample voltage (V R.NE) is higher than the second threshold voltage. Therefore, a double check is done, thereby improving the detection accuracy and allowing fulfilling the operators' safety requirement with a higher degree of accuracy compared with conventional methods. In a fifth implementation form of the apparatus according to the fourth implementation form of the first aspect, wherein the voltage sample and rectifying module comprises a voltage sample module and a voltage rectification module;

the voltage sample module includes a first string of resistors comprising a first sample resistor and a second sample resistor connected in series with each other, a first end of the first string of resistors being connected to the line conductor L and a second end of the first string of resistors being connected to the metal enclosure,

wherein the voltage sample module is configured to output the line-to-enclosure sample voltage (VS.LE) to the voltage rectification module, the line-to-enclosure sample voltage (VS.LE) being a voltage across the second sample resistor;

the voltage divider circuit includes a second string of resistors comprising a first divider resistor and a second divider resistor connected in series with each other, a first end of the second string of resistors being connected to the neutral conductor N and a second end of the second string of resistors being connected to the metal enclosure, wherein the voltage divider circuit is configured to output the neutral-to-enclosure sample voltage (VS.NE) to the voltage rectification module, the neutral-to-enclosure sample voltage (VS.NE) being a voltage across the second divider resistor;

a first end of the voltage rectification module is connected to the common end between the first sample resistor and the second sample resistor, a second end of the voltage rectification module is connected to the voltage comparison module, and a third end of the voltage rectification module is connected to the common end between the first divider resistor and the second divider resistor, the voltage rectification module is configured to rectify the line-to- enclosure sample voltage (VS.LE) and the neutral-to-enclosure sample voltage (VS.NE), and output the rectified line-to-enclosure sample voltage and the rectified neutral-to-enclosure sample voltage (VR.LE, VR.NE) to the voltage comparison module; and

the voltage comparison module is configured to compare the rectified line-to-enclosure sample voltage (VR.LE) with the first threshold voltage and the rectified neutral-to-enclosure sample voltage (VR.NE) with the second threshold voltage, and to output an alarm signal when the rectified line-to-enclosure sample voltage (VR.LE) is lower than the first threshold voltage and the rectified neutral-to-enclosure sample voltage (VR.NE) is higher than the second threshold voltage.

Therefore, the first string of resistors has two functions: firstly it is used to sample a line-to- enclosure sample voltage across the second sample resistor (VS.LE) and secondly it can be used to ensure an insulation between the line conductor L and the enclosure. In this manner, even though the operators touch one or more parts of the AC equipment, the electric current passing through the body of the operators will be reduced compared with the used main voltage, thereby further increasing the safety of the circuit;

Furthermore, the second string of resistors has three functions: firstly it is used to divide or split the main voltage (VL-N) between the line conductor L and the neutral conductor N, secondly it can be used to ensure an insulation between the neutral conductor N and the enclosure. In this manner, even though the operators touch one or more parts of the AC equipment, the electric current passing through the body of the operators will be reduced compared with the used main voltage, thereby further increasing the safety of the circuit, and thirdly it is used to sample a neutral-to-enclosure sample voltage (VS.NE) across the second divider resistor. A second aspect of the present invention provides an AC equipment, comprising a metal enclosure and a circuit for detecting electrical shock hazard on the metal enclosure of the AC equipment described above.

A third aspect of the present invention provides a communication system comprising an AC equipment described above and a power system for supplying power to the AC equipment.

A fourth aspect of the present invention provides a base station system, BSS, comprising an AC base station and a power system for supplying power to the AC base station, wherein the AC base station comprises a metal enclosure and a circuit for detecting electrical shock hazard on the metal enclosure of the AC equipment described above, wherein the AC base station is deployed on a tower of the BSS.

The AC equipment, the communication system and the base station system of the present invention achieves the same advantages as described above for the circuit for detecting electrical shock hazard on a metal enclosure of an alternating current equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

The above aspects and implementation forms of the present invention will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which Figure 1 illustrates an exemplary block diagram of a circuit for detecting electrical shock hazard on a metal enclosure of AC equipment according to an embodiment of the present invention;

Figure 2 illustrates an exemplary block diagram of a circuit for detecting electrical shock hazard on a metal enclosure of AC equipment according to another embodiment of the present invention;

Figure 3a illustrates an exemplary circuit schematic diagram of a circuit for detecting electrical shock hazard on a metal enclosure of AC equipment of Fig. 1 ;

Figure 3b illustrates an exemplary circuit schematic diagram of a circuit for detecting electrical shock hazard on a metal enclosure of AC equipment of Fig. 1 ;

Figure 4 illustrates an exemplary circuit schematic diagram of a circuit for detecting electrical shock hazard on a metal enclosure of AC equipment of Fig. 1 ;

Figure 5 illustrates an exemplary circuit schematic diagram of a circuit for detecting electrical shock hazard on a metal enclosure of AC equipment of Fig. 1 ;

Figure 6a illustrates an exemplary circuit schematic diagram of a circuit for detecting electrical shock hazard on a metal enclosure of AC equipment of Fig. 2;

Figure 6b illustrates an exemplary circuit schematic diagram of a circuit for detecting electrical shock hazard on a metal enclosure of AC equipment of Fig. 2;

Figure 6c illustrates an exemplary circuit schematic diagram of a circuit for detecting electrical shock hazard on a metal enclosure of AC equipment of Fig. 2;

Figure 6d illustrates an exemplary circuit schematic diagram of a circuit for detecting electrical shock hazard on a metal enclosure of AC equipment of Fig. 2;

Figure 7 illustrates an exemplary circuit schematic diagram of a circuit for detecting electrical shock hazard on a metal enclosure of AC equipment of Fig. 2;

Figure 8 illustrates an exemplary circuit schematic diagram of a circuit for detecting electrical shock hazard on a metal enclosure of AC equipment of Fig. 1 ;

Figure 9 illustrates an exemplary circuit schematic diagram of a circuit for detecting electrical shock hazard on a metal enclosure of AC equipment of Fig. 1 ;

Fig. 10 illustrates an exemplary block diagram of AC equipment according to an embodiment of the present invention; Fig. 11 illustrates an exemplary block diagram of a communication system according to an embodiment of the present invention; and

Fig. 12 illustrates an exemplary block diagram of a base station system, BSS, according to an embodiment of the present invention.

DETAILED DESCRIPTION

A clear and full description is given below to the solutions according to embodiments of the present disclosure, with reference to the accompanying drawings.

In order to conveniently understand embodiments of the present invention, several terms that will be introduced in the description of the embodiments of the present invention are illustrated herein first.

With the term line conductor L is meant a conductor which is energized in normal operation and capable of contributing to the transmission or distribution of electric energy but which is not a neutral or mid-point conductor. With the term neutral conductor N is meant a conductor electrically connected to the neutral point and capable of contributing to the distribution of electric energy.

With the term protective earthing conductor PE is meant a conductor provided for purposes of safety, for example protection against electric shock; protective conductor provided for protective earthing. With the term line-to-neutral voltage is meant a voltage between a line conductor and the neutral conductor at a given point of an a.c. electric circuit.

With the term electric shock is meant a physiological effect resulting from an electric current through a human or animal body.

Fig. 1 illustrates an exemplary block diagram of a detecting circuit 100 for detecting electrical shock hazard on a metal enclosure 10 of an AC equipment according to an embodiment of the present invention. As shown in FIG. 1 , the detecting circuit 100 includes a voltage sample and rectifying module 110 and a voltage comparison module 120. The voltage sample and rectifying module 1 10 is connected between a line conductor L and the metal enclosure 10, and is configured to sample a line-to-enclosure sample voltage VS.LE between the line conductor and the metal enclosure (called in the following description also line-to-enclosure sample voltage), rectify the line-to-enclosure sample voltage (VS.LE), and output the rectified line-to-enclosure sample voltage VR.LE (also in short rectified L2E sample voltage) to the voltage comparison module 120. The voltage comparison module 120 is configured to compare the rectified L2E sample voltage (VR.LE) with a first threshold voltage, and to detect an electrical shock hazard on the metal enclosure when the rectified L2E sample voltage (VR.LE) is lower than the first threshold voltage. In the previous paragraph and in the following description, reference is made to several modules included and/or forming the detecting circuit 100 or included in the metal enclosure 10. It will be clear that such modules are circuits or circuit portions. Accordingly, any module described to in the description may also be referred to as circuit. For instance, the voltage sample and rectifying module 1 10 and the voltage comparison module 120 may be as well referred to as a sample and rectifying circuit and a voltage comparison circuit.

Contrary to conventional solutions, in which a neutral-to-enclosure sample voltage between the neutral conductor and the metal enclosure is used as judging parameter for detecting an electrical shock hazard, the voltage sample and rectifying module 1 10 according to the present invention samples a line-to-enclosure sample voltage between the line conductor and the metal enclosure.

In normal working conditions the line-to-enclosure sample voltage (VS.LE) between the line conductor and the metal enclosure (in short L2E sample Voltage) is high regardless of whether the metal enclosure is earthed well or not, and will become low in case of an electric fault. Hence, the first threshold voltage is pre-defined to detect electrical shock hazard on a metal enclosure of an alternating current (AC) equipment. A possible example of how the first threshold voltage is defined will be described in the following paragraphs.

Accordingly, the judge criterion is that, if the rectified L2E sample voltage (VR.LE) is higher than the first threshold voltage, the metal enclosure is considered to be no electrical shock hazard. If the rectified L2E sample voltage (VR.LE) is lower than the first threshold voltage, the metal enclosure is considered to be electrical shock hazard.

In a further embodiment, the first threshold voltage (V _re f) is chosen from a range which is higher than 0 and lower than the predefined voltage value, the predefined voltage value being proportional to a calculated line-to-enclosure voltage (VLE. _C ) value between the line conductor L and the metal enclosure when a minimum line-to-neutral voltage value (Vi_N,min) among a set of predefined main voltages between the line conductor L and the neutral conductor N is used. The set of predefined main voltages may, for instance include, several voltages, which are available in the network and which depend on the standard used for the power supply network, such as but not limited to 220V, 1 10V and the like. A more detailed example will be given below.

The predefined voltage value may be defined by taking into account a limit (most unfavorable) scenario, in which the circuit can be used. In such a scenario, the AC equipment uses the minimum line-to-neutral voltage value (Vi_N,min) between the line conductor L and the neutral conductor N and it will be assumed that the metal enclosure is not earthed well. A minimum line-to-enclosure voltage value (VLE.min) between the line conductor L and the metal enclosure is a fraction of the minimum line-to-neutral voltage value VLN.min . However the specific value of the minimum line-to-enclosure voltage value (VLE.min) depends on how the minimum line-to- neutral voltage value (VLN.min) is shared by the minimum line-to-enclosure voltage (VLE.min) and a minimum neutral-to-enclosure voltage (VNE.min).

Correspondingly, the calculated line-to-enclosure voltage (VLE. _C ) may be a fraction of the minimum line-to-enclosure voltage (Vi_E,min). In an implementation, the proportionality factor may be a conversion ratio for rectifying half-wave AC to direct current (DC).

It is noted that, the set of predefined main voltages may be {100V to 240V}; the main voltage between the line conductor L and the neutral conductor N varies from country to country throughout the world. A widely used voltage choice is either 1 10-volt AC (1 10V) or 220-volt AC (220V). Note that 1 10 volts and 220 volts are averages, since the voltage does fluctuate during usage.

Considering, for example a system using 110V and 220V, if the minimum line-to-neutral voltage value (VLN.min) is 110V and the metal enclosure is not earthed well, the minimum line- to-enclosure voltage (VLE.min) and the minimum neutral-to-enclosure voltage (V _N E.min) are a fraction of the minimum line-to-neutral voltage value (VLN.min). As an example, the minimum line-to-enclosure voltage (Vi_E.min) and the minimum neutral-to-enclosure voltage (VNE.min) may be 55V respectively. A more specific explanation on how the minimum line-to-enclosure voltage (VLE.min) and the minimum neutral-to-enclosure voltage (V _N E,min) can be calculated for a specific configuration will be given in the following discussion. As can be seen from Fig. 1 , regardless of whether the metal enclosure of the AC equipment is earthed well or not, the line-to-enclosure sample voltage (VS.LE) is sampled, and an electrical shock hazard on the metal enclosure can be detected when the rectified line-to-enclosure sample voltage (VR.LE) is lower than the first threshold voltage, thus the detection accuracy can be improved or ensured, and the operators' safety requirement can be met. Furthermore, the limit scenario is considered for defining the predefined voltage value, and the first threshold voltage (V _re f) is chosen from a range which is higher than 0 and lower than the predefined voltage value. In this manner, if the AC equipment is used in the normal case (i.e. no hazard voltage is connected to the metal enclosure), it is not possible that the rectified L2E sample voltage (VR,LE) is lower than the first threshold voltage (V _re f). Hence, regardless of whether the metal enclosure is earthed well or not, as long as the rectified L2E sample voltage (VR.LE) is lower than the first threshold voltage, there must be an electrical shock hazard on the metal enclosure, thus the detection accuracy can be further improved or ensured, and the operators' safety requirement can be met.

Figure 2 illustrates an exemplary block diagram of a detecting circuit 200 for detecting electrical shock hazard on a metal enclosure of AC equipment according to another embodiment of the present invention. The detecting circuit 200 is a further development of the detecting circuit 100 and components therein, which are common between the two circuits will be indicated with same reference signs and will not be described again. As shown in FIG. 2, the circuit includes a voltage sample and rectifying module 110 and a voltage comparison module 120 as already described above. In addition, the detecting circuit 200 includes a voltage divider circuit 230.

The voltage sample and rectifying module 110 is connected between a line conductor L and the metal enclosure, and is configured to sample the line-to-enclosure sample voltage (VS.LE) between the line conductor and the metal enclosure and rectify the line-to-enclosure sample voltage (VS.LE) (in short L2E sample voltage), and output the rectified L2E sample voltage (VR.LE) to the voltage comparison module 120. The voltage comparison module 120 is configured to compare the rectified L2E sample voltage (VR.LE) with a first threshold voltage, and to detect an electrical shock hazard on the metal enclosure when the rectified L2E sample voltage (VR.LE) is lower than the first threshold voltage.

The voltage divider circuit 230 is connected between the neutral conductor N and the metal enclosure and is configured to divide a current line-to-neutral main voltage VL-N (in short main voltage) between the line conductor L and the neutral conductor N, so that a line-to-enclosure voltage (VL-E) between the line conductor L and the metal enclosure and a neutral-to-enclosure voltage (VN-E) between the neutral conductor N and the metal enclosure are a fraction of the main voltage (VL-N). In other words, the sum of the line-to-enclosure voltage (VL-E) and the neutral-to-enclosure voltage (VN-E) gives the current main voltage (VL-N) between the line conductor L and the neutral conductor N.

It can be understood that by means of the voltage divider circuit 230, the line-to-enclosure voltage (VL-E) and the neutral-to-enclosure voltage VN-E can be definite. Once the line-to- enclosure voltage V _L -E and the neutral-to-enclosure voltage VN-E are known, these can be used to define the predefined voltage value, thus it is more efficient to choose the first threshold voltage.

As shown in a dotted line of FIG. 2, according to a further embodiment, the voltage divider circuit 230 may be further configured to output a neutral-to-enclosure sample voltage VS.NE between the neutral conductor N and the metal enclosure (also called in the following neutral- to-enclosure sample voltage) to the voltage sample and rectifying module 110, wherein the neutral-to-enclosure sample voltage (VS.NE) is a fraction of the neutral-to-enclosure voltage (VN-E) between the neutral conductor N and the metal enclosure.

In such embodiments, the voltage sample and rectifying module 110 is further configured to rectify the neutral-to-enclosure sample voltage (VS.NE) and output the rectified neutral-to- enclosure sample voltage VR.NE (in short rectified N2E sample voltage) to the voltage comparison module 120.The voltage comparison module 120 is further configured to compare the rectified N2E sample voltage VR.NE with a second threshold voltage and to detect an electrical shock hazard on the metal enclosure when the rectified L2E sample voltage (VR.LE) is lower than the first threshold voltage and the rectified N2E sample voltage (VR.NE) is higher than the second threshold voltage.

Regarding with how to choose the first threshold voltage, reference can be made to the above embodiment depicted in figure 1.

Alternatively, in one example, the second threshold voltage may be chosen to be higher than a limit voltage value, the limit voltage value being proportional to a calculated neutral-to- enclosure voltage (VNE. _C ) value between the neutral conductor N and the metal enclosure when a maximum line-to-neutral voltage value (Vi_N,max) among a set of predefined main line-to- neutral voltages between the line conductor L and the neutral conductor N (in short main voltage) is used. The limit voltage value may be defined by taking into account a limit (most unfavorable) scenario, in which the circuit can be used. In such a limit scenario the maximum line-to-neutral voltage value Vi_N,max between the line conductor L and the neutral conductor N is used, and it will be assumed that the metal enclosure is not earthed well. The maximum line-to-neutral voltage value is chosen among a set of predefined main voltages between the line conductor L and the neutral conductor N as described above with reference to the implementation of Figure 1. In this scenario, a maximum neutral-to-enclosure voltage value (VNE.max) is a fraction of the maximum line-to-neutral voltage value (Vi_N,max).

The specific value of the maximum neutral-to-enclosure voltage value (V E.max) depends on how the maximum line-to-neutral voltage value (VLN.max) is shared by the maximum line-to- enclosure voltage (VLE.max) and the maximum neutral-to-enclosure voltage (V E.max).

Correspondingly, the calculated neutral-to-enclosure voltage (VNE. _C ) may be a fraction of the maximum neutral-to-enclosure voltage (VNE.max), and the proportionality factor may be a conversion ratio for rectifying half-wave AC to DC.

As can be seen from Fig. 2, not only the rectified L2E sample voltage (VR.LE) is compared with a first threshold voltage, but also the rectified N2E sample voltage (VR.NE) is compared with a second threshold voltage. According to this implementation an electrical shock hazard on the metal enclosure is detected when the rectified L2E sample voltage (V _R , _LE ) is lower than the first threshold voltage and the rectified N2E sample voltage (VR.NE) is higher than the second threshold voltage. Hence, a double check is done, thereby improving the detection accuracy and allowing fulfilling the operators' safety requirement with a higher degree of accuracy compared with conventional methods.

Figure 3a or 3b illustrates an exemplary circuit schematic diagram of a detecting circuit 300 for detecting electrical shock hazard on a metal enclosure of the AC equipment of Fig. 1. As shown in Fig.3a or Fig.3b, a voltage sample and rectifying module 1 10 of the circuit of Fig. 1 includes a voltage sample module 311 and a voltage rectification module 312.

The voltage sample module 311 comprises a first string of resistors comprising a first sample resistor (Rn ) and a second sample resistor (Ri ₂ ) connected in series with each other. A first end of the first string of resistors is connected to the line conductor L and a second end of the first string of resistors is connected to the metal enclosure. According to this implementation, the line-to-enclosure sample voltage (VS.LE) is a voltage across the second sample resistor (R12), and the voltage sample module 31 1 is configured to output the line-to-enclosure sample voltage (VS.LE) to the voltage rectification module 31 2. Although in the shown embodiment the voltage sample module 31 1 comprises a first sample resistor (Rn) and a second sampie resistor (R12), according to further embodiments two or more resistors may be alternatively used.

In the detecting circuit 300 a first end of the voltage rectification module 31 2 is connected to a common end between the first sample resistor (Rn) and the second sample resistor (R12). A second end of the voltage rectification module 31 2 is connected to the voltage comparison module 1 20, the voltage rectification module 31 2 being configured to rectify the line-to- enclosure sample voltage VS.LE.

The voltage comparison module 1 20 is configured to compare the rectified L2E sample voltage (VR.LE) with the first threshold voltage and output an alarm signal when the rectified L2E sample voltage (VR.LE) is lower than the first threshold voltage. Specifically, the voltage comparison module 1 20 outputs an output signal having a first value when the rectified L2E sample voltage (VR.LE) is lower than the first threshold voltage. The first value indicates an electrical shock hazard on the metal enclosure of the AC equipment. The output signal may be seen as an alarm signal. Furthermore, the voltage comparison module 1 20 may output an output signal having a second value when the rectified L2E sample voltage (VR.LE) is higher than the first threshold voltage. The second value indicates no electrical shock hazard on the metal enclosure of the AC equipment.

In a further embodiment, the voltage sample module 31 1 may include a number m of resistors connected in series where m is an integer and m>=2. In this case, the first threshold voltage, Vrefi may be represented by the following formula:

wherein Rn represents the resistance of the rth sample resistor in the first string of resistors in the voltage sample module 311 respectively, Vi.E,efr represents an effective voltage between the line conductor L and the metal enclosure when the minimum line-to-neutral voltage value (Vi_N,min) among the set of predefined main voltages between the line conductor L and the neutral conductor N is used. The voltage V _L E,eff is a fraction of the minimum line-to-neutral voltage value (VLN.min), and a represents a conversion ratio for rectifying half-wave AC to DC. It is noted that when the mt sample resistor is included in the first string of resistors and m=3, the third sample resistor may be connected in series to the first sample resistor, an end of the third sample resistor being connected to the line conductor L.

Also in this case, the limit scenario, in which the minimum line-to-neutral voltage value (Vi_N,min) is used and the metal enclosure of the AC equipment is not earthed well, is considered. Hence, the effective voltage VLE.eff is a fraction of the minimum line-to-neutral voltage value (V _L N,min), for example, the voltage Vi_E,eff may be half of the minimum line-to-neutral voltage value

In one example, as shown in Figure 3a or 3b or 4, the first string of resistors in the voltage sample module 311 may consists of a first sample resistor and a second sample resistor connected in series with each other. In this case a first end of the first sample resistor is connected to the line conductor L and a second end of the second sample resistor is connected to the metal enclosure, and the first threshold voltage, V _ie ii is represented by the following formula:

12

0 < F„„ < a * V,

^LE ' ^eff R -) ^■ #,, '

wherein Ru and Rn represent the resistance of the first and second sample resistor in the voltage sample module 311 , respectively.

In a further embodiment, the total resistance of the first string of resistors in the voltage sample module 311 is larger than 1 ΜΩ to limit the current. The first string of resistors in the voltage sample module 311 may include more than two resistors to ensure safety also in case that one of the resistors fails.

In any of the embodiments previously described, the detection circuit may further include an alarm module 140. The alarm module 140 is configured to generate an alarm based on the alarm signal, wherein the alarm is any one or combination of the following: buzzer, light and vibration.

In any of the embodiments previously described, the voltage comparison module 120 comprises any one or combination of the following: Silicon Controlled Rectifier (SCR), metallic oxide semiconductor field effect transistor (MOSFET), operational amplifier, Comparators, Relay and voltage regulator diode.

In one example, as shown in Fig.4, a possible implementation of a voltage rectification module of the circuit of Fig. 3a or Fig. 3b is described. The voltage rectification module 412 includes a rectify diode D1 , a filter capacitor C1 and a discharge resistor F¾, wherein the filter capacitor C1 is used for filtering a voltage waveform of the line-to-enclosure sample voltage VS.LE , and the discharge resistor is used for discharging the current from the filter capacitor C1 .

The anode of the rectify diode D1 is connected to the common end between the first sample resistor and the second sample resistor, and the cathode of the rectify diode D1 is connected to the voltage comparison module 420.

The filter capacitor C1 and the discharge resistor F¾ are connected in parallel, and both the filter capacitor C1 and the discharge resistor R3 are connected between the cathode of the rectify diode D1 and the metal enclosure. In the realization depicted in figure 4, the anode of the rectify diode D1 correspond to the first end of the voltage rectification module 412, and the cathode of the rectify diode D1 corresponds to the second end of the voltage rectification module 412.

Fig.4 illustrates a possible realization of a voltage comparison module, which can be used in the detection circuit according to the implementations of the invention described so far. The voltage comparison module 420 of the detection circuit 400 illustrated in Fig. 4 includes an operational amplifier 421 . A non-inverting input 422 of the operational amplifier 421 is connected to the cathode of the rectify diode D1 , or more generally to the output of the rectifying module 412. An inverting input 423 of the operational amplifier 421 is connected to the source of the first threshold voltage; a positive power supply of the operational amplifier 421 is connected to GND; a negative power supply of the operational amplifier 421 is connected to Vcc (i.e. the supply voltage for the operational amplifier 421 ). In an implementation, an output of the operational amplifier 421 is connected to the alarm module 140. The alarm signal, which may be a low electrical level signal (e.g. 0), is output when the rectified L2E sample voltage (VR, _LE ) is lower than the first threshold voltage, and a high electrical level signal (e.g. 1 ) is output when the rectified L2E sample voltage (VR.LE) is higher than the first threshold voltage.

In another example, which can be used in alternative to the implementation described above, the inverting input of the operational amplifier 421 may be connected to the cathode of the rectify diode D1 and the non-inverting input of the operational amplifier 421 may be connected to the source of the first threshold voltage. In this case, the positive power supply of the operational amplifier 421 is connected to Vcc; a negative power supply of the operational amplifier 421 is connected to GND. As described above, an output of the operational amplifier 421 is connected to the alarm module 140. According to this implementation, the alarm signal being a high electrical level signal (e.g. 1 ) is output when the rectified L2E sample voltage (VR.LE) is lower than the first threshold voltage, and a low electrical level signal (e.g. 0) is output when the rectified L2E sample voltage (VR.LE) is higher than the first threshold voltage.

Furthermore, as shown in Fig.3a, the voltage sample module 31 1 , the voltage rectification module 312 and the voltage comparison module 320 have a common terminal connected to a common voltage reference GND.

As shown in Fig.4, the voltage sample module 31 1 , the voltage rectification module 412 and the voltage comparison module 420 have a common terminal connected to a common voltage reference GND. Alternatively, as shown in Fig.3b, a voltage reference terminal of the voltage sample module

31 1 may be different from a common voltage reference of the voltage rectification module

312 and the voltage comparison module 320.

As can be seen from Fig. 3a or Fig.3b or Fig.4, the first string of resistors (including a first sample resistor and a second sample resistor) being connected between the line conductor L and the metal enclosure actually has two functions: firstly it is used to sample a line-to- enclosure sample voltage across the second sample resistor and secondly it can be used to ensure an insulation between the line conductor L and the enclosure. In this manner even through the operators touch one or more parts of the AC equipment, e.g. the circuit along the bold line shown in Fig. 4, the electric current passing through the body of the operators will be reduced compared with the used main voltage, thereby further increasing the safety of the circuit.

Figure 5 illustrates a schematic diagram of an example of a detecting circuit 500 for detecting electrical shock hazard on a metal enclosure of AC equipment of Fig. 1. As shown in Fig.5, a voltage sample and rectifying module includes a voltage sample module 511 and a voltage rectification module 412.

The voltage sampie module 511 comprises a string of components comprising a first resistor F¾i and a transformer T1 connected in series with each other, a first end of the string of components being connected to the line conductor L and a second end of the string of components being connected to the metal enclosure. The line-to-enclosure sample voltage (VS.LE) is a voltage output from the transformer T1 , and the voltage sample module 511 is configured to output the line-to-enclosure sample voltage (VS.LE) to the voltage rectification module 512. Although in the shown embodiment the voltage sample module 511 comprises a first resistor R51 configured to limit the current, according to further embodiments two or more resistors may be also used. In the implementation depicted in Fig. 5, a voltage rectification module 412 includes a rectify diode D1 , a filter capacitor C1 and a discharge resistor F¾. The anode of the rectify diode D1 is connected to an output of the transformer T1 , and the cathode of the rectify diode D1 is connected to the voltage comparison module 420;. The filter capacitor C1 and the discharge resistor F¾ are connected in parallel, and both the filter capacitor C1 and the discharge resistor R3 are connected between the cathode of the rectify diode D1 and the metal enclosure.

The anode of the rectify diode D1 corresponds to the first end of the voltage rectification module 412, and the cathode of the rectify diode D1 corresponds to the second end of the voltage rectification module 412.

The transformer T1 has a primary winding on a primary side of the transformer T1 and a secondary winding on a secondary side of the transformer T1. The primary winding's circle number and the secondary winding's circle number are represented by N1 and N2 respectively.

Considering an implementation in which the first resistor R51 , the second resistor R52 and the transformer T1 are included in the first string of component in the voltage sample module 511 , the first threshold voltage, V _re fi may be represented by the following formula:

R N

0 < F . _S < a * V * ^Ώ *— ^■ -— ,

' ^{e/I u} <* R + R _{57 +} R _n N _{l +} N ₂

wherein R51, R52, and Rn represent the resistance of the first resistor, the second resistor and the transformer T1 in the voltage sample module 511 respectively, represents an effective voltage between the line conductor L and the metal enclosure when the minimum line-to-neutral voltage value (Vi_N.min) among the set of predefined main voltages between the line conductor L and the neutral conductor N is used. The voltage VLE.etf is a fraction of the minimum line-to-neutral voltage value (VLN.min), and a represents a conversion ratio for rectifying half-wave AC to DC.

For other detailed implementation details, reference may be made to the above embodiments, which are not repeated herein.

Figure 6a, 6b, 6c, 6d or 7 illustrates an exemplary circuit schematic diagram of a circuit for detecting electrical shock hazard on a metal enclosure of AC equipment of Fig. 2. As shown in Figure 6a, 6b, 6c, 6d or 7, a voltage sample and rectifying module 1 10 of the circuit of Fig. 2 includes a voltage sample module 311 and a voltage rectification module 312.

The voltage sample module 311 comprises a first string of resistors comprising a first sample resistor and a second sample resistor connected in series with each other. A first end of the first string of resistors is connected to the line conductor L and a second end of the first string of resistors is connected to the metal enclosure. The line-to-enclosure sample voltage (VS.LE) is a voltage across the second sample resistor R12, and the voltage sample module 311 is configured to output the line-to-enclosure sample voltage (VS.LE) to the voltage rectification module 312. Although in the shown embodiment the voltage sample module 311 comprises a first sample resistor Rn and a second sample resistor R12, according to further embodiments two or more resistors may be also used.

A first end of the voltage rectification module 312 is connected to the common end between the first sample resistor Rn and the second sample resistor R12 and a second end of the voltage rectification module 12 is connected to the voltage comparison module120. The voltage rectification module 312 is configured to rectify the line-to-enclosure sample voltage (VS.LE) across the second sample resistor, and output the rectified L2E sample voltages (VR.LE) to the voltage comparison module 120.

The voltage divider circuit 630 comprises a second string of resistors comprising a first divider resistor R21 and a second divider resistor R22 connected in series with each other. A first end of the second string of resistors is connected to the neutral conductor N and a second end of the second string of resistors is connected to the metal enclosure.

Although in the shown embodiment the voltage divider circuit 630 comprises the first divider resistor, R21 and the second divider resistor, R22, according to further embodiments two or more resistors may be also used. According to a further embodiment, as shown in FIG. 6c or 6d, the voltage divider circuit 630 is configured to output a neutral-to-enclosure sample voltage (VS.NE) to the voltage rectification module 312, the neutral-to-enclosure sample voltage (VS.NE) being a voltage across the second divider resistor R22.

According to a further embodiment, as shown in Figure 6c or 6d, a third end of the voltage rectification module 312 may be connected to the common end between the first divider resistor R21 and the second divider resistor, R22 of the voltage divider circuit 630. The voltage rectification module 312 is further configured to rectify the neutral-to-enclosure sample voltage (VS.NE) across the second divider resistor R22, and output the rectified neutral-to-enclosure sample voltages (VR.NE) to the voltage comparison module 120.

The voltage comparison module 120 is configured to compare the rectified line-to-enclosure sample voltage VR.LE (in short rectified L2E sample Voltage) with the first threshold voltage and to compare the rectified neutral-to-enclosure sample voltage (VR.NE) with the second threshold voltage. The voltage comparison module 20 is further configured to output an alarm signal when the rectified L2E sample voltage (VR.LE) is lower than the first threshold voltage and the rectified N2E sample voltage (VR.NE) is higher than the second threshold voltage., The alarm signal indicates the electrical shock hazard on the metal enclosure of the AC equipment. In one example, the alarm signal, which may be a high electrical level signal (e.g. 1 ), is output when the rectified L2E sample voltage (VR.LE) is lower than the first threshold voltage and the rectified N2E sample voltage (VR.NE) is higher than the second threshold voltage; while a normal signal, which may be a low electrical level signal (e.g. 0), is output when the rectified L2E sample voltage (VR.LE) is higher than the first threshold voltage and the rectified N2E sample voltage (VR.NE) is lower than the second threshold voltage.

In another example, the alarm signal, which may be a low electrical level signal (e.g. 0), is output when the rectified L2E sample voltage (VR,LE) is lower than the first threshold voltage and the rectified N2E sample voltage (VR.NE) is higher than the second threshold voltage; while a normal signal, which may be a high electrical level signal (e.g. 1 ), is output when the rectified L2E sample voltage (V _R , _L E) is higher than the first threshold voltage and the rectified N2E sample voltage (VR.NE) is lower than the second threshold voltage.

In a further embodiment, the voltage sample module 311 may include a number m of sample resistors connected in series, and the voltage divider circuit 630 may include a number n of divider resistors connected in series, where m or n is an integer and m, n>-2. In this case, the first threshold voltage, V _re » may be represented by the following formula:

wherein Rn, R12, . and Ri,„ represent the resistance of the first sample resistor, and the mth sample resistor of the first string of resistors in the voltage sample module 311 respectively, VLE.eff represents an effecitive voltage between the line conductor L and the metal enclosure when the minimum line-to-neutral voltage value (VLN.min) among a set of predefined main voltages between the line conductor L and the neutral conductor N is used. The voltage is a fraction of the minimum line-to-neutral voltage value (VL .mm), and a represents a conversion ratio for rectifying half-wave AC to DC. The voltage Vi_E,eff may be given by:

wherein R21, ..., ¾ _> represent the resistance of the first divider resistor, the second divider resistor,... and the nth divider resistor of the second string of resistors in the voltage divider circuit 630 respectively. Here, the limit scenario, where a minimum line-to-neutral voltage value (V _L N,min) between the line conductor L and the neutral conductor N is used and the metal enclosure of the AC equipment is not earthed well, is considered. Hence the voltage Vi_E,eff is a fraction of the minimum line-to-neutral voltage value ( min) For example, the voltage Vi_E,eff is a half of the minimum line-to-neutral voltage value (VLN.min) if the resistance of the voltage sample module 311 are the same with the resistance of the voltage divider circuit 630.

In one example, as shown in Figure 6a, 6b, 6c, 6d or 7, the first string of resistors in the voltage sample module 311 may consist of a first sample resistor and a second sample resistor connected in series with each other. According to this configuration, a first end of the first sample resistor is connected to the line conductor L and a second end of the second sample resistor is connected to the metal enclosure. The first threshold voltage, V _rS fi will be given by the following formula:

wherein R and Rn represent the resistance of the first sample resistor, the second sample resistor in the voltage sample module 311 respectively. In yet another example, as shown in Figure 6a, 6b, 6c, 6d or 7, the first string of resistors in the voltage sample module 311 may consist of a third sample resistor Ri ₃ (dashed in the figures), a first sample resistor Rn and a second sample resistor R12 connected in series with each other. According to this configuration, an end of the third sample resistor R13 is connected to the iine conductor L and a second end of the second sample resistor R12 is connected to the metal enclosure. The first threshold voltage, V _re n is represented by the following formula:

wherein Ru, R12 and R13 represent the resistance of the first sample resistor, the second sample resistor and the third sample resistor in the voltage sample module 311 respectively.

In a further embodiment, the voltage divider circuit 630 may include a number n of divider resistors connected in series, where n is an integer and n>=2, the second threshold voltage, Vref2 may be represented by the following formula:

7=1 wherein R21, ... , Rzm represent the resistance of the first divider resistor, the second divider resistor and the mth divider resistor in the second string of resistors in the voltage divider circuit 630 respectively, V _N E,eff represents an effective voltage between the neutral conductor N and the metal enclosure when a maximum line-to-neutral voltage value (VLN.max) among a set of predefined main voltages between the line conductor L and the neutral conductor N is used. The effective voltage VNE.eff is here a fraction of the maximum line-to-neutral voltage value (VLN.ma ), and or represents a conversion ratio for rectifying half-wave AC to DC.

The effective voltage VNE.eff is given by:

VNE.eff = V _{LN n} * wherein R21 R∑n represent the resistance of the first divider resistor, the second divider resistor,... and the nth divider resistor of the second string of resistors in the voltage divider circuit 630 respectively.

Hence, the limit scenario, where not only the maximum line-to-neutral voltage value (VLN.max) between the line conductor L and the neutral conductor N is used and the metal enclosure is not earthed well, is considered. Hence the effective voltage V E.eff is a fraction of the maximum line-to-neutral voltage value (VLN.max), for example, the voltage V E.eff is a half of the maximum line-to-neutral voltage value (Vi_N,max) if the resistance of the voltage sample module 311 are the same with the resistance of the voltage divider circuit 630.

In one example, as shown in Figure 6a, 6b, 6c, 6d or 7, the second string of resistors in the voltage divider circuit 630 may consist of a first divider resistor F½ and a second divider resistor F½ connected in series with each other. In this case a first end of the first divider resistor R21 is connected to the neutral conductor N and a second end of the second divider resistor R22 is connected to the metal enclosure. The second threshold voltage, V _r0 f2 is represented by the following formula:

V > * V * ^

^K 2 \ + ^K 22 wherein R21 and R22 represent the resistance of the first divider resistor and the second divider resistor in the second string of resistors in the voltage divider circuit 630 respectively.

In another example, as shown in Figure 6a, 6b, 6c, 6d or 7, the second string of resistors may include a third divider resistor, a first divider resistor and a second divider resistor connected in series with each other in the voltage divider circuit 630. In this case an end of the third divider resistor being connected to the neutral conductor N and a second end of the second divider resistor is connected to the metal enclosure. The second threshold voltage, V _re f2 is represented by the following formula:

V^ > a * V,

R + R _n + R ₂₃ wherein R21, R22 and R23 represent the resistance of the first divider resistor, the second divider and the third divider resistor in the voltage divider circuit 630 respectively.

In a further embodiment, the total resistance of the first string of resistors in the voltage sample module 311 or the total resistance of the second string of resistors in the voltage divider circuit 630 is larger than 1 Ω to limit the current. The respective string of resistors in the voltage sample module 311 or the voltage divider circuit 630 includes more than two resistors to ensure safety in case one or more of the remaining resistors fails. In an advantageous realization, the total resistance of the first string of resistors in the voltage sample module 311 is the same as the total resistance of the second string of resistors in the voltage divider circuit 630.

According to a further embodiment, the detecting circuit further includes an alarm module 140, configured to generate an alarm based on the alarm signal, wherein the alarm is any one or combination of the following: buzzer, light and vibration. The alarm module 140 outputs the alarm to alert an operator of a failure which may cause an electrical hazard. By means of sampling the voltage between the line conductor L and the metal enclosure and using said voltage for judging whether a failure occurred or not, the present invention does not have to rely on the fact that the earth pin of the socket or the metal enclosure is earthed well. Even if said components are not earthed well, there will not be un-wanted or false alarms.

In such embodiments, the voltage comparison module 120 comprises any one or combination of the following: Silicon Controlled Rectifier (SCR), metallic oxide semiconductor field effect transistor (MOSFET), operational amplifier, Comparators, Relay and voltage regulator diode. In one example, as shown in Fig.7, a voltage rectification module 412 may include a rectify diode D1 , a filter capacitor C1 and a discharging resistor R ₃ .

The anode of the rectify diode D1 is connected to the common end between the first sample resistor and the second sample resistor, and the cathode of the rectify diode D1 is connected to the voltage comparison module 420. Further, the filter capacitor C1 and the discharging resistor are connected in parallel, and both the filter capacitor C1 and the discharging resistor are connected between the cathode of the rectify diode D1 and the metal enclosure.

In the realization depicted in figure 7, the anode of the rectify diode D1 corresponds to the first end of the voltage rectification module 412, and the cathode of the rectify diode D1 corresponds to the second end of the voltage rectification module 412.

In one example, as shown in Fig.7, the voltage comparison module 420 may include an operational amplifier 421. A non-inverting input 422 of the operational amplifier 421 is connected to the cathode of the rectify diode D1 ; an inverting input 423 of the operational amplifier 21 is connected to the source of the first threshold voltage V _re fi ; a positive power supply of the operational amplifier 421 is connected to GND (i.e. ground); a negative power supply of the operational amplifier 421 is connected to Vcc (i.e. the supply voltage for the operational amplifier 421 ). According to an advantageous realization, an output of the operational amplifier 421 is connected to an alarm module 140. The alarm signal being a low electrical level signal (e.g 0) is output when the rectified L2E sample voltage (VFUE) is lower than the first threshold voltage, and the normal signal being a high electrical level signal (e.g 1 ) is output when the rectified L2E sample voltage (VFUE) is higher than the first threshold voltage. In another example, a inverting input 423 of the operational amplifier 421 is connected to the cathode of the rectify diode D1 ; an non-inverting input 422 of the operational amplifier 421 is connected to the source of the first threshold voltage V _re fi ; a positive power supply of the operational amplifier 421 is connected to Vcc; a negative power supply of the operational amplifier 421 is connected to GND; preferably, an output of the operational amplifier 421 is connected to an alarm module 140. Correspondingly, the alarm signal being a high electrical level signal (e.g 1 ) is output when the rectified L2E sample voltage (VR.LE) is lower than the first threshold voltage, and the normal signal being a low electrical level signal (e.g 0) is output when the rectified L2E sample voltage (VR.LE) is higher than the first threshold voltage. Furthermore, as shown in Fig.6a or Fig.6c, the common terminal of the voltage sample circuit 311 , the voltage division circuit 630, the voltage rectification module 312 and the voltage comparison module 120 is a common voltage reference.

As shown in Fig.7, the common terminal of the voltage sample circuit 311 , the voltage division circuit 630, the voltage rectification module 412 and the voltage comparison module 420 is a common voltage reference.

Alternatively, as shown in Fig. 6b, a common voltage reference of the voltage sample circuit 311 and the voltage division circuit 630 is different from a common voltage reference of the voltage rectification module 312 and the voltage comparison module 120.

As shown in Fig.6d, a common voltage reference of the voltage sample circuit 311 , the voltage division circuit 630 and the voltage rectification module 312 is different from a voltage reference terminal of the voltage comparison module 120.

As can be seen from Fig. 6a or Fig.6b or Fig.6c or Fig.6d or Fig.7, the first string of resistors (including a first sample resistor and a second sample resistor) connected between the line conductor L and the metal enclosure has two functions: firstly it is used to sample a line-to- enclosure sample voltage across the second sample resistor (VS.LE) and secondly it can be used to ensure an insulation between the line conductor L and the enclosure. In this manner, even through the operators touch one or more parts of the AC equipment, e.g. the circuit along the bold line shown in Fig. 7, the electric current passing through the body of the operators will be reduced compared with the used main voltage, thereby further increasing the safety of the circuit. Furthermore, the second string of resistors (including a first divider resistor and a second divider resistor) connected between the neutral conductor N and the metal enclosure has three functions: firstly it is used to divide or split the main voltage (VL-N) between the line conductor L and the neutral conductor N, secondly it can be used to ensure an insulation between the neutral conductor N and the enclosure. In this manner, even through the operators touch one or more parts of the AC equipment, e.g. the circuit along the bold line shown in Fig. 6c, the electric current passing through the body of the operators will be reduced compared with the used main voltage, thereby further increasing the safety of the circuit, and thirdly it is used to sample a neutral-to-enclosure sample voltage (VS.NE) across the second divider resistor R22. Furthermore, not only the rectified L2E sample voltage (VR,LE) across the second sample resistor is compared with a first threshold voltage, but also the rectified N2E sample voltage (VR.NE) across the second divider resistor is compared with a second threshold voltage. According to this implementation an electrical shock hazard on the metal enclosure is detected when the rectified sample voltage (V R.LE) is lower than the first threshold voltage and the rectified sample voltage (V R.NE) is higher than the second threshold voltage. Hence, a double check is done, thereby improving the detection accuracy and allowing fulfilling the operators' safety requirement with a higher degree of accuracy compared with standard methods.

Figure 8 or 9 illustrates an exemplary circuit schematic diagram of a circuit for detecting electrical shock hazard on a metal enclosure of AC equipment of Fig. 1. As shown in Figure 8, a voltage sample and rectifying module 810 includes: a rectify diode D1 , a first resistor R ₈ i and a second resistor Rs2 connected in series with each other. The anode of the rectify diode D1 is connected to the line conductor L and an end of the second resistor Rs2 is connected to the metal enclosure.

A filter capacitor C1 and a discharging R3 resistor are connected in parallel, and both the filter capacitor C1 and the discharging resistor R3 are connected between the common end between the first resistor Rsi and the second resistor Rs2, and the metal enclosure.

The voltage sample and rectifying module 810 is configured to output the rectified L2E sample voltage (VR.LE) to the voltage comparison module 420, the rectified L2E sample voltage (VR, _L E) being a voltage across the second resistor Rs2. In such embodiment, the voltage comparison module 420 is connected to the common end between the first resistor R ₈ i and the second resistor Rs2.

As shown in Figure 9, a voltage sample and rectifying module 910 includes a first resistor R91 , a rectify diode D1 and a second resistor R92 connected in series with each other, an end of the first resistor R91 being connected to the line conductor L and an end of the second resistor R92 being connected to the metal enclosure.

A filter capacitor C1 and the discharging resistor f¾ are connected in parallel, and both are connected between the common end between the cathode of the rectify diode D1 and the second resistor R92, and the metal enclosure. The voltage sample and rectifying module 910 is configured to output the rectified L2E sample voltage (VR.LE) to the voltage comparison module 420, the rectified L2E sample voltage (VR,LE) being a voltage across the second resistor R92. In such embodiment, the voltage comparison module 420 is connected to the common end between the cathode of the rectify diode D1 and the second resistor R92. For other details about the implementation of the detection circuit, reference may be made to the above embodiments, which are not repeated herein.

Fig. 10 illustrates an exemplary block diagram of AC equipment according to an embodiment of the present invention. As shown in FIG. 10, the AC equipment includes a metal enclosure 10 which is a device housing and a detection circuit for detecting electrical shock hazard on the metal enclosure of the AC equipment as described above.

According to a further embodiment, the AC equipment includes a power supply unit (PSU) 150 being configured to convert an input AC voltage (for example, 220Vac or 110Vac) to a DC voltage (for example, 3.3Vdc or 5Vdc) being a power supply for internal use in the AC equipment. In a example, the AC equipment may be an AC base station, the detection circuit for detecting electrical shock hazard on the metal enclosure of the AC equipment may be deployed in an area which is provided by a power supply unit (PSU) included in the AC base station.

It can be understood that, the circuit for detecting electrical shock hazard on the metal enclosure of the AC equipment of this embodiment may be specifically implemented according to the above embodiments; reference may be made to related description in the above embodiment, which is not repeated herein.

As can be seen from Fig. 10, regardless of whether the metal enclosure of the AC equipment is earthed well or not, a line-to-enclosure sample voltage (VS.LE) between the line conductor and the metal enclosure is sampled, and an electrical shock hazard on the metal enclosure can be detected when the rectified L2E sample voltage (V R.LE) is lower than the first threshold voltage, thus the detection accuracy can be improved or ensured, and the safety requirement of the AC equipment can be met. FIG. 11 illustrates an exemplary block diagram of a communication system according to an embodiment of the present invention. As shown in FIG. 1 1 , the communication system includes an AC equipment 1100 and a power system 1101 for supplying power to the AC equipment.

Furthermore, according to further embodiment implementations, as shown in FIG. 11 , the power system 1101 may be deployed at a distance away from a foundation of a tower of the communication system, for example, the distance may be larger than 300 meter, and the AC equipment 1100 is deployed on a tower of the communication system (for example, deployed on a top of the tower of the communication system), the power system 1101 is configured to convert the high voltage A.C. (e.g 10KVac) from the electricity distribution system to a normal low mains voltage A.C. (for example, 220Vac or 1 0Vac) for supplying power to the AC equipment 1100.

In such embodiment, the AC equipment 1 100 includes a metal enclosure and a detection circuit for detecting electrical shock hazard on the metal enclosure of the AC equipment as described above. Preferably, the AC equipment 1 100 further includes a power supply unit (PSU) being configured to convert an input AC voltage (for example, 220Vac or 110Vac) to a DC voltage (for example, 3.3Vdc or 5Vdc) being a power supply for internal use in the AC equipment. It can be understood that, the detection circuit for detecting electrical shock hazard on the metal enclosure of the AC equipment of this embodiment may be specifically implemented according to the above embodiments; reference may be made to related description in the above embodiment, which is not repeated herein.

It can seen from the above, regardless of whether the metal enclosure of the AC equipment is earthed well or not, the line-to-enclosure sample voltage (VS.LE) between the line conductor and the metal enclosure is sampled, and an electrical shock hazard on the metal enclosure can be detected when the rectified L2E sample voltage (V .LE) is lower than the first threshold voltage, thus the detection accuracy can be improved or ensured, and the operators' safety requirement of the communication system can be met. Fig. 12 illustrates an exemplary block diagram of a base station system (BSS) according to an embodiment of the present invention. As shown in FIG. 12, the base station system (BSS) comprising an AC base station 1200 and a power system 1201 for supplying power to the AC base station, wherein the AC base station 1200 comprises a metal enclosure and a detection circuit for detecting electrical shock hazard on the metal enclosure of the AC equipment described above, wherein the AC base station is deployed on a tower of the BSS.

Furthermore, according to further embodiment implementations, as shown in FIG. 11 , wherein the power system is deployed at a distance away from a foundation of a tower of the BSS, for example, the distance may be larger than 300 meter, and the AC base station is deployed on a top of the tower of the BSS, the power system is configured to convert the high voltage A.C. (e.g 10KVac) from the electricity distribution system to a normal low mains voltage (for example, 220Vac or 110Vac) for supplying power to the AC base station.

Preferably, the AC equipment may further includes a power supply unit (PSU) being configured to convert an input AC voltage (for example, 220Vac or 110Vac) to a DC voltage (for example, 3.3Vdc or 5Vdc) being a power supply for internal use in the AC equipment.

Furthermore, the base station system (BSS) further comprises a base station controller 1202; reference may be made to the known prior art, which is not repeated herein.

The power system is further configured to convert the high voltage A.C. (e.g 10KVac) from the electricity distribution system to a normal low mains voltage (for example, -48Vdc) for supplying power to the base station controller 1202.

It can be understood that, the detection circuit for detecting electrical shock hazard on the metal enclosure of the AC equipment of this embodiment may be specifically implemented according to the above embodiments; reference may be made to related description in the above embodiment, which is not repeated herein. It can seen from the above, regardless of whether the metal enclosure of the AC equipment is earthed well or not, the line-to-enclosure sample voltage (VS.LE) between the line conductor and the metal enclosure is sampled, and an electrical shock hazard on the metal enclosure can be detected when the rectified L2E sample voltage (V R.LE) is lower than the first threshold voltage, thus the detection accuracy can be improved or ensured, and the operators' safety requirement of the BSS can be met.

It is noted that the term "voltage" in the present invention refers to an effective voltage.

In the previous embodiments, the term "sample resistor", "divider resistor" are used for identifying the different resistors based on their location in the circuit. However, it should be understood that, the term "sample", "divider" is not intended to limit the resistor to any specific type of resistors.

In the above embodiments, the description of each embodiment has its emphasis, and for the part that is not detailed in an embodiment, reference may be made to the relevant description of other embodiments. The objectives, technical solutions, and advantages of the present invention are further illustrate above in detail through the exemplary embodiments, but it should be understood that the above descriptions are merely exemplary embodiments of the present invention, but are not intended to limit the present invention. Any modification, equivalent replacement, or improvement made without departing from the principle of the present invention should fall within the protection scope of the present invention.