This invention relates to a method and apparatus for monitoring the earth loop impedance of an electrical installation that is protected by a circuit protection device, such as a residual current device. Said method and apparatus enabling an earth loop impedance test to be performed on an electrical installation without tripping the circuit protection device.

Domestic systems are required to have their earth loop impedance tested as such is essential in verifying the safety of electrical installations. Typically, a test setup is used similar to that shown in Figure 1, where a test box 10 supplying a test voltage is applied at a customer's premises 12, usually connected directly to a socket outlet 14. The test voltage generates a current which flows via the neutral lead to the power station or local sub station 16 and then returns via the earth connection 18 back to the premises 12. By measuring the current flowing, the earth loop impedance can be determined.

However, a problem exists when circuit protection devices such as earth leakage circuit breakers (ELCBs) or residual current devices (RCDs) 20a, 20b are fitted in the customer's premises. Generally speaking, an RCD consists of an toroid having the supply phase and neutral passing therethrough, which acts as the primary winding of a current transformer; a secondary winding around the

toroid is connected to a trip mechanism. Under normal conditions, the phase and neutral currents are equal and opposite so that no flux is induced in the toroid and hence no current flows in the secondary winding. If a fault condition occurs and current flows through the earth path back to the energy source, the phase and neutral currents will no longer be balanced and a flux will be induced in the toroid, which will activate the trip mechanism and disconnect the electrical supply.

Unfortunately, as RCDs 20a, 20b are present to detect leakage current to earth, the connection of an earth loop impedance tester to check the integrity of the earth loop impedance trips the RCD whenever a test current is passed. In order to address this, loop impedance testers 10 have been developed using very low test currents, such that any RCDs present do not trip, but these, as a result, are not particularly accurate. Current practice is to short circuit any RCDs present 20a, 20b before performing a test. This is time- consuming for the person performing the test and dangerous if the short circuits are forgotten and left in place afterwards .

Other examples of prior art arrangements have been disclosed in GB 2 239 530 and EP 0 295 800 which describe methods and apparatus for temporarily desensitising the operation of the RCD by injecting a DC current or a very low frequency AC current into the sensing circuit of the RCD to saturate the toroid core, and so that a loop impedance test can then be made. EP 0 278 174 describes an earth monitoring apparatus whereby the earth loop

impedance is measured by injecting a high frequency test signal into the earth conductor, and measuring the resultant current flowing through the earth conductor, so as to provide a measurement of the earth impedance. The integral earth leakage protection is insensitive to this high frequency test signal and therefore does not trip.

These approaches however suffer from a number of disadvantages in that the DC test signal or the high frequency AC test signal shift the operating point of the toroid to a point on its B-H curve where it can be insensitive to a genuine earth fault current. Hence, the sensitivity of the RCD will be reduced which, of course, could be extremely dangerous should a fault condition occur shortly after an earth loop impedance test has been performed.

These approaches also suffer from poor measurement accuracy, as the measurement frequency is not representative of the normal operating frequency of 50Hz or 60Hz.

It is the object of the present invention to provide a method and apparatus for monitoring the earth loop impedance of an electrical installation that is protected by a circuit protection device, such as a residual current device. Said method and apparatus enabling an earth loop impedance test to be performed on an electrical installation without tripping the circuit protection device and to maintain the sensitivity of the device after an earth loop impedance test has been performed. It is a further object of the present

invention to provide a method and apparatus which automatically signals to any circuit protection devices present in the electrical installation that an earth loop impedance test is about to be performed. It is a further object of the present invention to provide a circuit protection device, such as a residual current device, which can detect the earth loop impedance test signal and not trip for a predetermined interval. After the predetermined interval, the circuit protection device reverts to its normal operation.

According to the present invention there is provided a method of temporarily disabling the operation of a circuit protection device having an electrical installation connected thereto and determining the earth loop impedance of said electrical installation, comprising the steps of: injecting a signal sequence into a conductor of said electrical installation under test from a source providing said signal sequence, said signal sequence comprising a first part of said sequence to disable operation of said circuit protection device and a second part of said sequence to monitor the earth loop impedance of said electrical installation; receiving said signal sequence from said source; verifying if said signal sequence comprises said first part of said sequence and disabling operation of said circuit protection device for a predetermined period of time; and measuring the current flowing in said second part of said sequence to determine said earth loop impedance of said electrical installation.

Also according to the present invention there is provided an apparatus for determining the earth loop impedance of an electrical installation that is protected by a remote circuit protection device, comprising: a source for injecting a signal sequence into a conductor of said electrical installation under test, said signal sequence comprising a first part of said sequence to disable operation of said remote circuit protection device and a second part of said sequence to monitor the earth loop impedance of said electrical installation; and measuring means for measuring the current flowing in said second part of said sequence to determine said earth loop impedance of said electrical installation.

Further according to the present invention there is provided a circuit protection device for detecting a residual current on an electrical installation, comprising: detection means for sensing said residual current on said electrical installation, said residual current also comprising a signal sequence generated by a remote source connected to said electrical installation, said signal sequence comprising a first part of said sequence to disable operation of said circuit protection device and a second part of said sequence which, in use, determines the earth loop impedance of said electrical installation by said remote source; processing means, connected to said detection means, for processing if said signal sequence comprises said first part of said sequence and disabling operation of

said circuit protection device for a predetermined period of time; and relay means, operative from said processing means, for disconnecting the electrical supply to said electrical installation if said residual current exceeds a predetermined fault condition.

Preferably, said circuit protection device is a residual current device. In use, said relay means further comprises a trip coil controlling main contacts in the phase and neutral of said electrical supply. Said relay means further comprising overload and earth fault elements .

Further preferably, said circuit protection device is a digital residual current device, implemented using a microprocessor or digital signal processor. In use, the output of said detection means is sampled and converted from analogue to digital for processing by said processing means.

In a preferred embodiment, the signal sequence may be any form that is producible by said source and detectable by said circuit protection device. The signal sequence may be unique, such that said circuit protection device is disabled only when an earth loop impedance test is about to be performed. In use, said second part of said sequence, which is used to determine the earth loop impedance of the electrical installation, has a normal mains frequency of 50Hz or 60Hz .

In a further embodiment, the source for injecting said signal sequence into a conductor of said electrical installation under test may be implemented using a first source for injecting said first part of said sequence to disable operation of said remote circuit protection device and a second source for injecting said second part of said sequence to monitor the earth loop impedance of said electrical installation. In this embodiment, said first source may be located in a remote ancillary unit situated between said apparatus and said remote circuit protection device.

In use, said detection means comprises a toroid having a supply phase and neutral passing therethrough. The frequency response of said toroid means that the amplitude of the first part of said sequence may be higher for higher signalling frequencies. A lower signalling frequency may be easier to detect.

Preferably, the signalling sequence is within the bandwidth of the RCD detection system (i.e. below approximately 2kHz assuming a digital RCD with a 4kHz sampling frequency) . In use, the frequency of said first part of said sequence may be widely separated from said second part of said sequence to allow ease of discrimination in the presence of standing residual currents .

As it is possible for harmonics of the electrical supply to be present in residual currents, discrimination between the first and second parts of the signalling sequence is easier if the said first part of said

sequence was also not a harmonic of the mains frequency. All practical residual currents have higher fundamental amplitude than harmonic amplitude, so the presence of a harmonic of the mains frequency with the fundamental would itself provide discrimination.

In use, there is a possibility of any signalling frequency being generated by other appliances connected to the mains supply. As such, the apparatus for monitoring the earth loop impedance further comprises some form of signal modulation and coding.

Preferably, the magnitude of the signal sequence current has to be set to a level sufficient to ensure detection by connected RCDs. Given the unknown earth loop impedance, the apparatus for monitoring the earth loop impedance further comprises some form of feedback control system to provide a signal current source.

In use, the amplitude of said first part of the signal sequence may be set at a level below the minimum trip level of the circuit protection device, or by applying a larger second part of the signal current for a period shorter than the trip detection time, or by preventing the second part of the signal sequence being included in the earth loop impedance test.

It is believed that a method and apparatus for monitoring the earth loop impedance of an electrical installation that is protected by a circuit protection device in accordance with the present invention at least addresses the problems outlined above. The advantages of

the present invention are that said method and apparatus enable an earth loop impedance test to be performed on an electrical installation without tripping the circuit protection device and thus maintain the sensitivity of the device after an earth loop impedance test has been performed. Advantageously, the present invention provides a method and apparatus which automatically signals to any circuit protection devices present in the electrical installation that an earth loop impedance test is about to be performed. Furthermore, the present invention provides a circuit protection device, such as a residual current device, which can detect the earth loop impedance test signal and not trip for a predetermined interval. After the predetermined interval, the circuit protection device reverts to its normal operation.

A specific non-limiting embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram illustrating the principle of a known earth loop impedance tester connected to an electrical installation which is protected by residual current devices;

Figure 2 illustrates schematically how the present invention is implemented;

Figure 3 shows a schematic diagram of the general arrangement of the circuit protection device according to present invention; and

Figure 4 shows one example of a signalling sequence Isignai generated by the earth loop impedance tester to disable operation of the circuit protection device.

The present invention comprises a method and apparatus for monitoring the earth loop impedance of an electrical installation that is protected by a circuit protection device, such as a residual current device. Such an arrangement is capable of carrying out a test of the earth loop impedance of an electrical installation protected by at least one residual current device, as exemplified, for example, in Figure 1.

Figure 2 illustrates a preferred embodiment of the present invention that enables an earth loop impedance test to be performed on an electrical installation 100 without tripping the circuit protection device, and which maintains the sensitivity of the device after an earth loop impedance test has been performed. In use, an apparatus for determining the earth loop impedance of an electrical installation 100, such as an earth loop impedance tester 102, is connected to the electrical installation 100 under test, usually via a socket outlet

(not shown) . The electrical installation 100 under test is protected by at least one circuit protection device

104.

In use, the present invention is achieved using an earth loop impedance tester 102 that automatically signals to any circuit protection devices 104 present in the electrical installation 100 that an earth loop impedance test is about to be performed. The circuit

protection device 104 can detect the earth loop impedance tester' s signal sequence I _S ignai which comprises a first part of the sequence to disable operation of the circuit protection device 104 for a predetermined interval and a second part of the sequence to monitor the earth loop impedance of said electrical installation 100. After this predetermined interval, the circuit protection device 104 reverts back to its normal operation.

In a further aspect of the invention, it is envisaged that an ancillary unit (not shown) is placed between the earth loop impedance tester 102 and the circuit protection device 104. In use, the ancillary unit includes a transmit button which, when depressed by an operator, sends the first part of the signal sequence to disable operation of the circuit protection device 104 for a predetermined interval. The operator then transmits the second part of the signal sequence using the earth loop impedance tester 102. Co-ordination between the earth loop impedance tester 102 and the ancillary unit to transmit the first and second parts of the signal sequence is either achieved manually or by a communications bus. The ancillary unit can be either powered from the line supply 100 or by a local power source, such as, a battery.

The circuit protection device 104 shown in Figure 2 is implemented using a residual current device which comprises a toroid 106 having the supply phase and neutral passing therethrough, which acts as the primary winding of a current transformer; a secondary winding 108 around the toroid 106 is connected to a trip mechanism

(not shown) . Upstream from the residual current device is an isolation transformer 110, typically found in a sub station. In use, the residual current device continuously monitors both the current flowing into and out of the electrical installation 100; any difference in the current flow is detected and processed by the residual current device.

The earth loop impedance tester 102 includes a current source 112 arranged so that it is connected between neutral and earth conductors. On performing an earth loop impedance test on the electrical installation 100, a signalling current or sequence I _S i _gπ ai is injected down the neutral conductor and returns via the earth connection at the sub station 110. The residual current device senses the injected current I _S i _gna i as an imbalance between the phase conductors and processes the signal. To ensure that the signalling current I _S ig _na i does not trip the residual current device, the signalling current I _S ig _na i comprises a first part of the sequence which is detectable by the residual current device to disable its operation for a predetermined period of time, and a second part of sequence which, in use, determines the earth loop impedance of the electrical installation under test. After the predetermined interval, the residual current device reverts back to its normal operation.

The signal sequence I _S i _gn ai can be any form that is producible by the earth loop impedance tester 102 and detectable by the circuit protection device 104. The signal sequence I _S i _g nai may be unique, such that the circuit protection device 104 is disabled only when an

earth loop impedance test is about to be performed. The particular signal sequence and frequency Isignai used to communicate between the earth loop impedance tester 102 and the residual current device can take a number of forms .

The skilled person will appreciate that the amplitude of the first part of the signal sequence I _S i _gn ai can be set at a level below the minimum trip level of the circuit protection device, or by applying a larger second part of the signalling current Isignai for a period shorter than the trip detection time, or by applying a signalling current I _S i _gna i having a unique signalling frequency widely separated from the normal mains frequency to allow discrimination in the presence of, say, standing residual currents on the electrical installation 100.

In one embodiment, a test current I _S i _gn ai below 10mA is used. However, in some circumstances where high current devices (e.g. 50OmA RCDs) are utilised, the residual current device may not be sensitive enough to detect the signalling current I _s ignai • For example: a 50OmA RCD, allowing for a crest factor of 5 and using a 10-bit converter, gives a resolution of 5mA, so the signalling current I _s ig _na i would only be twice the resolution and may be hidden within the converter noise and hence undetectable.

In order to address this, a signalling sequence I _s ign _a i having a current of up to around 5OmA may be utilised, providing that it is applied for less than say

20ms (assuming that half of the minimum trip time is used

to determine if the device should trip) . Code sequences would then have to span several mains cycles so that the overall RMS signal level is less than the trip threshold.

In another embodiment, a notch filter is included in the RCD measurement path, centred around the signalling frequency Isignal- This would prevent modified residual current devices tripping on detection of the signalling current Isig _na i and hence allow much larger signalling amplitudes, which would be easily detectable by the whole range of devices.

In a further embodiment, a signalling current I _s i _gn ai of around IkHz, 5OmA, 100% amplitude modulated, with a specific on/off sequence spread over several mains cycles can be utilised. This could be detected within existing RCDs using a simple filter and tone decoder circuit. However, with a digital RCD, the signal presence could be determined by analysis of the residual current sampled data stream using standard data processing techniques, either pattern matching or fast Fourier transform (FFT) analysis .

The amplitude of the signalling pulse I _s ig _na i is dependent on the construction of the electrical installation 100. Since the installation 100 is protected by a residual current device, ground fault interrupter or earth current monitor, then it is necessary to ensure the maximum amplitude of the signalling pulse Isig _na i is below the threshold of the protection device 104. For example, where IEC 61008, 61009 or 60947 type devices are used, the maximum signalling amplitude Isignai should be around

half the rated threshold of the device to ensure that the signalling current Isignai does not trip the residual current device.

The skilled person will also appreciate that the signalling sequence I _S i _gn ai could be modulated in a number of different ways in order to convey information to the circuit protection device 104; typical modulation schemes include: time, frequency, missing pulse and amplitude modulation.

Figure 3 shows the schematic arrangement of a circuit protection device 104, such as a residual current device, for detecting residual current on the electrical installation 100 in accordance with the present invention. The device includes a current sensor 200, which could be the secondary winding 108 around toroid 106 in Figure 2, or a solid state transducer. The output of the current sensor 200 senses the current imbalance between the phase and neutral on the electrical installation 100 is then taken to an acquisition stage 210 in which this analogue signal is converted to a digital signal which is subsequently filtered 220. To limit the bandwidth and remove any spurious ^■ signals, a processor 230 looks for the transmitted signals from the filtered data stream 220. In use, the processor 230 can either be hardware or software-based and any number of detection algorithms could be used. In one example, a correlation algorithm is used to determine the statistic probability of a signalling current I _S i _g nai being present.

Although not shown in Figure 3, the output of the processor 230 also controls relay means for disconnecting

the electrical supply to the electrical installation 100, if the detected residual current exceeds a predetermined fault condition.

Further detail of one modulation scheme that can utilised in the present invention is depicted in Figure 4. Figure 4 shows the phase voltage 300 on the electrical installation under test together with the discrete signalling sequence I _s ign _a i generated by the earth loop impedance tester 102, as shown in Figure 2.

In use, the signalling current I _S i _g nai is phase locked to the line voltage 300 and applied at the zero cross of the phase voltage. The skilled person will appreciate that whilst it is possible to inject the signalling current I _s ign _a i ^a t any time during the phase voltage cycle, indeed in terms of detecting the signalling current I _S i _g n _a i it may be better to inject the signalling current I _s ign _a i at the peek of the voltage wave, however the necessary processing required to detect the signalling pulse I _S i _gna i is less demanding at the zero cross.

As shown in Figure 4, the signalling pulse I _S i _gna i itself is in the form of a sine wave with positive and negative half cycles of equal periods and equal amplitudes. The transition between positive cycles and negative cycles must coincide with the zero cross of the voltage waveform 300. The maximum tolerance for the transition point is ± half the sampling period T _sam pi _e of the circuit protection device 104. In practice, this tolerance may be increased by using multiple sampling

points during both half cycles of the current signal

J-signal •

In this embodiment, missing pulse modulation is used to transmit binary data to the circuit protection device 104. This data could be an addressing, configuration, control or any other form of data. As the signalling current I _s ignai is unidirectional, there is no possibility to send an acknowledgment from the circuit protection device 104 to the earth loop impedance tester 102, so some form of error detection may be required in order for the circuit protection device 104 to determine the validity of the transmitted data. It is also be advantageous to transmit the signalling sequence I _s i _gna i a number of times to ensure the message is received correctly by the circuit protection device.

Various alterations and modifications may be made to the present invention without the departing from the scope of the invention. For example, although particular embodiments refer to implementing the present invention on a domestic consumer's electrical installation, this is no way intended to be limiting as, in use, the present invention could be incorporated into larger installations, both signal and multi-phase.