"Circuit for protection against the risk of electric shocks"

The invention relates to a circuit for protection against the risk of electric shocks.

Numerous electrical protection devices associated with power supply conductors (usually the phase P and neutral N in the mains network) are known, said devices interrupting the electrical connection thereof should a person accidentally make contact with the conductor which has a dangerous voltage, see for example WO 2005/101605. Since the human body may be passed through by a current which, from the phase or neutral, flows to earth, the protection devices are equipped with a differential current meter which disconnects the phase and the neutral from the mains network when the differential current exceeds a certain threshold value. Further details are in any case given in WO 2005/101605, the general principle of which need only be mentioned here.

These devices may intervene only if the currents which the flow in the two conductors P and N are different, something which happens, for example, if an alternative path for the current between the conductor P and earth is formed.

In Class 2 electrical devices (C2EDs), for example, Italian law prohibits the earth connection. Since the C2EDs cannot have metal parts subject to the risk of contact with the user which are separated from the electrical mains by means of a single insulation, the regulations consider them to be safe. However, if they fall into a container with water which is isolated from earth, they are unable to trigger a differential protection switch because there is precisely no leakage current which flows to earth. The currents passing through a differential instrument transformer would in this case be balanced with a zero output signal.

The differential protection could be activated only if the user were to place his/her hands in the water in order to recover the C2ED, forming with his/her body the dispersion path for the current. But, in doing so, for the entire time during which the differential switch intervenes, the user suffers an electric shock which is limited only by the impedance of his/her body and the contact resistance with earth.

This condition occurs fairly frequently in view of the use of pipes which are made of plastic, with the result that the earth isolation of sinks, wash-basins and bath-tubs is very common.

Movable differential switches also do not solve this problem, for the same reason: until the user generates the leakage current by putting his/her hands in water (an instinctive and therefore highly likely reaction) there is no leakage current to earth needed to trigger the differential switch. In a tool which contains a motor the normal condition is that there is operational

insulation between the live parts and the motor casing and also further insulation between the motor casing and the exterior, thus providing a double insulation. Subsequent failure of the operational insulation, with consequent energization of the motor casing, eliminates one of the levels of insulation without there being, however, any external indication and without the protection circuits being able to intervene.

One solution could be to provide the tool with an earth connection, but this is not possible for C2EDs. Also, the earth connection, even though compulsory by law, is not always present in the installation.

The object of the invention is to provide a protection system which solves these problems.

This object is achieved with a protection circuit applicable to a sensor for the differential current flowing in power supply conductors connected to a load which has an internal conducting part electrically subject to accidental electrical contact with the power supply conductors, characterized in that it comprises an auxiliary node with a voltage different from that of the power supply conductors, to which said internal part is connected by means of an auxiliary conductor designed to induce in the sensor a differential current signal following accidental electrical contact with the power supply conductors.

Preferably, the auxiliary node is the terminal of a voltage generating circuit.

Preferably, the voltage generating circuit has a terminal connected to one of the power supply conductors so as to relay the voltage of the auxiliary node to this conductor.

By way of a very simple and effective embodiment the voltage generating circuit may comprise a voltage divider for the voltage at the terminals of the power supply conductors and the auxiliary node corresponds to the output of the divider.

The advantages of the invention will become clearer from the following description of a preferred embodiment illustrated in the accompanying drawing where:

Fig. 1 shows an electrical protection according to the invention;

Fig. 2 shows a variant of the electrical protection. Fig. 1 shows the functional diagram of an electrical protection device 10 according to the invention (depicted inside a box) which is powered by the phase conductor P and neutral conductor N of the mains network and in turn powers a load 20, for example a C2ED, which has an internal metal part 22 which is not accessible from outside (for example the casing of a motor or a protective cage).

A known automatic safety switch 34 for isolating or connecting the load 20 to the line P, N is arranged on the conductors P and N. The switch 34 is driven by a known control circuit CC connected to a differential current sensor 12, preferably a differential transformer. According to the invention the components described above are supplemented by an

additional protection circuit composed of:

- a resistive voltage divider Rl, R2 connected to the output of the differential transformer 12 and branched between the lines P and N; and

- a conductor or cable W which connects the centre (or also output) of the divider Rl, R2 to the metal part 22.

The cable W forms one or more turns 16 on the core of the differential transformer 12 so as to allow the latter to detect a current which passes through the cable W. If the load 20 falls into water or if there is a failure of the operational insulation, a current flow is created between the metal part 22 connected to the cable W and the other parts energized by the lines P, N. This current flows inside the cable W and is detected by the differential transformer 12 and by the circuit CC which may therefore operate the switch 34 without the need for a leakage current to earth, namely before the user may be exposed to any risk. The circuit CC may also or only emit a luminous or acoustic warning signal.

As can be seen, the circuit increases the level of protection of differential switches in the case where there is no earth circuit and achieves the predefined object.

The sensor 12 may also be a known differential switch movable on the power supply plug of the load 20 or a known differential switch which is fixed (for example to a wall). In this case, the switch 34, the circuit CC and the transformer 12 are already included in the known differential switch and it is merely required to add to it the additional protection circuit described. It is evident that the invention therefore allows a known protection device to be provided with an additional protective system in an easy manner and without radically altering the circuit layout thereof.

The voltage divider may be provided with two capacitors for reducing the dissipation, and the differential transformer 12 may be arranged before or after the switch 34 without altering the functional capability of the system.

The added wire W is preferably connected to the differential transformer 12 with several turns 16 in order to increase the sensitivity of the system and reduce the dissipation of the resistors Rl and R2 or the capacitance values.

The divider branched between P and N is a simple way of creating a virtual voltage generator having a node with a voltage different from that of P and N to be connected to the cable W. In this way it is possible to induce a current in the cable W.

The divider may be replaced with any circuit or element which generates a node with a similar voltage. This voltage may be alternating so as to induce an output current on the differential transformer without saturating it, but temporary saturation thereof also with a direct-current voltage is acceptable.

For example, it is possible to use an alternating voltage generator Vg (provided by modulation of a battery) with one terminal connected to P or N and the other terminal connected to the cable W (see Fig. 2 which shows this variant - the reference numbers indicated therein refer to components the same as those already described). If the generator Vg is arranged upstream of the transformer 12, it is no longer necessary to provide turns 16 on the transformer 12 since a differential current is directly induced on P and N via the cable W which essentially derives the current from a winding of the transformer 12.

It should be noted that if the power supply conductors carry a direct- current voltage, the invention is equally applicable, it being sufficient to replace (for greater efficiency) the transformer 12 with a direct-current differential current sensor or combining non-differential current sensors in circuit form, for example using LEM transducers.