A CONTROL CIRCUIT FOR A DIFFERENTIAL SWITCH

Abstract: A control circuit for controlling a differential switch designed to protect users by acting upon the power supply, includes a toroidal transformer effective to sense leakage currents to earth and to operate opening of the switch through a solid state switch (I₁,) and a trip coil (L₁). Provided between the transformer (T₁) and the solid state switch (I₁) is a matching capacitor (C₁) that is so dimensioned as to give a maximum power transfer down­ stream of said transformer (T₁). Also provided is a passive network (P₁) the proper dimensioning of which permits operation of the switch even in presence of unidirectional and partialized loss currents.

Description: A control circuit for a differential svitch. This invention relates to a device for controlling differential svitches, particularly but not essentially, designed to safeguard single users against possible fault or leakage currents, which device is comprised of passive, inexpensive components, and is capable of responding to unidirectional (do-components) and 'partialized' (pulsanting) loss currents. As is known, differential svitches are utilized to protect one or more users against possible leakage currents, by acting directly upon the pover supply to the circuit of the user(s) to be safeguard. The differential svitches commercially available may be divided into two basic types: the electro-mechanical type, and the type utilizing electronic circuits. The electro-mechanical type of differential svitches makes use of a toroidal transformer to sense the occurrence of a loss current to earth and to directly operate an electromechanical relay that is capable of causing the main svitch to open through a servomechanism. A disadvantage vith this type of switch is that it requires a toroidal transformer that must be of rather large size to be able to transfer an adequate operating power to the relay. It also requires use to be made of a sophisticated relay and a servomechanism capable of increasing the operating pover for opening the main svitch, as vell as a resetting system for said relay. All of the above, apart from a consideration of the reliability of the described circuit, uill inevitably have a consequential effect on the cost of this latter. The other type of switch know, utilizes a toroidal transfor mer to sense the occurrence of a leakage current, and an electronic circuit that operates a semiconductor element effective for closing the circuit of a trip coil operating the switch. Both the electronic circuit and the trip coil require to be fed from individual supplies of power that may be taken from the netvork or an auxiliary source. In this case, there is no need for a largely dimensioned core, because the electronic chain dounstream of the transformer enables the signal to be amplified up to the desired levels for operation of the switch through said coil. Hovever, as already said, two additional power supplies are needed: one power supply for maintaining an adequate voltage on the operating coil, and the other one for providing adequate current to bias the electronic circuits. in general, the electronic circuits known are of an active type and utilize integrated circuits in order to attain the desired funcional results, which increases the complexity and, thus, the cost of the device, vhile reducing its reliability, due to the multiplicity of functions these circuits are to perform. Reliability is also reduced in that said electronic circuits being fed from their own power supply, obviously tend to get hot, giving lover performance than do practically cold circuits. Moreover, said power fed electronic circuits require a rise -to-uorking-level time -f their supply voltage and this may cause delay in operation, in the event of closure of the dif ferential svitch due to pre-existing fault. A drawback that is frequently experienced with this type of circuits utilizing a solid state switch upstream of the trip coil, is that application for operating power to said switch may be in a pulsating form with periodicity of 20ms, if the network frequency is 50 Hz. As is known, a solid state switch will close upon receiving the operating signal and open when current flowing across the anodic circuit is brought to zero. Should a phase difference exist between the loss-toearth current and the main voltage to such an extent that the solid state switch will close a few milliseconds before the main voltage going through zero, then the time during which the current is flowing across the trip coil would be insufficient for the contacts to open, and this would cyclically occur at each period of the main voltage, thereby causing operation of the differential switch to fail. 2h differential switch control circuit according to this invention obviates to the drawbacks described above, it is a low-cost, high reliable device, and it utilizes a rather small core for its earth-fault responsive toroidal transformer, vithout requiring, otherwise, the use of said electronic chain. The above results are attained according to the invention which provides a control circuit for a differential svitch designed to protect users by acting upon the power supply, which control circuit includes a toroidal transformer having a reduced size core, effective for sensing leakage currents to earth and for operating through a solid state switch a trip coil to cause opening of the switch; there being provided, between the secondary of said transhrEmer and the solid state switch, a matching capacitor, a recti Bier and a passive, resistance-capacitance network; said trip coil being from its own power supply; a second passive network being arranged between said coil and the solid state switch in order to protect the circuit against overvoltages and electromagnetic disturbances, The matching capacitor is such dimensioned that it will reach a resonance, or near-to-resonance, condition with the inductance of the secondary of torsional transformer, which gives a maximum power transfer to the solid state switch for operating the trip coil. Said passive network that is arranged between the secondary of the toroidal transformer and the solid state switch, is dimensioned such that the switch will be able to operate even in presence of unidirectional and partialized currents. The above and further features of the invention will be best understood when reading the following description in connection with the accompanyng drawings which show, as a non restrictive example, a preffered embodiment of the invention. In the drawings: Figure 1 is a block diagram of the differential switch control circuit according to the invention; and Figure 2 is a wiring diagram oP the control circuit in figure 1, showing the different circuit components. Reference vill be made now tithe above figures and first to figure 1, vhere the control circuit of the invention is designated by 1 as a whole. F and N are the power supply conductors. T1 is a toroidal transformer designed to sense the existence of leakage currents to earth and to operate, via a chain to be described herein blew, a trip coil L1 through a solid state switch to to cause opening of movable contacts 2 and 3 on power supply. Said chain that is interposed between the secondary 4 of transformer T1 and the solid state switch I1, consist of a capacitor C1, a rectifier R1, a passive network and second capacitor C2, the functions of which are explained herein after. The trip coil L1 is provided vith its own supply of power A1 that may be drawn out oP the same supply network or an auxiliary source. A second rectifier R is arranged on the circuit section 2 where the trip coil L1 is placed. Provision of rectifier R2 is necessary if the solid state switch I1 is a SCR but not essential vhen I1 is a TRIAC. Provided between the solid state switch I1 and the trip coil, is a second passive netvork P2 which performs the functions of preventing the circuit from being subjected to electromagnetic disturbances and protecting it against possible overvoltages. It is to be noted that because of the circuit 1 being formed by passive networks, the circuit is, in itself, liable to little disturbances. The capacitor C1 that is arranged right downstream of the secondary 4 of transformer T1, is dimensioned so that it will attain a resonance, or near-to-resonance, condition with the inductance of secondary 4. In this way, indeed, a maximum power transfer is obtained downstream of the tran sformer T1 which gives adequate voltage and current values to the operation of the solid state switch Correct dimensioning oP the rectifier R1 and, particularly, the passive network P1 upstream of the solid state switch enables the circuit to respond to both unidirectional (do-components) and partialized or pulsating less currents. The capacitor C2, vhen arranged as shown in the diagram, permits achievement of the desired operate time. Clearly, with a rather high capacitance C2, delays of the order of the tens and even hundreas of milliseconds may be obtained over the time of operation of the switch. This may be of utility when a selectivity in operation is desired, i.e. when it is desired that the differential operations o± an entire installation be coordinated. Mereover, the capacitor C2, provided it is of a proper value relatively to the features of the solid state switch, enables this latter to be at any time maintained in a closed position, once the operating condition is reached, thereby overcoming any problem of unfavourable differenc*f phase between the less-to-earth current and the main voltage, a drawback that is experienced with the electronic circuits mentioned above. Referring now to the circuit diagram in figure 2 shoving the different circuit components, given herein below are the principle values of said components for a correct functioning of the circuit, the commercial symbols for some of them being given tee. In said diagram, the toroidal transformer T1 is a 21 .5x8. gmm BTicino-29023 with N1 (number of turns of the primary) = 5 and N2 (number of turns of the secondary) = 1000. When such a transformer is used, in order to be able to work under near-to-resonance conditions, i.e. to have a maximum power transfer downstream of said transformer, it is necessary that the capacitor C1 should have a capacitance of 0.22jar F with 20% tolerance and 50 dcwv (direct current wor- king volts). The rectifier R1 is a 7 N 4148 diode. The passive network P1 is a 50kit potentiometer. C2 is a 0,47F capacitance with 20% tolerance, and 50 dcit. The solid state svitch I1 is a C106M1, The passive network P2 is a RC-series: the resistance is of 22wL with 20% tolerance and 0.25W maximum pover dissipation; the capacitor is of 0.1jAF with 20% tolerance and 400 dcwV. The rectifier R2 is a WL08 diode bridge. Finally, the trip coil B1 is a BTicino 25053 trip coil. Logically, the above values, that are the values for an optimal operation of the circuit, are at least in part related to one another and result from actual tests carried out. Thus, for example, the capacitance C1 is calculated as a function of the size of the core of transformer T1 the type of material from which it is made, and the number of turns in the secondary. As a matter of fact, even though the values ascribed to the different circuit components are given as optimal values, they may be varied, uhen necessary, taking into account the mutual relationships. It should be apparent that many modifications may be made, as to the details of construction, to the differential sKi- tch control circuit described above, without departing from the spirit and scope of the invention, as defined in the following claims.

Claims: Claims 1. A control circuit for a differential switch designed to protect users by acting upon the power supply, said control circuit including a toroidal transformer with a smaller si- ze core than usually needed for direct operation, which is able to sense loss-earth current of higher value than a preset thereshold value, and to operate through a solid state switch (I1 )a power suppled trip coil (L1) to cause opening of the switch by acting upon movable contacts(2,3),uherein between the secondary of said transformer (T1) and the solid state switch (I1) there is provided a matching capacitor (C1), a rectifier (R1), a passive network (p1) and a further capacitor (C), while a second passive network is arranged between said coil (L1) and the solid state switch (I1) in order to protect the circuit against overvoltages and electromagnetic disturbances. 2. The differential switch control circuit according to claim 1, wherein for optimum performance of said circuit the different circuit components have the following values as determined from pratical tests: - Toroidal transformer (T1): BTicino - 29023 of 21.5 x 8.5mm, N1 = 5, N2 = 1000; - C1 = 0.22 F with 20% tolerance, 50 dctV; - Rectifier R1: 1N4148; - Passive network P1 : 50frpotentiometer, C2 = 0.47 F, 20% tolerance, 400 dcwV; - Rectifier R2 :WL08 diode bridge; - Trip coil L1 : BTicino 25853; such a dimensioning of the circuit, and, particularly, the capacitor (C1), the rectifier (R1), the passive network (p1) and the capacitor (C ), enabling said circuit to respond 2' to both unidirectional and partialized, or pulsating, leakage currents. 3. The differential switch control circuit according to claim 2, wherein said dimensioning of the circuit and, in particular, of the capacitor (C2) relatively to the solid state switch (I1) utilized, results in that, once the operating condition is reached, continuous operation of the switch (I1) is obtained vhatever the phase difference between the loss-to-earth current and the main voltage may be. 4. The control circuit according to claim 1 and 2, wherein adjusting the value of capacitance (C2) permits the desired operate time to be obtained.

Inventors: BALLADA, ROMANO, TASSINARI, VITALIANO
Applicants: BASSANI TICINO S.P.A.
Application year: 1983
Main classification: H02H3
IPC codes: H02H3