NZ270016A

NZ270016A - Differential current switch for isolation monitoring

Info

Publication number: NZ270016A
Application number: NZ270016A
Authority: NZ
Inventors: Hans-Jurgen Fleckenstein; Norbert Holscher
Original assignee: Kopp Heinrich Ag
Priority date: 1993-11-30
Filing date: 1994-11-29
Publication date: 1996-12-20
Also published as: AU679574B2; EP0655820A1; ATE152299T1; NO943983D0; NO943983L; DE59402511D1; DE4340737A1; FI944860A0; EP0655820B1; FI944860A; AU7883694A

Abstract

A residual current device, in particular for insulation monitoring of electrical circuits, having a summing current transformer which has a secondary winding and a primary winding which is formed from a phase conductor and a neutral conductor. The secondary winding controls a relay (which acts on switching contacts) via an amplifier and a limiter circuit. An oscillator is provided between the limiter circuit and the relay and is connected to a voltage supply circuit such that a resistance is connected to the electrical mains power supply, which consists of a live conductor and a neutral conductor. A fault current is produced which flows continuously through the primary winding, as a result of which a low-frequency signal is produced in the secondary winding, which signal causes the oscillator to oscillate. <IMAGE>

Description

<div class="application article clearfix" id="description"> Priority Date(s): ...S.Q.\1).:2&, C^mploie Specification Filed: :IL3,H C-iass: Publication Data:. ■2..fl..D£C . J996 P.O. Journal No:..... JfihU ..... Patents Form No. 5 Our Ref; DT203922 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION DIFFERENTIAL CURRENT PROTECTIVE SWITCH We, HEINRICH ROPP AG, a Germany company of Alzenauer StraBe 6b -ftt-72, 63796 Kahl Am Main, Federal Republic of Germany hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: PT0580701 ";>T OFFICE nm 1394 I.JO - 1 - (followed by page la) 27 - la - Differential Current Protective Switch The invention relates to a differential current switch, particularly for isolation monitoring of electrical circuits according to the preamble of claim 1. A differential current switch of the above-mentioned type is known from EP 220 408. This differential current switch is based on the use of an additional winding at the sum-current transformer by means of which independent monitoring through feeding of periodic short-termed impulsive currents can be carried out. A differential current switch with a sum-current transformer and an additional winding, wherein the sensitivity of the differential current detection depends on how the differential current between the return conductor of the electrical circuit and earth is formed, is known from the US 3,786,356. This differential current switch is provided with an AC-power supply which supplies via a gate the additional winding with an AC-current signal detected via a secondary winding, an amplifier and a limiting circuit. The known differential current switches have the disadvantage that an additional winding by means of which a differential current simulation is effected has to be provided besides the primary and secondary winding which are formed by the phase conductor and the zero conductor. The invention refers to a differential current switch of the above-mentioned type wherein the secondary winding via an amplifier and a limiting circuit controls a relay acting on a main switch contact in such a way that monitoring of interrupted, bridged or of components deviating from tolerance (followed by page 2) - 2 - and of their corresponding parts is guaranteed. It is the object of the present invention to provide a differential current switch which monitors itself and comprises a simple construction. This object is achieved according to the features in the characterizing portion of claim 1. Further embodiments of the present invention are characterized by the subclaims. The invention provides a differential current switch wherein the oscillation frequency for an oscillator is supplied from the mains so that the oscillator oscillates independently and regardless whether the switch contacts are opened or closed. An essential advantage consists in that for the self-monitoring a permanently flowing differential current is produced through which the electronics of the differential current switches is monitored in such a way that on a defect of a component the relay assigned to the switching contacts is deactivated and the differential current switch is switched off by means of the limiting circuit and the oscillator. In a preferred embodiment of the differential current switch it is no longer necessary to provide a test resistor because the test key as a break key in serial connection to a resistor switches the current which permanently passes the sum-current transformer. Preferably, the differential current flows also when the switch contacts of the main contact are still opened. According to the present invention, the monitoring is effected by the differential current switch with a low frequency, 2700 is - 3 - preferably the mains frequency. In the following, the differential current switch of the present invention will be described with reference to the attached drawings. Fig. 1 is a block diagram of the differential current switch according to the invention; and Fig. 2 shows details of a preferred embodiment of the differential current switch. According to Fig. 1, a preferred embodiment of the differential current switch according to the present invention comprises a sum-current transformer the primary winding of which is formed by phases and zero conductors indicated as 3, 4. Besides the primary winding 3, 4, a secondary winding 5 is provided which leads to an input circuit 7 to which an amplifier 8 is connected. The output of the amplifier 8 is connected to a limiting circuit 16, the output signal of which in turn is fed to an oscillator 18. The oscillator 18 is connected to a holding magnet 20. Further, a current supply is provided which is switchable via an auxiliary switch 24. The current supply 14 feeds the input circuit 7, the amplifier 8, the limiting circuit 16 and the oscillator 18. A conductor 4 is connected to a resistor 10 in series to which a break key 12 is provided as shown in detail in Fig. 2. The series connection formed by the resistor 10 and the test key 12 is connected to the current supply 14 which in turn is connected to the amplifier 8, the limiting circuit 16 and the oscillator 18. The limiting circuit 16 receives as input the output signal of the amplifier 8 and supplies its output signal 27001 - 4 - ■to the oscillator 18. The oscillator 18 is connected to the relay 20 which controls the switches 22 and 24 respectively. The current supply 14 includes an auxiliary switch 24 which is coupled to the main switch 22 in such a way that the main switch 22 closes with a delay in time to the auxiliary switch 24. The break key 12 serves as a test key for opening the connection between the conductor 4, the auxiliary switch 24 and the conductor 3. With the break key 12 closed, a differential current passes permanently the sum-current transformer via the resistor 10 and the break key 12, which makes an additional test resistor unnecessary. However, if an additional test resistor is used, the resistor 10 can be connected between the conductor 4 via the auxiliary switch 24 to the conductor 3. It can be seen from Fig. 1 and Fig. 2 that with the auxiliary switch 24 closed, the current supply circuit 14 is connected to the mains supply. Details of a preferred embodiment of the differential current switch according to the present invention are shown in Fig. 2. As can be seen from Fig. 2, behind the resistor 10 the break key 12 is situated which is connected to a connection 26 from which the auxiliary switch 24 provides the connection to the phase conductor. The current supply 14 comprises a resistor 28 which serves as an input resistor to the remaining components of the current supply circuit 14. The current supply circuit 14 contains among others a variable resistor 30, a bridge rectifier 31 as well as a condensator 35 as shown in detail in Fig. 2. 27001 - 5 - The input circuit 7 substantially consists of a parallel circuit formed by a resistor 37, diodes 38, 39 and an RC-network 40. The diodes 38, 39 are connected in opposite direction to each other. The input circuit 7 as well as the amplifier 8, the limiting circuit 16 and the oscillator 18 are connected to the output of the current supply circuit 14. The limiting circuit 16 is in the form of a window comparator and comprises operation amplifiers 42, 43 which function as voltage comparators. The switching point of the operation amplifier is adjusted by means of the resistors 44, 45, 46 in such a way that the comparison voltages slightly differ from the DC-voltage at the output of the amplifier 8 which is substantially formed by the operation amplifier 46 and that they are symmetrical. Hereby the output signals of the amplifiers 42, 43 have zero volt, if the alternating signal is too small. If the alternating signal at the input of the limiting circuit 16 exceeds the adjusted switching points, the outputs of the amplifier 42, 43 switch to almost operating voltage. The amplifier 42 generates an output voltage, if the positive halfwave of the alternating signal exceeds the switching point and the operational amplifier 43 supplies an output signal, if the negative halfwave of the alternating signal comes. By connecting the outputs of the amplifiers 42, 43 via diodes 48, 49, a square wave signal is generated which preferably provides an impulse every 10 ms, if the provided test current flows through the resistor 10. The impulses are supplied to an evaluation logic 60 which performs an evaluation of the impulse length. The impulse length depends on the level of the differential current. The evaluation logic 60 provides permanently short impulses to its output, if the input impulses have a length which is shorter than a pre-adjusted value. The value can be adjusted by means of a condensator 61 and a resistor 62. The short impulses are generated by means of a transistor 62 as well as resistors 63 to 65 and a capacitor 66. If the length of an input impulse of the evaluation logic 60 is bigger than an adjusted value, i.e. exceeds the differential current a predetermined tripping value, the evaluation logic provides a long impulse at the output. The length of the impulse can be adjusted by means of a resistor 67 and a capacitor 68 and is calculated in such a way that the oscillator 18 stops to oscillate, in case that the differential current exceeds the tripping value and consequently the relay 20 is de-excited. The output impulses of the evaluation logic 60 are fed to a level shifter consisting of resistors 70, 71 and 72, 73 as well as a transistor 74 and an operation amplifier 75. The level shifter is necessary because the oscillator 18 is supplied with a higher voltage than the input circuit 7, amplifier 8 and the limiting circuit 16. The composition of the level shifter is shown in detail in Fig. 2. The oscillator 18 comprises according to a preferred embodiment electronic switches 50, 51. The oscillator 18 oscillates if it is constantly triggered by impulses which as a result of the main frequency occur out at the circuit point indicated with 53 in Fig. 2 which is situated on the ouput side of a diode 54. If the impulse fails to appear at the switch point 53, then the oscillator 18 falls out of cycle and the relay 20 is released. If one of the electronic switches 50, 51 fails due to a defect, the oscillation is also ended and the current through the relay is interrupted or limited to such a small amount that the magnetic force of the relay is not sufficient to hold the 2700 16 - 7 - switching contacts or the main switch contacts 22 closed. Electronic switches, preferably in form of switching transistors 50, 51 of the oscillator 18 are connected in such a way that the switching device 51 is connected in series to a relay 20. The two switches 50, 51 are monitoring each other for which the short impulses are used being outputted by the limiting circuit 16 or rather its evaluation logic 60. In the embodiment shown in Fig. 2, the collector of the transistor 50 is on the one hand connected to earth via a relay 20 and a capacitor 57 to a resistor 59, and on the other hand via a diode 58. The resistor 59 is a base resistor of the transistor 51. The evaluation logic 60 of the limiting circuit 16 generates on the output side only short voltage impulses as long as the tripping differential current is under a predetermined tripping value, while the evaluation logic 60 upon exceeding the tripping value by the differential current provides no output voltage until the main switches 22 are opened via the relay 20. The tripping value is determined according to a preferred embodiment of the invention by means of the resistors 44, 45, 46. The monitoring is performed according to the invention with a frequency which corresponds, for example, to the mains frequency or another low frequency. Consequently, according to the invention, self-monitoring with a permanent flowing differential current and upon low frequency is guaranteed without employing an additional winding for feeding a differential or fault current to the sum-current transformer. 270016 - 8 - The oscillator 18 is fed via the oscillation frequency of the mains, namely via the resistor 10 and the test key 12. It is of importance that the oscillator 18 starts swinging by itself, even if the contact switches of the main switch 22 are opened because via the resistor 10 and the test key 12 a voltage is applied to the oscillator 18 via the current supply circuit 14. Accordingly, the mains frequency is the real impulse generator for the oscillator, i.e. no resonant circuit is necessary. Upon interruption of the impulses derived from the mains frequency, the relay 20 is permanently de-excited due to the opening of the switching contacts of the switch 22. From the above description follows that the circuit of the differential current switch according to the present invention does monitor itself. Upon failure of a component being important for the differential current switch, the current through the relay 20 is interrupted so that it is released and the main switch contacts 22 are opened. </div>

Claims

<div class="application article clearfix printTableText" id="claims"> 270016 what Reclaim is: - 9 - 1. Differential current switch, especially for self-monitoring of electrical circuits, with a sum-current transformer (1) comprising a primary winding (3, 4) being formed by a phase conductor (L) and a zero conductor (N) and a secondary winding (5) which controls via an amplifier (8) and a limiting circuit (16) a relay (20) which in turn acts on switching contacts (22), characterized in that between the limiting circuit (16) and the relay (20) an switching circuit stage (18) is provided which is connected to a current supply circuit (14), that a resistor (10) is connected to the mains consisting of said phase conductor (L) and said zero conductor (N) so that a differential current which is permanently flowing through the primary winding (3,4) is generated causing a low frequency signal in said secondary winding (5) which switches said switching circuit stage (18) via an input circuit (7), said amplifier (8) and said limiting circuit (16). 2. Differential current switch according to claim 1, characterized in that the monitoring is performed at a low frequency. 3. Differential current protective switch according to claim 1 or 2, characterized in that the limiting circuit (16) comprises two operation amplifiers (42, 43) the respective outputs of said operational amplifiers being connected to each other via a diode (4G, 49). n.z. patent office 8 - NOV 1995 RECEIVED 270016 - 10 - Differential current switch according to at least one of the preceding claims, characterized in that the limiting circuit (16) comprises an evaluation logic (60) which only provides short voltage impulses as long as the differential current is below a predetermined tripping value while when the differential current exceeds the tripping value, the evaluation logic (60) does not provide any voltage output until the main switching contacts (22) are opened by means of the relay (20). Differential current switch according to any of the preceding claims, characterized in that a resistor (10) can be connected to a break key (12) which can be used as a test device. Differential current switch according to any of the preceding claims, characterized in that an electronic switch (50) is connected in parallel and a further electronic switch (51) is connected in series to a relay (20) and that said electronic switches are permanently controlled by the impulses supplied from the limiting circuit (16) and consequently control each other. 270016 - 11 - 7. Differential current switch, especially for selfmonito-ring of electrical circuits, with a sum-current transformer (1) comprising a primary winding (3,4) being formed by a phase conductor (L) and a zero conductor (N) and a secondary winding (5) which controls via an amplifier (8) and a limiting circuit (16) a relay (20) which in turn acts on switching contacts (22) being located in said phase and zero conductors (L, N) characterized in that a resistor (10) is provided connecting said phase conductor (L) and said zero conductor (N), wherein a differential current is permanently flowing by way of said resistor through said primary winding (3,4) said differential current inducing a signal in said secondary winding (5) that upon the amplitude of said differential current being below a predetermined tripping value said limiting circuit (16) generates a control signal which switches a switching circuit stage (18) holding said switching contacts (22) closed via said relay (20) and that upon said amplitude of said differential current being above said tripping value as well as upon failure of an element of said differential current switch said switching circuit stage (18) falls out of cycle and opens said switching contacts (22) by de-exciting said relay (20). 8. Differential current switch according to claim 7 characterized in that said switching circuit stage (18) starts swinging by itself independently from the switching -position of said switching contacts (22). ' 270016 - 12 - 9. Differential current switch according to one of the preceding claims characterized in that said limiting circuit (16) consists of two operation amplifiers (42, 43) the outputs thereof being connected respectively via a diode (48, 49). 10. Differential current switch according to one of the preceding claims characterized in that said limiting circuit (16) comprises a level shifter (70 - 75) through which said switching circuit stage (18) is connected to said limiting circuit (16) and said current supply circuit (14). 11. Differential current switch according to one of the preceding claims characterized in that said control signal generated by said limiting circuit (16) consists of a sequence of short impulses. 12. Differential current switch according to one of the preceding claims characterized in that said resistor (10) is connected in series to a test key (12) by means of which said permanent flowing differential current is interruptable. 13. Differential current switch according to one of the preceding claims characterized in that said current supply circuit (14) is connected to said phase conductor (L) by way of an auxiliary switch (24). n •? PATENT office, 8 - U0V 1395 RECEIVED 27001 - 13 - 14. Differential current switch according to one of the preceding claims characterized in that said switching circuit stage (18) comprises electro nic switches (50, 51) said first electronic switch (50) being connected in parallel and said second electronic switch (51) being connected in series to said relay (20) 15. A differential current switch substantially as herein described with reference to the accompanying Drawings. HEINRICH KOPP AG By its attorneys BALDWIN, SON & CAREY N.Z. PATENV^ b - m m | ..... ... . '1 </div>