GB2176069A

GB2176069A - A switch for protection against fault currents

Info

Publication number: GB2176069A
Application number: GB08609254A
Authority: GB
Inventors: Gottfried Biegelmeier
Original assignee: Brown Boveri und Cie AG Germany; BBC Brown Boveri France SA
Current assignee: BBC Brown Boveri AG Germany; BBC Brown Boveri France SA
Priority date: 1985-04-16
Filing date: 1986-04-16
Publication date: 1986-12-10
Also published as: EP0218648A1; ATE57448T1; AU5778686A; WO1986006222A1; ES553997A0; DE3674877D1; GB8609254D0; ES8703678A1; NZ215855A; GR860989B; GB2176069B; AU597252B2; EP0218648B1; AT383906B; ATA113785A

Abstract

The circuit breaker consists of a housing and contact apparatus (17) arranged with terminals, a fault current releasing device (15), a summation current converter (10), an electronic circuit (19) dependent of network voltage, and an energy storage circuit (13) independent of network voltage. For greater simplicity in assembling and manufacture, the device has only one summation current converter (10) and one fault current releasing device (15), which are used not only for release by the circuit dependent of the network voltage, but also by the circuit independent of the network voltage.

Description

SPECIFICATION A switch for protection against fault currents.

The invention relates to a fault-current protective switch. Such a device commonly comprises a contact device disposed in a suitable housing having terminals for the wiring cables and an associated switch latch, test device and actuating means, a fault-current tripping device with at least one tripping coil, a summing current transformer, and electronic energy store circuit independent of the mains voltage and an electronic circuit dependent on the mains voltage, the energy store circuit containing a capacitor which, when a.c. or pulsating d.c. fault currents occur, is charged in dependence on the fault current via the summing current transformer and a rectifier circuit and when a given charging voltage is reached, discharges via a voltage-dependent semi-conductor element, in the form of pulses through a tripping coil of a fault-current tripping device and thus triggers the fault-current protective switch, or alternatively the triggering is brought about by the electronic circuit dependent on the mains voltage when a d.c. fault current or an a.c. fault current with DC components occurs, the direct current or the DC components in the summing current transformer alter the voltage generated in a winding of the transformer by an oscillator in the electronic circuit, by asymmetry or by displacing the operating point, so that a switching process is brought about via the electronic circuit so as to use the mains energy to actuate a fault-current tripping device.

The stimulus towards the invention was given by experience obtained in the use of the fault-current protective circuit, which requires new technical solutions to close the gaps now known to exist in the protection given by this device, which is nowadays used in many countries.

The three possible basic circuits used for constructing fault-current protective switches were described twenty years ago (1). Fault-current protective switches with pulse or energy-storage tripping, a trend-setting invention first used in Austria (AT-PS 197 368) were installed in large numbers and have been found reliable.

The switching device in the energy storage circuit was initially a glow lamp, and consequently the device was expensive (a large number of secondary terms in the summing current transformer) and too bulky.

Today however suitable semi-conductor components are available and are cheap, small and can replace the glow lamp as a switching element. At switching voltages of about 1 0V they require relatively small numbers of secondary turns in the summing current transformer, and the protective switch can have almost any tripping characteristics. In order to make the storage capacitor small also, it is advantageous to use polarised permanent-magnet tripping devices, which are less critical to adjust and can operate with greater tripping forces than circuits independent of the mains voltage and without electric energy storage. However, a modern fault-current protective switch must also be tripped by fault currents which may occur in the form of pulsating or smoothed direct currents. Of course it has alway been known that protective switches are effective only with alternating fault currents.If the fault current has DC components, the tripping sensitivity of the switch is adversely affected. Since more and more electronic components are being used in domestic appliances, the DC problem also has to be solved. It was long believed that if a short to earth occured in the circuits used in practice in domestic electric appliances, the resulting fault current could occur only in the form of a pulsating direct current. In the case of fault currents of this kind, e.g. occurring during half-wave rectification without a smoothing capacitor, it is possible to construct protective switches independent of the mains voltage, using conventional circuits, i.e. with energy storage tripping, (GB-PS 2 082 408 B).It has been found impracticable, however, to impose restrictions in the selection of electronic circuits on the manufacturers of electric appliances because of fault-current protective switches. Half-wave rectification with smoothing capacitors and three-phase current recrificationarn particularly relevant in this connection.

Owing to the transformer tripping principle, however, circuits independent of the mains voltage, even circuits with electric energy storage, cannot detect fault currents in the form of a smoothed direct currents. In that case it is necessary to use electronic circuits which naturally depend on the mains voltage and can process direct fault currents in all forms via summing current transformers.

Circuits of this kind are described e.g. in U.S. P.S. 3,768,011 and German OS 27 30 874. However, there is the question of the possible dangers of using the mains as an auxiliary voltage supply for tripping fault-current protective switches. If the auxiliary voltage is taken only by an external conductor and the neutral conductor, the switch may fail if the corresponding external conductor or neutral conductor is disconnected, (e.g. if a fuse operates in the external conductor or if the neutral conductor breaks), and it will then be impossible to detect fault voltages resulting from shorts to earth in the other two external conductors. However, even if all three external conductors in a three-phase network are used for the alternative voltage supply, the neutral conductor may still break.In an attempt to remedy this situation, the protective switch was tripped when the neutral conductor broke (German OS 28 25 881). If an additional connection, e.g. the earthed conductor, is. used for this purpose, the disadvantage is that the installation becomes more complicated and the earth potential is introduced into the switch. This makes the circuit more sensitive to over-voltages, which mainly occur to earth. However the main disadvantage of tripping in dependence on the mains voltage is that when shorts to earth occur in the protected apparatus the strength of the auxiliary voltage supply depends on the relation between (a) the mains loop resistance between the transformer and the places where the auxiliary voltage supply is connected and (b) the total resistance in the fault loop.This is shown in Figure 1 of the drawings in the case of a short to earth in an installation returned to zero.

If ZL is the line impedance of the external conductor from the transformer to the fault-protective switch, ZPEN is the line impedance of the PEN line from the transformer to the protective switch, Z1 is the.line impedance of the external conductor from behind the protective switch to the fault.

Z2 is the line impedance ofthe earthed conductorfrom the fault to the PEN conductor.

UN iS the external conductor-PEN mains voltage and Ua is the supply voltage of the electronic circuit dependent on the mains voltage, then the value of the supply voltage is: ZI Z, + Z2 Ua = UN X ZL + ZPEN + Z1 + Z2 If therefore the fault to earth occurs near the place where the protective switch is installed, the auxiliary voltage may become zero and tripping will be impossible. In such cases the overcurrent preventing means have to eliminate the fault to earth and the protective switch becomes inoperative. For this reason, in countries using switches with tripping systems dependent on the mains voltage, the fault-current protection gives only "additional" protection and corresponding restrictions are planned in international requlations (2,3).

There is also no use in monitoring the neutral-conductor voltage relative to the earthed conductor, as disclosed in German OS 28 25 881. The reason is that the circuit fails if a short circuit occurs simultaneously with the fault to earth. In that case, as shown in Figure 2, the output terminals of the protective switch are short-circuited together with the earthed conductor and the switch cannotftip, even though the protected parts of the installation are receiving the fault voltage of e.g. 1 0V.

This is shown in Figure 2 where ZL' = Line impedance of the external conductor from the transformer to the protective switch ZB' = Line impedance of neutral conductor from transformer to the protective switch, ZL"-= Line impedance of external conductor from protective switch to the short-circuit ZN" = Line impedance of neutral conductor from protective switch to the short-circuit, UN = External conductor-neutral conductor mains voltage Ua = Supply voltage of electronic circuit dependent on mains voltage PE = Terminal for first conductor in protective switch ilk = Short-circuit current 1A = Fault current RA = Earthing resistance of protected installation and RB = Earthing resistance of neutral conductor in the transformer station (system earth) Ua=UNX ,ZL"+ZN" ZL + ZL + ZN + ZN If therefore a short-circuit occurs nearthe protective switch, the supply voltage breaks down and the PE-terminal also receives the potential earth short-circuit. The switch therefore cannot trip under any circumstances. The short-circuit current Ikflows through the external conductor and neutral conductor and the resulting voltage drop in the neutral conductor, relative to the earth connection, RA to the system earth RB, acts as a fault voltage. Owing to the absence of the auxiliary voltage, UA, however, the fault current I, flowing as a result of the fault voltage cannot trip the switch.In order therefore to use a fault-current protective switch for protection in the event of indirect contact (fault protection) the following conditions- must be fulfilled.

1 ) The switch must trip if alternating and/or d.c. fault currents occur at the normal mains-voltage supply. If it is triggered by alternating fault currents at the rated value ln of the tripping fault current, it is sufficient for physiological reasons (4) if tripping in the event of pulsating d.c. fault currents occurs at rZ x An' four half-wave rectification, 2 x lAnforfull-wave rectification and 2.8 x li,n in the event of a smoothed d.c. fault current For this purpose electronic circuits dependent on mains voltage are necessary. Other auxiliary voltage sources such as batteries-are impracticableforthis purpose.

2) The switch must be tripped by alternating fault currents even if the external conductor and/or neutral conductor fail and if there is a simultaneous short-circuit and short to earthed. In that case if does not need to remain operational for d.c. fault currents, firstly because the simultaneous occurrence of a mains fault and a d.c. fault current is a negligible safety risk and secondly because during short-circuits the fault voltage, as a result of the voltage distribution between external conductor and neutral conductor in networks up to 240V, remains below 1 20v to earth and consequently does not exceed the conventional contact-voltage limit, which is 1 20v for direct current.

As explained, circuits independent of the mains voltage cannot detect smoothed direct current, whereas constructions dependent on the network voltage fail if the external conductor and/or neutral conductor is disconnected and if shorts to earth occur simultaneously with short-circuits. The technical requirementsregarding protection against indirect contact exclude circuits independent of the mains voltage and also circuits dependent on the mains voltage. These apparently contradictory requirements on a switch giving protection against indirect contact (fault protection) are economically met by the invention described hereinafter.

It would of course be possible to use two protective switches in series, one connected independently of the mains voltage to fulfill condiction 2) and one construction dependent on the mains voltage to satisfy the conditions in 1). This complication is uneconomic. It is also uneconomic and takes up too much space to incorporate both systems in one switch with two summing current transformers and/or two fault-current tripping devices. This is why D-PS 2348 881 has never been applied in practice.It discloses a protective switch an summing current transformer system having a secondary winding connected to the field winding of the fault-cu rrent tripping device, the secondary winding being supplied by an external AC source with a static current for premag netising the moving or transformer core, the operative feature being that an additional separate summing current transformer system is provided and has a number of primary windings corresponding to the first summing current transformer system and a secondary winding acting on the switch catch associated with the first summing current transformer system. The characteristic feature of this patent therefore is the use of two summing current transformers, and this was the very reason why this patent has not become important in practice.

Therefore EP Laid-open Specification 113026 discloses a fault-current protective circuit in which a relay is connected only by a rectifier circuit to the secondary winding of the summing current transformer. The sensitivity of the tripping circuit can be increased by using a capacitor, which cooperates with the inductance (relay or secondary winding) in the tripping circuit to form an LC-circuit tu ned to the frequency on a secondary side. This known circuit is not an energy-storage circuit.

US-PS 4320433 also fails to disclose an energy storage circuit.

The object of the invention is to construct a fault-current protective switch of the initially-mentioned kind which is equally suited for alternating and direct fault currents and for alternating fault currents with DC components, and is also simpler in construction than e.g. the switch according to German PS 2348881.

The invention resides in a fault-current protective switch comprising a fault-current tripping device with at least one tripping coil, a summing current transformer, an electronic energy store circuit independent of the mains voltage and an electronic circuit dependent on the mains voltage, the energy store circuit containing a capacitor which, when a.c. or pulsating d.c. fault currents occur, is charged in dependence on the fault current via the summing current transformer and a rectifier circuit and when a given charging voltage is reached, discharges via a voltage-dependent semi-conductor element, in the form of pulses through a tripping coil of a fault-current tripping device and thus triggers the fault-current protective switch, or alternatively the triggering is brought about by the electronic circuit dependent on the mains voltage when a d.c. fault current or an a.c. fault current with DC components occurs, the direct current or the DC components in the summing current transformer alter the voltage generated in a winding of the transformer by an oscillator in the electronic circuit, by asymmetry or by displacing the operating point, so that a switching process is brought about via the electronic circuit so as to use the mains energy to actuate a fault-current tripping device, characterised in that only one summing current transformer and only one fault-current tripping device, preferably in the form of a permanent-magnet tripping device, are used both for triggering independently of the mains voltage by means of the energy store circuit comprising an electronic switching element, a storage capacitor and a rectifier circuit, and also for tripping in dependence on the main voltage, using an electronic circuit.

Preferably the switch has a contact device disposed in a suitable housing having terminals for the wiring cables and an associated switch latch, test device and actuating means.

In contrast to the circuit according to German PS 2348881, the system according to the invention described here uses only one summing current transformer and the fault-current tripping device, preferably a permanent magnet, used for the energy storage circuit independent of the mains voltage is also used for triggering by the voltage-dependent electronic circuit. This can be done either if a tripping coil of the tripping device is tripped via the electronic circuit directly with AC or DC, using energy from the mains, or alternatively the electronic circuit charges the storage capacitor from the mains when the corresponding fault current flows, and the tripping pulse occurs after the corresponding charging voltage is reached.The electronic circuit can be dimensioned so that the switch has the same tripping characteristic when tripping in dependence on the mains voltage via the electronic system after direct fault currents occur, or then tripping via the energy storage circuit independently of the mains voltage, i.e. both tripping systems have the same rated values.

The secondary winding of the summing voltage transformer is used for charging the storage capacitor in the event of triggering independently of the mains voltage if alternating fault currents or pulsating d.c. fault currents occur, and also for actuating the electronic circuit for detecting smoothed or pulsating d.c. fault currents if the energy storage circuit is designed only for alternating fault currents. If the summing current transformer is supplied with a secondary winding and a tertiary winding, the secondary winding can be used for the energy storage circuit and the tertiary winding for actuating the electronic circuit.

By this means the energy storage circuit can be electrically isolated from the electronic circuit if the fault-current tripping device is provided with two tripping coils.

The tripping characteristic of the protective switch can also be influenced in a simple, known manner (AT-PS 205 574) if an artificial fault current below the response threshold of the switch is permanently produced by the electronic circuit in the secondary or tertiary winding of the summing current transformer.

The summing current transformer will then be pre-energised by this fault current and the storage capacitor will be partially charged. If a real fault current exceeding the tripping circuit of the switch then occurs in the installation, it will supply the rest of the charge to the storage capacitor needed for the energy storage circuit to come into action.

To this end, the signal current generated at a suitable frequency the electronic circuit dependent on the mains voltage and flowing through the secondary or tertiary winding of-the summing current transformer, induces it and therefore pre-stores energy in the storage capacitor via the rectifier circuit.

Embodiments of the invention will now be described in detail by way of example with reference to the drawings, which show prior-art circuits and embodiments of the invention. In the drawings: Figures land 2 show known circuits which have already been described, and Figures 3, 4, and 5 each show an embodiment of a fault-current protective switch according to the invention.

The phase conductors La, L2, L3 and the neutral conductor N of a network form primary windings extending through a summing current transformer 10. Transformer 10 has a secondary winding 11 connected to a rectifier circuit 12 connected in parallel to an energy store in the form of a capacitor 13, which after reaching a given charging voltage trips a threshold-value switch 14. A perm-anent-magnettripping device 15 connected in parallel with energy store 13 trips a switch latch 16 which opens contact members 17 in conductors Li to L3 and N.Supply lines 18 connected to phase conductor L1 and neutral conductor N supply voltage to an electronic detecting device 19 which detects fault currents at the secondary winding and is connected by lines 20,21 to the energy store.

The secondary winding 11 is thus used for charging capacitor 13 if a.c. fault currents or pulsating d.c. fault currents occur, and also for controlling the electronic circuit 19 dependent on the mains voltage, circuit 19 being for example so arranged that it is parallel to capacitor 13, so that it also operates with tripping pulses via the voltage dependent semi-conductor element 14 and the tripping coil of the fault-current tripping device 15.

The circuit in Figure 4 differs from the circuit in Figure 3 in thatthe summing current transformer 10 has a secondary winding 11 and a tertiary winding 22, the secondary winding being used for the energy storage circuit 13 and the tertiary winding 22 being used for controlling the electronic circuit 19. The tripping device 13 has e.g. two tripping coils 15a and 15b. In this manner the tripping circuit ofthe energy storage circuit is electrically isolated from the tripping circuit of the electronic circuit and therefore also from the mains.

A more detailed embodiment of the invention is shown in Figure 5, which as before shows the phase conductors Ln, 12, L3 and the neutral conductor N, which form primary windings extending through transformer 10. The secondary winding 11 is connected to the energy storage circuit, represented by a timing element 13 and a threshold value switch ortriggercircuit 14. The output of circuit 13/14 is connected to relay 15 and when alternating fault currents occur, capacitor 13 is charged and when the charge reaches a certain value it is discharged via threshold switch 14 to relay 15, thus opening contact members 17. Lines 18 connect a rectifier 30 to phase conductor L1 and to N, rectifier 30 being used for supplying the electronic circuit 19 shown surrounded by chain-dotted lines.Circuit 19 substantially comprises an amplifier part 31, a timing element or integrator 32, a threshold switch 33 and an oscillator 34. The secondary winding 11 of the summing current amplifier 10 is magnetised and demagnetised at a frequency of preferably 500 Hz, corresponding to the hysteresis loop of the transformer material, by the oscillator via a line 35 to a tap 36 on the secondary winding 11. If a d.c. fault current or an a.c. fault current with DC components occurs, the transformer moves towards saturation direction i.e. to an asymmetrical state, and the resulting voltage drop - is detected and processed by circuit 31/32/33 and supplied to the energy storage circuit 13/14, thus triggering relay 15 and opening contacts 17.

In known electronic circuits-fbr detecting DC fault currents, the frequency used for premagnetising the core is generally from 1000 to 5000 Hz. To enable the fault switch to detect AC fault currents also, it is usual to provide a second summing current transformer, e.g. in German PS 2348881. In order to reduce the oscillator frequency to about 500 Hz, summing currenttransformers suitable for ACfault currents can be used. If the -transformer is made of F80 material, the secondary side needs 700 turns and the tap occurs after 100 turns, so thatthere are 100 turns of the secondary winding between connection 40 and tap line 35.

Literature (1) Biegelmeier, G.: Moderner Fehlerstromschutz. E.u.M., 75. Jg. (1958), H.8, S. 157 ...164 (2) Biegelmeier, G.: Gedanken tuber die Nullung (TN-System) al & ptimalen Fehlerschutz (Schutz maRnahme bei indirektem Beruhren) in elektrischen Anlagen. ÖZE 37. Jg. (1984), H.12, S.483 (3) IEC 64 (Central Office) 151, January 1985, IEC Publ. 364 Part 5, Chapter 53 switchgear and controlgear (4) IEC-Report 479, second Edition: Effects of electric shock on the human body, Part 2, ChapterS

Claims

1. A fault-current protective switch comprising a fault-current tripping device with at least one tripping coil, a summing current transformer, an electronic energy store circuit independent of the mains voltage and an electronic circuit dependent on the mains voltage, the energy store circuit containing a capacitor which, when a.c. or pulsating d.c. fault currents occur, is charged in dependence on the fault current via the summing current transformer and a rectifier circuit and when a given charging voltage is reached, discharges via a voltage-dependent semi-conductor element, in the form of pulses through a tripping coil of a fault-current tripping device and thus triggers the fault-current protective switch, or alternatively the triggering is brought about by the electronic circuit dependent on the mains voltage when a d.c. fault current or an a.c. fault current with DC components occurs, the direct current or the DC components in the summing current transformer alter the voltage generated in a winding of the transformer by an oscillator in the electronic circuit, by asymmetry or by displacing the operating point, so that a switching process is brought about via the electronic circuit so as to use the mains energy to actuate a fault-current tripping device, characterised in that only one summing current transformer and only one fault-current tripping device, preferably in the form of a permanent-magnet tripping device, are used both for triggering independently of the mains voltage by means of the energy store circuit comprising an electronic switching element, a storage capacitor and a rectifier circuit, and also for tripping in dependence on the main voltage, using an electronic circuit.

2. A fault-current protective switch according to claim 1, characterised in that the storage capacitor is charged by means of the mains energy via the electronic circuit and summing current transformer when d.c.

fault currents or a.c. fault currents with DC components flow, and after reaching a corresponding charging voltage energises a tripping coil of the fault-current tripping device in pulsed manner and thus, via the fault-current tripping device, initiates the process of opening the protective switch.

3. A protective switch according to claim 1 or 2, characterised in that the electronic circuit is constructed so that it trips the protective switch with the same tripping characteristic as during tripping by the energy storage circuit independent of the mains voltage.

4. A protective switch according to claim 1,2 or 3, characterised in that the summing current transformer in addition to the secondary winding has a tertiary winding, the secondary winding being provided for tripping by the energy storage circuit independently of the mains voltage and the tertiary winding being used for tripping via the electronic circuit in dependence on the mains voltage.

5. A protective switch according to claim 4, characterised in that the fault-current tripping device has two tripping coils, one of which is energised by the energy storage circuit independent of the mains voltage and the other is energised by the mains via the electronic circuit.

6. A protective switch according to any of claims 1 to 5, characterised in that an exciting current of suitable frequency is permanently made to flow via the electronic circuit in the secondary or tertiary winding of the summing current transformer, and the induction voltage of the current pre-chargesthe storage capacitor via the rectifier circuit in the mains voltage-dependent energy storage circuit.

7. A fault current protective switch as claimed in any of claims 1 to 6 having a contact device disposed in a suitable housing having terminals for the wiring cables and an associated switch latch, test device and actuating means.

8. A protective switch substantially as herein described with reference to Figures 3,4 or 5 of the accompanying drawings.