GB2108339A

GB2108339A - Electric protective circuits

Info

Publication number: GB2108339A
Application number: GB08219086A
Authority: GB
Inventors: Leslie Charles Miller
Original assignee: SECR DEFENCE; UK Secretary of State for Defence
Current assignee: SECR DEFENCE; UK Secretary of State for Defence
Priority date: 1981-07-07
Filing date: 1982-07-01
Publication date: 1983-05-11
Also published as: GB2108339B

Abstract

In an electric shock protection circuit for use with a load which may have permissible currents to earth (e.g. via the r.f. suppression filter F of a mobile transmitter), the core-balance transformer 3 includes both neutral and earth primary windings P3, P2, and a bypass shunt impedance (Z1) is connected between the output side of the neutral primary P3 and the input side of an the earth primary P2. Any filter current to earth point CE passes through the transformer via the extra primary (P2), to maintain current balance. Leakage to any other earth point (c) trips the circuit in the normal way. In the event of power neutral discontinuity, neutral current bypasses the transformer via impedance Z1. In the event of a power earth discontinuity, filter current is routed twice through the transformer (3), ie once through the extra primary (P3) via the impedance (21). Either event causes current imbalance and operation of the trip-switch (S). Further monitoring functions may be provided, by adding additional impedance shunts (Z2 to Z5) between primaries, and between primaries and circuit earth. <IMAGE>

Description

SPECIFICATION An electric shock protection circuit and circuit breaker adapted therefor The present invention concerns an electric shock protection circuit of the type including a current operated earth leakage circuit breaker (ELCB), and concerns also, circuit breakers specifically adapted for such use.

Electrical and electronic equipments when deployed outdoors, or otherwise in mobile installations, require effective protection to safeguard both personnel and equipment from dangers inherent in the use of electrical power under such conditions. To provide protection, an ELCB can be included in circuit between power supply and equipment.

A current operated ELCB of the conventional kind comprises the combination of a current balance transformer and an input trip switch. The transformer includes a first primary coil connected at one end to the neutral line input of the ELCB via a first pole of the switch. For use with single phase power input, the transformer includes a second primary coil connected at one end to the live line input of the ELCB via a second pole of the switch. For use with 3-phase power input the transformer includes a further two primary coils connected to the phase line inputs via further switch poles. The primaries are connected at their remote ends to the neutral, and to the live or 1-2-3 phase outputs of the ELCB. The secondary of the transformer is connected across the control inputs of the trip switch.Under operating conditions the trip switch is closed and, provided the current flowing through the primaries is balanced, this switch remains closed. To avoid initial trip of the switch, it is normal practice to bias the contacts of the switch so that the live or phase contacts are iast to make. However, should the current flowing through the primaries deviate significantly from balance such that the net current exceeds a predetermined threshold - usually 30 mA or thereabouts, sufficient current is induced in the secondary to trip the switch.

In compliance with United Kingdom domestic regulations, it is usual to connect a test-switch and series connected limiting resistance between the live output, (single phase) of the ELCB to the input end of the first primary coil or between any two of the 1-2-3 phase outputs (3-phase). Providing the ELCB is functioning correctly under operating conditions, the trip switch should then immediately cut out when the test switch is closed, for in each case, a primary is by-passed and the current balance disturbed.

A conventional circuit installation of such an ELCB is described in "Defence Standard 61-5 (Part 3) Sept 1979 pp 11,12 and 20, issued by the Ministry of Defence, directorate of Standardisation, First Avenue House, London WC1V 6HE. As incorporated in this circuit, the ELCB outputs (live and neutral) are connected to user equipment via an equipment switch. The equipment is bonded to a supporting chassis which in turn is connected to a local safety earth spike. Single phase power is supplied by a 3-wire cable, the live and neutral wires being connected to the ELCB inputs. The third supply wire -the earth continuity conductor- is connected at the supply source to the neutral conductor and to a system main earth - an earth spike. At the installation end of the cable, the earth wire is connected to the chassis.In the event of a discontinuity in the earth wire, the equipment and chassis remain earthed by the local safety earth, to the extent that this latter is effective. This installation serves to protect personnel and equipment from excessive leakage current from the live conductor to earth.

However, the circuit installation described has a number of disadvantages: (i) The circuit is not compatible with equipments which inherently utilise earth leakage as part of their function - eg equipments incorporating RF suppression filters- mobile transmitters, etc. When such equipment is fitted to the output side of the ELCB, the high earth leakage current normally drawn by the filter is sufficient to cause the ELCB to cut out.

(ii) Accidental direct connection of the live supply wire to the installation earth will inevitably cause the chassis to assume a lethal potential, despite the safeguard of an earth spike.

(iii) In the event of a discontinuity in the earth continuity conductor- an event not uncommon in the field where trailing cable is exposed - the installation is wholly dependent on the connection to the local safety earth and again the chassis can nonetheless assume a lethal potential.

(iv) When such installations are employed in the interconnection of a number of mobile equipments, a fault in the system earth connected to one installation may go undetected if earth currents become diverted along interconnecting signal cable.

When such a cable is disconnected, the metal shell ofthe plug can assume lethal potential.

The present invention is intended to provide a circuit that is compatible with a filtered equipment which furthermore is sensitive to misconnection of the live or phase supply to either the neutral or earth inputs of the circuit. Furthermore, by the preferred embodiments of the invention as described hereinafter, it is intended to provide circuits capable of monitoring the continuity of the earth and neutral conductors of a connected supply.

The present invention is also intended to provide a circuit breaker adapted for use in such a circuit.

In accordance with the invention there is provided an electric shock protection circuit including a current operated earth leakage-circuit breaker comprising: a trip-switch having at least two poles; and, a current balance transformer having a secondary connected to control the trip-switch, a first primary connected at its near and to a first one of the switch poles for connection to power neutral, a second primary connected at its near end to a second one of the switch poles, for connection to power live or phase, and a third primary; the trip-switch being arranged such that the second pole shall make contact following the making of contact corresponding the first pole; the circuit beihg characterised by a first impedance connected to the near end of the third primary and to the remote end of the first primary.

For use in conjunction with 3-phase power, the transformer may include an additional two primaries, and the trip-switch an additional two poles connected each to one of the additional primaries, the additional poles likewise making contact following the first pole.

To afford fail-safe protection it is preferable that the trip-switch includes a third pole, the third primary being connected to the remote side of this pole, the first impedance being connected to the third primary on the remote side of this pole, it being arranged that the second pole, and any additional pole aforesaid, shall make contact following the third pole.

To extend the versatility of this circuit, it is preferable that the circuit shall also include one or more additional impedances as follows: a second impedance connected at its near end to the near end of the first-impedance, for connection at its remote end to a local safety earth; a third impedance connected between the remote ends of the first and second primaries; a fourth impedance connected between the remote ends of the second and third primaries; and, a fifth impedance connected to the remote end of the third primary for connection at its remote end to the local safety earth or to the remote end of the second impedance.

In further accordance with the invention there is provided a current operated earth leakage circuit breaker for use in the circuit above defined and for use with single phase power supply, the circuit breaker comprising: a trip-switch having at least three poles; and a current balance transformer having a secondary connected to control the trip-switch, and at least three primaries, a first, a second, and a third, each connected at its near end to a corresponding one of the switch poles the trip-switch being arranged such that the pole corresponding the second primary shall make contact following the pole corresponding the first primary.

the circuit breaker being characterised by an input tap connected to the third primary on the remote side of the corresponding switch pole, it being arranged that the pole corresponding the second primary shall likewise make contact following the pole corresponding the third primary.

Thus, as a matter of convenience, a 4 - pole, 4 primary - ELCB may be modified to this end by the provision of an input tap and the adaptation of the arrangement of the trip - switch; one of the poles and one of the primaries then being redundant. In yet further accordance with the invention there is provided a current operated earth leakage circuit breaker for use in the ciruit above defined and for use with 3 - phase power supply, the circuit breaker comprising: a trip-switch having at least four poles; and, a current balance transformer having a secondary connected to control the trip-switch, and four pri maries a first, a second, and two in addition, each connected at its near end to a corresponding one of the switch poles, the switch being arranged such that the poles corresponding the second and the additional two primaries shall make contact following the pole corresponding the first primary; the circuit breaker being characterised in that the transformer has yet a further primary.

Preferably, the trip-switch has yet a further pole connected to the further primary, and an input tap is connected to the near end of the further primary on the remote side of the trip-switch.

Particular embodiments of the invention will now be described for the purpose for example only, with reference to the accompanying drawings of which : Figure lisa circuit diagram of a single - phase shock protection circuit shown connected between mains supply and user equipment; and, Figure 2 is a circuit diagram of a 3 - phase protection circuit, a variant of the circuit shown in the preceding figure.

There is at the heart of the shock protection ciruit shown in figure 1 a sensitive, current operated, earth leakage current breaker (ELCB) 1. This ELCB 1 is adapted from an ELCB of the type usually used in circuit between a 3 - phase mains supply and user equipment, and comprises the combination of a current balance transformer 3 and an electro-mechanical, solenoid operated, 4 - pole trip - switch 5. The transformer includes four primary coils P1 to P4, three of which, P1, P2 and P3 are utilised in this circuit. The secondary of the transformer 3 is connected across the solenoid control 7 of the trip switch 5. The three primaries P1, P2, P3, are connected at one end to each one of three poles TL, TE, TN, respectively, of the trip - switch 5.The remote end of each of these three primaries P1, P2 and P3, is connected to an output OL, OE, and ON, respectively, of the ELCB 1. The input side of each pole of the switch 5, input SL, SE and SN, corresponding to the tree primaries P1, P2 and P3, is connected to one of the live ML, earth ME and neutral MN conductors, respectively, of a single phase 3 - wire mains supply through a plug and socket arrangement M. The three selected outputs OL, ON and OE, of the ELCB are connected to three corresponding circuit outputs CL, CN and CE where, as shown in figure 1, they are in turn connected across the load and to the filter earth of an rf suppression filtered equipment F.

To conform with United Kingdom domestic regulations a test - switch TS1 and limiting resistor R1 are connected in series between the remote end of the live - connected primary P1 and the near end of the neutral-connected primary P3, thereby providing a test by-pass of the neutral - connected primary P3 for testing operation of the trip - switch 5.

The ELCB has been adapted by the provision of a tap connection TI to the earth - connected primary P2 of the transformer 3, this connection being made between the output side of the trip-switch 5 and the transformer 3. It is also arranged, by appropriate mechanical biasing of the poles of the switch 5, that the live - connected pole SL-TL of the switch 5 "makes" not only after the neutral - connected pole SN-TN, but also after the earth - connected pole SE-TE. This avoids tripping of the switch under norma! start transient conditions.

The tap-connection TI is connected, via a first impedance Z1, to the remote end of the neutral-connected primary P3. The impedance Z1 is intended to function as a bypass, under certain fault conditions, as will be explained below. As best utilised, the circuit is connected to a local safety earth and, as shown, the tap-connection TI is connected to local chassis metalwork C and to an earth spike ES via a second impedance Z2, of impedance value comparable with that of the first impedance Z1. The remote end of this second impedance Z2 is also connected to a circuit output CC to facilitate the earthing of user equipment.

The circuit so far described is compatible with rf-filtered equipment, if the equipment itself provides adequate impedance across the circuit outputs CL and CN, and CL and CE. However, it is preferable to include shunt impedances Z3 and Z4 internally, between CL and CN and between CL and CE. With these additions the circuit is then more versatile in its application to equipments of different kinds, and furthermore is self-contained and capable of monitoring the integrity of the mains supply even when isolated at its outputs from equipment.

An additional by-pass route is provided by a fifth impedance Z5 connected between the remote end of the earth-connected primary P2 and the chassis safety earth connection to the second impedance Z2.

To provide a test of the local earth-spike impedance and integrity, a second - test - switch TS2 has been inserted in the connection between chassis and the safety earth. This is normally biased to connect chassis and earth, but when operated switches the earth-spike to connect it to the remote end of the live-connected primary P1 through a drop resistance R2.

Suitable component values for a 240 V single-phase circuit including a 30 milliamp trip ELCB are as follows: R1 - 6.8 K ohm + 10% (Internal to ELCB) R2-5.6 K ohm 10% (if required) Z1 - 820 ohm resistor + 10% Z2- " " Z3 -1 F capacitor + 20% Z4- " " " Z5 - 2 F capacitor f 20% To remove residual charge when the circuit is switched off, the capacitors Z3 and Z5 are shunted by resistors each of value 270 K ohm or thereabouts.

For the sake of clarity, these resistors have been omitted from the figures.

The circuit described, functions as follows: i. Under normal circumstances, current from the live input ML of the power supply flows through the live - connected primary P1 of the transformer and thence through the third impedance (the load short impedance Z3) and any paraliel load - eg the equipment connected across the circuit outputs CL, CN,-to return, in part, to mains neutral MN via the neutral-connected primary P3. A part of the live current however is directed by the fourth impedance Z4 onto the earth return path and together with any equipment leakage current (ie filter capacitor leakage) returns to mains earth via the earth - connected primary P2.The sum of the return currents flowing in the neutral- and earth - connected primaries P3, P2 almost exactly balance the supplied current flowing through the live - connected primary P1, and the trip - switch remains in its initial closed state.

ii. Under a fault condition, any leakage current from the live terminal of the load to chassis must by-pass the retrun primaries P2, P3 and return to the mains source either via EARTH (Chassis Earth Spike ES) or the safety earth impedance Z2 to the earth continuity conductor ME at the mains input. If this current is in excess of the ELCB trip current, the ELCB will trip and isolate the circuit and equipment form the mains supply.

iii. Cross-connection of the first impedance Z1 across the ELCB as described, enables monitoring of the integrity of mains earth and mains neutral. With NEUTRAL disconnected, current through Z3 and the load is by-passed through Z1 to EARTH (ME) causing transformer current unbalance and the ELCB to cut out. With EARTH disconnected, current through Z4, plus any filter leakage current, returns twice through the transformer, once via the earth primary P2, and once via the neutral - connected primary P3 after passing through Z1, causing ELCB unbalance.

iv. With both EARTH and NEUTRAL supply conductors ME, MN, disconnected, current flowing through the live - connected primary P1 can find its way to chassis C through a number of parallel paths, and from there to EARTH via the chassis Earth spike ES. With no load connected to the outputs of the circuit CL, CN, the current flowing through the shunt impedances Z3, Z4 is sufficient to cause the ELCB to cut out and isolate the circuit.

v. If mains live is incorrectly connected, for example, to the Neutral switch pole input SN, current through the neutral primary P3 is directed to earth via Zl,thus by-passing the transformer 3 and causing the ELCB to cut-out.

vi. If mains live is incorrectly connected, for example to the earth Switch pole input SE, part of the current is returned to EARTH via Z1 and P3, again causing the ELCB to trip. Since the impedances Z1 and Z2 are connected on the remote side of the trip-switch, they, and the whole of the circuit, are then isolated from the live supply, and the circuit fails safe.

vii. Should an excessive neutral to earth voltage arise between the input terminals MN and ME, an unbalanced current will be set up in primary P3 via impedance Z1 sufficient to cause the ELCB to cut out and isolate the circuit.

The circuit described above may be used to connect equipment to single - phase two conductor supply, provided a common connection - a short across MN and ME or across the neutral and earth switch pole inputs SN and SE is employed.

In the circuit shown in figure 2, which is intended to connect equipment to 3 - phase five conductor supply, the single primary coil P1 and the corresponding pole SL-TL of the trip - switch, have been replaced by three phase-connected primaries Fll, P12, P13 and three poles Sl-T1, S2-T2 and S3-T3.

The phase-connected poles are all arranged to "make" after switching of the neutral - connected pole SN-TN. The ELCB 1, shown in figure 2, is thus similar to the type normally used for connecting equipment to 3 - phase supply but differs by the inclusion of an additional primary, the primary P2.

Though the first and second impedances Z1 and Z2 may be connected directly to mains earth, it is preferable-so that the circuit fails safe-that a fifth pole SE-TE is included in the trip-switch 5 between mains earth and the connection of Zi, Z2 and the earth - connected primary P2. The trip-switch 5 is arranged so that the phase - connected poles S1-T1, S2-T2, and S3-T3 all make after the added pole SE-Te. The phase - connected primaries P11, P12 and P13 are connected to corresponding circuit outputs C1, C2 C3. Shunt impedances Z3, Z4 are provided between C2 and CN, and between C2 and CE, respectively, to provide the capability of monitoring the integrity of the M2, MN and ME supply when isolated from equipments at the outputs.

Suitable component values for a 440 V 3 - phase supply designed circuit including a 30 milliamps trip ELCB are given as follows: R1 - 6.8 Kohm + 10% (Internal to ELCB) R2- 5.6 K ohm + 10. (if required) Z1 -820 ohm resistor 10% Z2- " Z3- 1.0 11 F capacitor 1 20% Z41.0 ,u F " " Z5-2.011F " " It is to be understood that the electro-mechanical trip-switch may be replaced by an equivalent electronic switch and reference to switch poles and to the "making" of contact, shall be construed accordingly.

Claims

1. An electric shock protection circuit including a current operated earth leakage circuit breaker comprising: a trip-switch having at least two poles; and, a current balance transformer having a secondary connected to control the trip-switch, a first primary connected at its near end to a first one of the switch poles for connection to power neutral, a second primary connected at its near end to a second one of the switch poles, for connection to power live or phase, and a third primary; the trip-switch being arranged such that the second pole shall make contact following the making of contact corresponding the first pole; the circuit being characterised by a first impedance connected to the near end of the third primary and to the remote end of the first primary.

2. A circuit as claimed in claim 1 wherein the transformer includes an additional two primaries, and the trip - switch an additional two poles connected each to one of the additional primaries, the additional poles being arranged to make contact following the first pole.

3. A circuit as claimed in either claim 1 or 2 wherein the trip switch includes a third pole, the third primary being connected to the remote side of this pole, the first impedance being connected to the third primary on the remote side of this pole, it being arranged that the second pole, and any additional pole, shall make contact following the third pole.

4. A circuit as claimed in any one of the preceeding claims including one or more additional impedances as follows a second impedance connected at its near end to the near end of the first - impedance, for connection at its remote end to a local safety earth; a third impedance connected between the remote snds of the first and second primaries; a fourth impedance connected between the remote ends of the second and third primaries; and, a fifth impedance connected to the remote end of the third primary for connection at its remote end to the local safety earth or to the remote and of the impedance.

5. -A circuit constructed, arranged and adapted to operate siibstantially as described hereinbefore with reference to, and as shown in, either one of the accompanying drawings figure 1 orfigure2.

6. A current operated earth leakage circuit breakear for use in the circuit claimed in claim 1 above, the circuit breaker comprising: a trip - switch having at least poles; and, a current balance transformer having a secondary connected to control the trip - switch and at least three primaries, a first, a second, and a third, each connected at its near end to a corresponding one of the switch poles, the trip - switch being arranged such that the pole corresponding the second primary shall make contact following the pole corresponding the first primary; the circuit breaker being characterised by an input tap connected to the third primary on the remote side of the corresponding switch pole, it being arranged that the pole corresponding the second primary shall likewise make contact following the pole corresponding the third primary.

7. A current operated earth leakage circuit breakerfor use in the circuit claimed in claim 2 above, the circuit breaker comprising: a trip-switch having at least four poles; and a current balance transformer having a secondary connected to control the trip - switch, and four primaries a first, a second, and two in addition, each connected at its near end to a corresponding one of the switch poles, the switch being arranged such that the poles corresponding the second and the additional two primaries shall make contact following the pole corresponding the first primary; the circuit breaker being characterised in that the transformer has a further primary.

8. A circuit breaker as claimed in claim 7 wherein the trip switch has a further pole connected to the further primary, and an input tap connected to the near end of the further primary on the remote side of the trip - switch.