GB2078000A - Circuit breaker - Google Patents

Abstract

An earth leakage circuit breaker may be controlled via a differential transformer with two primaries, one carrying the live and one the neutral current and its secondary is connected to the circuit-breaker trip coil H. The circuit breaker has a toggle linkage of two levers B, C which normally move together with the pin E on the lever C against the side of the lever B. When the trip coil H attracts its armature J the armature strikes a member K which acts on the lever B to move the toggle linkage B,C overcentre to allow a spring G to move a lever D to a contact open position. <IMAGE>

Description

SPECIFICATION Circuit breaker The present invention relates to an earth leakage circuit breaker and its associated circuitry.

Such circuit breakers as hitherto available tend to be complex and expensive, and it is an object of the invention to provide such a breaker which is com pact and relatively simple.

According to the present invention there is pro vided an earth leakage circuit breaker, which in cludes a first lever which is driven by an on-off switch and which is pivoted to a point between the ends of a second lever, a pin on the second lever adjacent to an edge of the first lever so that when operated by the on-off switch the two levers move as a single lever, switching means controlled by the second lever so that operation of the switch causes on-off switching, means for detecting an earth leakage in the circuit to be protected, a relay coil to which an energising current is applied when an earth leakage is detected and which moves an armature, and a cam which is struck by the armature when the latter is moved on energisation of the relay coil, which cam operates on the first lever to rotate it with respect to the second lever so that the two levers no longer function as a single lever, the second lever then rotating with respect to the first lever to break the contacts.

An embodiment of the invention will now be described with reference to the accompanying draw ings, in which Figure 1 is the electronic circuitry associated with an earth leakage circuit breaker embodying the invention, Figure 2 is an "exploded" view of a circuit breaker unit embodying the inven tion, and including circuity such as that of Figure 1 and Figure 3 shows certain elements of Figure 2 extracted for clarity of explanation.

First we consider the operational principle, with special reference to Figure 1. When the leakage current occurs, it flows from either the live orthe neutral wire to earth, which causes an imbalance between live and neutral currents. The difference current thus produced is sensed by a differential transfomerT,which has two primarywindings connected in opposition, one of them carrying the live and the other the neutral line current. The voltage produced at the secondary winding depends on the value of the difference current, if any. An electrostatic shield is used between the primaries and the secondary to prevent false triggering due to capactive couplings. Also a clamping filter CF is connected across the secondary for the same reason.

After filtering the leakage current signal is fed ta a dual comparator DC, which responds whenever the signal exceeds a positive or a negative threshold.

This enables the circuit to detect faults due to rectifier-coupled leaks, and also with minimum delay. If the detector circuitry detects a leak of sufficient level, one or other of the comparators turns the SCR on, which energises the relay coil RC, which, as will be seen below, causes the mechanism to switch the output off.

To protect the circuitry from high mains transients, a surge suppressor SS is connected across the bridge rectifier which supplies the circuitry. Hence if a large enough surge occurs, SS begins to conduct, dropping the excess voltage across the resistor R1.

Atest arrangement, consisting of a test swtich TS in series with a resistor R2 is included, so that the unit can be periodically tested. Operation of this switch, e.g. by a push button induces a known leakage current into the transformer DT, and this should trip the breaker.

We now consider the mechanical arrangement of the breaker, see Figures 2 and 3. This mechanism consists in the main of moulded links, which is relatively cheap and controls a two-pole switch whose movable element is a hinged member D. In the design of this mechanism it was necessary to ensure that it functions within a safe time period when: (a) a short appears to earth while the protected apparatus is in use, and the switch is free to return to its OFF position, (b) as (a), but the switch is held ON, (c) when there is a short on the apparatus prior to switching on, As will be seen, the methods of both normal and "leakage" switching are based on the principle of "over-centre" levers. Figure 2 shows the state of the switch with the circuit open and Figure 3 shows it with the switch contacts and hence the circuit closed.

For normal switching, links B and C act together as a single lever because the pin Eon the lever C acts on the lever B to maintain B and C in this condition.

To switch the circuit on, lever A is reversed from its position in Figure 2 and, via a pin on its inner end it moves lever B leftwards until the link C abuts against a stop F. It is held in this position by the pressure of the movable contact return spring G. The levers have now reached the condition shown in Figure 3, in which the arm D is horizontal, with the contacts closed.

Normal switching off reverses this procedure so that we regain the condition shown in Figure 2.

When a leakage is detected with the switch ON, the relay coil H (coil RC of Figure 1) is energised, which causes its armature J to move downwards to strike a cam member K. This strikes the end of the link B, which pivots about the pin on which it is connected to the lever C. In so doing it passes overcentre, and so is no longer, "solid" combined link with C. As a result, the movable contact spring G is freed, and breaks the spring contacts. The torsion spring L return the-levers A, B and C to the OFF position.

A similar action occurs when there is an existing leakage, but in this case the coil J is energised as soon as the line contacts are made.

To ensure that the power is available to the electronic circuitry, the neutral contacts are arranged to make an advance of the line controls.

The blocks M and N indicate the location of the electronics and circuitry in the assembly of the breaker.

In its initial version, the above-described arrangement has been shown to detect a 20 mA leakage with a response time of 20 ms.

Claims (6)

1. An earth leakage circuit breaker, which includes a first lever which is driven by an on-off switch and which is pivoted to a point between the ends of a second lever, a pin on the second lever adjacent to an edge of the first lever so that when operated bythe on-off switch the two levers move as a single lever, switching means controlled by the second lever so that operation of the switch causes on-off switching, means for detecting an earth leakage in the circuit to be protected, a relay coil to which an energising current is applied when an earth leakage is detected and which moves an armature, and a cam which is struck by the armature when the latter is moved on energisation of the relay coil, which cam operates on the first lever to rotate it with respect to the second lever so that the two levers no longer function as a single lever, the second lever then rotating with respect to the first lever to break the contacts.

2. A breaker as claimed in claim 1, and which inciudes a fixed stop against which the levers move when switched on and which defines the levers' travel.

3. A breaker as claimed in claim 1 or 2, and in which a leakage is detected by a differential transformer having two primary windings one carrying the live current and the other the neutral current, the secondary output of the transformer being a voltage proportional to a leakage, if any.

4. A breaker as claimed in claim 3, and in which the output of said secondary is applied to comparison means adapted to detect a transformer output indicative of a positive or negative leakage, the output of said comparator controlling the relay coil.

5. An earth leakage circuit breaker substantially as described with reference to the accompanying drawings.

New claims or amendments to claims filed on 5th February 1981 Superseded claims New or amended claims:

6. An earth leakage circuit breaker, which includes a first lever which is driven by an on-off switch and which is pivoted to a point between the ends of a second lever, a pin on the second lever adjacent to an edge of the first lever so that when the pin and the edge of the first lever causes the two levers to move as a single lever, switching means controlled by the second lever such that operation of the switch causes off-on or on-off switching, a relay coil to which an energising current is applied when an earth leakage is detected in the circuit to be protected, which coil moves an armature when its coil is thus energised, and a cam which is struck by the armature when the latter is moved on energisation of the relay coil, which cam operates on the first lever to rotate it about said cam with respect to the second lever so that the two levers no longer function as a single lever, the second lever then rotating with respect to the first lever to break the contacts.