[0001] This invention is concerned with improvements relating to the circuit breakers, particularly
of the kind known as "miniature" circuit breakers.
[0002] A conventional miniature circuit breaker (hereinafter referred to as being of the
kind specified) comprises a first contact (conventionally fixed) and a second contact
movable relative to the first between an open position and a closed position, a manually
operable control member to move the movable contact between open and closed positions,
and a tripping mechanism to move the movable contact from its closed to its open position
in the event of an overload.
[0003] Conventionally the tripping mechanism comprises two overload devices to open the
circuit in the event of overload.
[0004] The first of these is a bimetal device the temperature of which rises under overload
conditions, and which moves in consequence of such temperature rise to open the switch,
usually under the action of a spring. However such devices are slow to operate and
are used only to open the circuit under moderate overloads.
[0005] On short circuit overloads a solenoid device is used, through which current passing
through the circuit breaker flows, and which, under short circuit conditions, produces
a magnetic field to move a tripping element (conventionally termed a "slug") to open
the switch, the slug moving to delatch a tripping mechanism and physically to move
the movable contact to an open circuit position.
[0006] Circuit breakers are limited by the current overload they can accommodate and still
be re-usable, the current overload being known as the "breaking capacity". This however
makes it desirable that the circuit breaker be capable of moving to an open circuit
condition in a very short time (typically 5 milliseconds).
[0007] On the movement of a conventional circuit breaker to en open condition under short
circuit conditions, the movable contact is physically moved away from a fixed contact
at high speed. However current still passes between the contacts in the form of an
arc, and conventionally the circuit is arranged so that the arc is subjected to pressure
to move from the two contacts onto "arc runners", which diverge and lead the arc into
an arc stack, at which the arc is extinguished.
[0008] Thus in a conventional circuit breaker, one arc runner is electrically integral with
the fixed contact, and the other is engaged by the movable contact on tripping.
[0009] Whilst adequate magnetic forces may be generated by the solenoid to disengage the
movable contact from the fixed contact in sufficiently short periods of time, increasing
the breaking capacity of circuit breakers is presently limited by the difficulties
of transferring the arc to the arc runners and quenching in the arc stack, and it
is one of the various objects of this invention to provide a means by which the arc
may be extinguished more rapidly than is presently practiced.
[0010] It is to be noted that in larger circuit breakers, for example the type known as
the moulded case circuit breaker, it is known to split to arc between three arc runners.
However whilst such a technique may be used to advantage where the operating parameters
are as present in the moulded case circuit breaker (including a higher breaking capacity,
typically 25 kA to 100 kA, and a breaking time of 10 to 30 ms) this technique cannot
be used to advantage in a miniature circuit breaker, in view of the requirements of
small size and higher response speed (specifically less than 5 ms).
[0011] According to this invention there is provided a circuit breaker of the kind specified
wherein the arc is transferred to an arc runner which provides a higher impedence
to the flow of electric current.
[0012] The said arc runner may be electrically connected into the circuit at a point therein
on the side of the solenoid which is remote from the fixed contact.
[0013] The said arc runner may be afforded by a supplementary runner, to which the arc is
transferred from a primary runner integral with the fixed contact.
[0014] Preferably there is no direct physical connection between the primary arc runner
and said supplementary arc runner.
[0015] This invention is particularly useful in a miniature circuit breaker comprising specifically
two arc runners which diverge relative to one another from the contacts, and onto
which the arc generated is transferred as the contacts are opened, the arc being stretched
as it is moved between the diverging runners towards the arc stack. Thus it is to
be appreciated that the are is tansferred onto the supplementary runner from a first
runner (so that the arc extends between the supplementary runner and the second runner),
and the additional impedence reduces the rate of current build-up immediately prior
to entry of the arc into the arc stack.
[0016] Thus according to this invention there is provided a method of increasing the breaking
capacity of a miniature circuit breaker of the kind in which the arc is transferred
from the contacts to two divergent runners which stretch the arc as it is moved towards
the arc stack, wherein the arc is transferred from a first of said two divergent runners
to a supplementary runner so that the arc extends between said supplementary runner
and the second of the two divergent runners, the supplementary runner being connected
in parallel with the said first runner and having associated therewith a higher impedence
whereby the supplementary runner reduced the rate of increase of the arcing current
as the arc enters the arc stack.
[0017] There will now be given a detailed description, to be read with reference to the
accompanying drawings, of a circuit breaker which is the preferred embodiment of this
invention, having been selected for the purposes of illustating the invention by way
of example.
[0018] In the accompanying drawings:
FIGURE I is schematic view of a conventional circuit breaker;
FIGURE 2 is a schematic view of the circuit breaker which is the preferred embodiment
of this invention; and
FIGURES 3 and 4 are enlarged views of part of the preferred embodiment.
[0019] The circuit breaker shown in Figure I is of the kind specified, being a miniature
circuit breaker comprising a housing 6 in which is located a fixed contact 8, a movable
contact 10, and a manually operable control member 12 to move the contact 10 between
its open and closed positions. Located within the housing is overload mechanism, comprising
in series a bimetal device 14 and a solenoid device 16, the solenoid device 16 comprising
a coil 18.
[0020] Integral with the fixed contact 8 is a first arc runner 20, and engaged by the second
contact 10 when it moves to its open position is a second arc runner 22, which runners
diverge from their contacts towards an arc stock 24. The circuit breaker comprises
terminals 26 and 28, and the primary circuit extends from the terminal 26 to the movable
contact 10, the fixed contact 8, the coil 18, the bimetal element i4, and the terminal
28.
[0021] In known manner when the circuit protected by the breaker is subjected to a moderate
and continuing overload, the element 14 deflects under .temperature rise, and causes
the tripping mechanism (not shown) to move the contact 10 to its open position. Under
severe overload conditions, such as short circuit, the magnetic field produced by
the coil 18 causes a tripping element to move rapidly to trip the control mechanism,
and physically to move the contact 10 towards its open position. Any arc which is
established between the contacts 8 and 10 is moved by flux pressure (the flow path
being in the form of a U) onto the arc runners 20 and 22, and into the arc stack 24
at which the arc is extinguished.
[0022] In the circuit breaker which is the preferred embodiment of this invention, illustrated
in Figures 2 to 4, similar numerals are used to denote like parts.
[0023] However as distinct from the circuit breaker shown in Figure I, in the preferred
embodiment the first arc runner 20 extends towards a supplementary arc runner 30,
which is not in direct electrical contact with the runner 20. The supplementary arc
runner 30 is (located adjacent to the entry into the arc stack, and is electrically
connected into the circuit at a point therein on the side of the solenoid coil l8
which is remote from the fixed contact 8, preferably at a point between the solenoid
coil 18 and the bimetal element 14. However between the supplementary arc runner 30
and its connection into the primary circuit, a high impedence element 32 is provided,
specifically in the form of a secondary coil preferably wound around one of the formers
of the solenoid 16.
[0024] As will be seen in Figure 3, the secondary coil 32 is preferably provided of high
resistivity ferrous material. As will be seen in Figure 4, whilst the primary runner
20 necessarily passes close to the supplementary runner 30, desirably relatively large
gaps (shown at 34) ensure that metal particles carried on the arc do not become fused
to the arc runner system to provide a short circuit between the primary and supplementary
runner.
[0025] Thus the arc produced on opening of the contacts is initially transferred between
the first and second runners, the arc being stretched as it is moved between the diverging
runners towards the arc stack. As the arc is transferred between the supplementary
runner 30 and the second runner 22, it encounters higher impedence, which is effective
to reduce the rate of increase of the arcing current as the arc enters the arc stack.
[0026] It has been found that by this relatively simple expedient, the time in which an
otherwise conventional circuit breaker may terminate the flow of - electric current
may significantly be reduced, and the breaking capacity of the circuit breaker increased.
I. A circuit breaker comprising a first contact (8) and a second contact (10) movable
relative to the first between an open position and a closed position, a manually operable
control member (12) to move the movable contact (10) between open and closed positions,
and a tripping mechanism (16) to move the movable contact (10) from its closed to
its open position in the event of an overload wherein the arc is transferred to an
arc runner (30) which provides a higher impedence to the flow of electric current.
2. A circuit breaker according to Claim I wherein the arc runner (30) is electrically
connected into a circuit at a point therein on the side of the solenoid (18) which
is remote from the fixed contact (8).
3. A circuit breaker according to one of Claims and 2 wherein said arc runner (30)
is afforded by a supplementary runner, to which the arc is transferred from a primary
runner (20) integral with the fixed contact (8).
4. A circuit breaker according to Claim 3 wherein there is no direct physical connection
between the primary arc runner (8) and said supplementary arc runner (30).
5. A circuit breaker according to Claim 3 or Claim 4 wherein the supplementary runner
(30), and the point at which the arc is transferred to the supplementary runner, is
adjacent to the entrance of the arc stack (24).
6. A method of increasing the breaking capacity of a miniature circuit breaker of
the kind in which the arc is transferred from the contacts (8, 10) to two divergent
runners (20, 22) which stretch the arc as it is moved towards the arc stack (24),
wherein the arc is transferred from a first (20) of said two divergent runners to
a supplementary runner (30) so that the arc extends between said supplementary runner
(30) and the second (22) of the two divergent runners, the supplementary runner (30)
having assosciated therewith a high impedence whereby the supplementary runner reduces
the rate of increase of the arcing current as the arc enters the arc stack.
7. A circuit breaker constructed and arranged substantially as hereinbefore described
with reference to Figures 2, 3 and 4 of the accompanying drawings.
8. A method of increasing the breaking capacity of a circuit breaker, when carried
out substantially as hereinbefore described with reference to Figures 2, 3 and 4 of
the accompanying drawings.