The invention relates to a circuit breaker. More particularly, the invention relates to a circuit breaker of the type having an arc chamber means into which arc gases generated on tripping of the breaker under load arc delivered.

Circuit breakers are used to break an electrical circuit between a source of electricity and an electrical load in response to a fault condition. Generally the circuit breaker includes a stationary contact coupled to the electrical load and a movable contact coupled to the electrical source. In the event of a fault the circuit breaker opens the contacts . An arc is generated which is generally directed by an arc arrester plate and an arc deionisation stack to exhaust from the rear of the circuit breaker. Generally, a number of circuit breakers are arranged in a load centre having a number of bus bars to which the circuit breakers are mounted. The bus bars require to be protected against contact with the arc gases as the gases are ionised and can lead to short circuits between phases (known as cross- phasing) if the gases contact with the unprotected busbars .

The invention is directed towards providing an improved circuit breaker which will overcome this difficulty.

This invention is characterised in that the arc chamber means comprises :-

a first chamber into which the arc gases are delivered, the first chamber having an arc gas outlet; and

a second chamber into which arc gases from the first chamber are delivered through the outlet from the first chamber.

In a particularly preferred embodiment of the invention the arc gases are substantially retained within the second chamber and are not exhausted from the circuit breaker.

In one particular embodiment of the invention the breaker includes a housing defined by a base, a cover, a rear wall, a front wall, and upper and lower side walls extending between the front and rear walls, the arc chambers being defined within the housing.

Preferably the gas outlet is defined by a single opening between the first and second chambers.

In one embodiment of the invention the second arc chamber is provided adjacent the rear wall of the housing.

In a particularly preferred arrangement the second arc chamber includes a longitudinally extending dividing wall spaced-apart from the rear wall and separating the second arc chamber from components of a circuit breaking device, the longitudinal dividing wall having an aperture defining an arc gas outlet for delivery of arc gases from the first chamber into the second arc chamber.

Preferably the aperture is provided remote from the generation of the arc within the housing.

In one embodiment of the invention the second chamber is defined by a transversely extending dividing wall which extends between the rear wall of the housing and the longitudinally extending dividing wall.

Preferably the or each dividing wall comprises a cover portion extending from the cover and a base portion extending from the base to engage with the cover portion. Most preferably the cover portion and base portion of the dividing wall interengage to define the second chamber on assembly of the cover to the base.

In a preferred arrangement the length of the second chamber is at least 40% of the length of the rear wall.

In one embodiment of the invention the circuit breaker includes a baffle wall having a plurality of spaced- apart apertures through which arc gases which have passed through the arc stack are delivered into the first arc chamber.

In this case preferably the first arc chamber is defined at least partially between the baffle wall and portion of a longitudinally extending dividing wall spaced-apart from the rear wall of the circuit breaker housing.

In a preferred arrangement the first arc chamber is also defined by a generally Z-shaped dividing wall in-board of the longitudinally extending dividing wall, the Z- shaped dividing wall extending between the baffle wall and the longitudinally extending dividing wall. Preferably a lower leg of the Z-shaped dividing wall is spaced-apart from the baffle wall and an upper leg of the Z-shaped dividing wall extends generally parallel to the upper side wall to direct the arc gases to the aperture in the longitudinal dividing wall.

In a preferred embodiment of the invention the circuit breaker includes an additional choking baffle in the first arc chamber. Preferably the additional choking baffle comprises an additional choking baffle which comprises an additional baffle wall extending into the first arc chamber from the Z-shaped dividing wall.

In one embodiment of the invention the or each dividing wall and the or each baffle wall comprise a cover portion extending from the cover and a base portion extending from the base to engage with the cover portion. Preferably the cover portion and base portion of the dividing wall and/or baffle wall interengage to define the first chamber, an assembly of the cover to the base.

The invention will be more clearly understood from the following description thereof given by way of example Only with reference to the accompanying drawings in which :-

Fig. 1 is a view of a circuit breaker according to the invention;

Fig. 2 is a perspective view of a base of the circuit breaker;

Fig. 3 is a perspective view of a cover of the circuit breaker;

Fig. 4 is a perspective view of a detail of the circuit breaker;

Fig. 5 is a perspective view of a detail of part of the cover of the circuit breaker housing;

Fig. 6 is a perspective of a detail similar to Fig. 5 of part of the base of the circuit breaker housing;

Fig. 7 is a perspective, partially cut-away view of the parts of the base and cover of Figs. 5 and 6, assembled; and

Fig. 8 is a cross sectional view on the line VIII-VIII of Fig. 6, assembled.

Referring to the drawings, there is illustrated a circuit breaker 1 including a line terminal 2 for coupling to a source of electricity and a load terminal 3 for coupling to a load. The current path includes a coil 4 and an arc runner 6. An arc stack 5 assists in breaking any arc formed when contacts 7 are separated under load. The coil 4 causes separation of the contacts 7 in the event of a current surge. The contacts 7 may also be separated in response to operation of a bimetal 10. The bimetal 10 is adjusted by means of a calibration screw 12.

The circuit breaker 1 comprises a base 20 and a cover 21. The base 20 and cover 21 both have rear wall portions 22a, 22b respectively, front wall portions 23a, 23b, upper sidewall portions 24a, 24b and lower sidewall

portions 25a, 25b respectively which cooperate on assembly of the base 20 and cover 21 to form an enclosure for the various components of the circuit breaker.

Upper and lower rail mounting devices 26,27 extend from the rear of the housing. In this case the upper rail mounting device 26 is defined by the line terminal 2 and includes a pair of spaced-apart jaws 31,32.

The circuit breaker 1 includes a first baffle wall 40 having a plurality of spaced-apart apertures 41 through which arc gases which have passed through the arc stack 5 are driven. The arc gases are then led through a first arc gas chamber 44 defined between a generally Z- shaped dividing wall 45, the baffle wall 40 and portion 46 of a longitudinal dividing wall 51. The portion 46 of the longitudinal dividing wall 51 has an inlet aperture 52 through which the arc gases are led into a second arc gas chamber 50. It will be noted that the generally Z-shaped dividing wall 45 has a lower leg 47 which is spaced-apart from the baffle wall 40 and includes an end in-turned lip 48. An upper leg 49 of the Z-shaped dividing wall 45 extends generally parallel to the upper side wall 25 to direct the arc gases to the aperture 52 in the longitudinal dividing wall 51.

An additional choking baffle provided by an additional baffle wall 42 extends into the first arc chamber 44 from the dividing wall 45. This choke/baffle wall 42 acts to increase the upstream pressure thus increasing the blow off force which increases the speed of contact opening and thus the interrupting ability. The explosive nature of the exhaust gases are controlled by reduction in the volume of the first chamber 44.

The chamber 50 is defined by the longitudinal dividing wall 51, the rear walls 22a, 22b of the housing and a transverse diving wall 53. The transverse dividing wall 53 extends between the longitudinal dividing wall 51 and the rear wall 22a,22b of the housing, approximately half-way along the rear wall 22a,22b. Preferably the

length of the second chamber 50 is at least 40% and most preferably at least 50% of the length of the rear wall 22a,22b. This maximises the effective volume into which the neutralised arc gases finally exhaust, thus reducing the pressure on the retaining walls .

In this case the second arc chamber 50 is closed and does not have an exhaust outlet. This avoids a surge of hot and conductive gases passing out from the circuit breaker reducing the impact and potential destructive effect on parts outside the enclosure.

Referring particularly to Fig. 8, it will be noted that the longitudinal dividing wall 51 comprises a base portion 51a and a cover portion 51b which have interengaging portions 60a,60b which interengage as illustrated in Fig. 5 on assembly of the base 20 and cover 21. Similarly, the transverse dividing wall 53 comprises a base portion 53a and a cover portion 53b which have angled mating surfaces 61a, 61b for interengagement as illustrated in Fig. 5 on assembly of the base 20 and cover 21. The baffle wall 40 also has a base portion 40a and a cover portion 40b which have interengaging portions for interengagement, on assembly. Similarly the Z-shaped dividing wall 45 and the additional baffle wall 42 each have a base portion and a cover portion which have interengaging portions which interengage on assembly of the base 20 and cover 21. In this way effectively sealed first and second arc gas chambers 44,50 are formed.

The flow of arc gases following short circuit interruption and separation of the contacts is illustrated by arrows in Fig. 1 and Figs. 5 to 7. It will be noted that the arc is directed through the arc stack 5 by the arc runner 6. The debris and gases are then led through the apertures 41 in the baffle wall 40 and into the first chamber 44. In this chamber 44 the arc gases are cooled which assists in neutralising the destructive nature of the gases .

The gases then pass from the first chamber 40 through the inlet 52 into the second chamber 50.

In this case, the gases are confined within the second chamber 50 and are thus prevented from exiting the breaker. This eliminates cross-phasing in the load centre in which the breaker is mounted. The second arc chamber 50 is created within the boundaries of the circuit breaker housing which results in exhaust gases which can carry considerable debris from contact erosion being retained within the chamber 50. Because an external outlet for the gases is not required the housing may also be effectively sealed to prevent dust ingress .

It is also anticipated that the performance of the circuit breaker will be improved because having no outlet creates a back pressure for the arc gases. Tests to date have indicated an improved performance under short circuit conditions when the gases vent into a closed chamber 50. The two chambers 44,50 are volumes for gas expansion.

It will be appreciated that while the invention has been described with reference to a miniature circuit breaker, it will be appreciated that it may be applied to any type of circuit breaker including a moulded case circuit breaker or other devices in which arc gases are generated.

The invention is not limited to the embodiments hereinbefore described which may be varied in both construction and detail .