Circuit breaker having variable arc gas venting

Circuit breaker arc exhaust apertures in the line end of the circuit breaker case transfer arc gases from the circuit breaker interior during overcurrent circuit interruption. A flexible shield protects the circuit breaker terminal lug from arc gas contamination.

BACKGROUND OF THE INVENTION 
Circuit breakers as described in U.S. Pat. No. 4,731,921 entitled "Method 
of Fabricating a Molded Case Circuit Breaker" are capable of interrupting 
circuit current over a wide range of ampere ratings. One of the limits, to 
still higher ampere ratings is the problems that occur with the venting of 
the arc gases that are generated during short circuit interruption. Larger 
arc chutes are correspondingly required to cool and quench the gases and 
to limit and control the amount of gas that is exited from the line end of 
the circuit breaker enclosure. It would be desirable to use the same 
circuit breaker components and enclosure over still higher ampere ratings 
without having to enlarge either the arc chute, current carrying 
components or the enclosure. 
U.S. Pat. No. 3,582,966 entitled "Venting Means for Circuit Breaker Arc 
Quencher" describes a high ampere-rated circuit breaker that employs a 
flexible arc shield arranged intermediate the end of the arc chute and the 
apertures that vent the arc gases from the circuit breaker enclosure. The 
apertures are vented during so-called "long time" and "short time" 
overcurrent occurrences but become exposed during short circuit conditions 
by the forces generated by the arc gases during the short circuit 
interruption. The associated line terminal lug or terminal screw is 
situated under the vent apertures and is separated from the effluent gases 
by means of a line terminal compartment barrier. It would be economically 
advantageous to vent the effluent gases upon short circuit interruption at 
higher ampere ratings without having to limit the number of apertures by 
incorporating a line terminal compartment with in the circuit breaker 
enclosure to protect the associated line terminal screw. 
One purpose of the invention is to enlarge the circuit interruption 
capacity of a circuit breaker without having to correspondingly increase 
the size of the circuit breaker components or the size of the enclosure. 
SUMMARY OF THE INVENTION 
A circuit breaker arc exhaust arrangement whereby a large number of 
apertures are provided in the line end of the circuit breaker case and 
cover for arc gas transfer out from the case and cover during overcurrent 
circuit interruption. A flexible shield is interfaced between the circuit 
breaker line terminal lug and the apertures to prevent contamination of 
the line terminal screw without diminishing the circuit breaker ampere 
interruption capacity.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A molded case circuit breaker 10 is shown in FIG. 1 comprising a case 12 
with the cover removed to show the interior components. The current 
carrying components include a fixed contact 13 and a movable contact 14 
attached to an operating mechanism 16 by means of a movable contact arm 
15. The operating mechanism is refrained from driving the movable contact 
arm and movable contact to the open position under the bias provided by a 
pair of powerful operating springs 17 by interference between the cradle 
21 and the latch system 20. An explanation of the operation of the latch 
system is found in the aforementioned U.S. Pat. No. 4,731,921 entitled 
"Method of Fabricating a Molded Case Circuit Breaker". The load lug 18 is 
connected with an external poser distribution circuit such that circuit 
current transfers through a load strap 25 and bimetal 22 and from the 
braid conductor 23 to the movable contact arm 15 and contacts 13,14 to the 
line terminal screw 19. The bimetal 22 upon the occurrence of an 
overcurrent condition contacts the trip bar 24 to articulate the operating 
mechanism to separate the contacts. Upon the separation of the contacts, 
the arc that occurs therebetween is quickly extinguished within the arc 
chute 27 to completely interrupt the circuit current. The operating 
mechanism is separately controlled by means of an operating handle 28 
depicted with the contacts 13, 14 in their "ON" condition. In accordance 
with the invention, the enclosure for the arc plates 29 is in the form of 
a front 30, top 31, bottom 32, and rear 33 integrally-formed with the case 
12. The elongated slot 35 formed within the base accepts the deflector 36 
shown in FIGS. 2-4 to block the apertures 34 formed in the front of the 
arc plate enclosure. 
An important function of the deflector 36 is to protect the interior of the 
circuit breaker 10 from environmental conditions during quiescent 
operating conditions as indicated in FIG. 2, along with shielding the line 
terminal screw 19 during intense overcurrent conditions when copious arc 
gases must be released from the arc chamber. The circuit breaker 10 is 
shown in FIG. 3 with the contacts 13, 14 closed and the contact arm 15 in 
its closed position. Upon severe overcurrent conditions, such as a short 
circuit fault occurrence within the protected electrical distribution 
system, the contact arm moves to the phantom position resulting in the 
formation of an intense arc between the separated contacts 13, 14. The 
associated arc gases generate sufficient gas pressure to blow the 
deflector 36 to the phantom position to release the arc gases to the 
outside atmosphere. It is noted that the line terminal screw 19 is 
protected from the gases and hence is not subjected to deformation and 
welding as otherwise could occur. The deflector 36 is fabricated from a 
fibrous material such as Delrin, which is a trademark of DuPont Co. for a 
high-temperature fiber, although other heat-resistant resilient materials 
may also be employed. The mechanical memory of the fiber material allows 
the deflector to immediately return to the home position indicated in 
solid lines after the arc is cooled and quenched within the arc chute. 
Earlier flexible deflectors have been limited to a slight displacement 
from the home position by the positional relationship between the height 
of the elongated slot 35 and the location of the deflector within the 
slot. 
As best seen in FIG. 4, the line end of the circuit breaker 10 is shown in 
relation to the line terminal screw 19 to depict the positional 
relationship between the deflector 36 and the top of the elongated slot 
35. The apertures 34 extend along the line end of the circuit breaker 
cover and case 11, 12 below the top of the elongated slot to provide the 
maximum egress to the arc generated gases, and the flex of the deflector 
is arranged to provide the corresponding maximum exposure to the apertures 
in the following manner. The length L of the deflector 36 is selected such 
that the top part I is greater than the height H of the elongated slot 35 
as measured from the bottom of the cover and case. The center of gravity 
for the deflector 36 as viewed in the upright position, as shown in FIG. 
4, is approximately located at half the width W/2 and half the height L/2 
of the deflector 36 Accordingly, the top of the elongated slot 35, at H, 
should be below the center of gravity of the deflector 36 for maximum 
deflection to occur. 
An inexpensive and simple arrangement has herein been described for 
allowing a circuit breaker to interrupt overcurrent conditions at 
increased ampere ratings without having to enlarge the circuit breaker arc 
chute and enclosure. The increase in the out-gassing facility at the 
higher interruption currents is provided by the corresponding increase in 
the rate of egress of the arc gases from the circuit breaker enclosure by 
means of a flexible deflector having mechanical memory and high 
temperature withstand properties.