The figure below shows a part of a low voltage current-limiting circuit breaker (CB) in
closed (left) and open (right) position. Conductor A represents the fixed contact member (does not move), while conductor B represents the movable contact member. The
length of the contact blades (L) are 5.0 cm. The distance between center of the parallel
conductors (d) is 1.0 cm. The weight of the movable contact arm is about 35 g.
During normal operation, the switch is in closed position and should be able to carry a load current of 800 A. In case of a short circuit fault, the current level might be
up to 30 kA. In such a case, the switch should open to clear the fault by interrupting
the current. In order to interrupt (and limit) the short-circuit current, the CB needs to
open quickly and create a 5.0 mm long gap within 3.5 ms. Assume a force of 350 N is
required to open the contacts fast enough.
(a) Consider the current flowing through conductors A and B. Explain whether the
electromagnetic force generated by this current will cause repulsive or attractive
forces between the two conductors.
(b) Estimate the the force between the conductors during normal load current AND
during a short circuit current. Explain what simplifications you make. Compare
your results with the weight of the movable arm and the force required to achieve
current interruption (and limitation) and comment.
Answers
Answer:
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the risk of fire. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.
Circuit breakers are made in varying sizes, from small devices that protect low-current circuits or individual household appliance, up to large switchgear designed to protect high voltage circuits feeding an entire city. The generic function of a circuit breaker, or fuse, as an automatic means of removing power from a faulty system is often abbreviated as OCPD (Over Current Protection Device).
Explanation:
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