What Is a Circuit Breaker?
Where a fuse is the simplest Over Current Protection Device (OCPD) in electrical circuits, a circuit breaker (CB) is a much more evolved kind of OCPD. The primary difference between a fuse and a circuit breaker is that the circuit breaker is resettable. Where the fuse sacrifices itself while protecting an asset, the circuit breaker has a long life extending over hundreds and thousands of operations. Because of this, the circuit breaker is obviously a more expensive class of OCPD. Another big difference between a fuse and a circuit breaker is that the latter is used for relatively higher currents.
How Is a Circuit Breaker Constructed?
A circuit breaker protection system can consist of up to three parts: 1) a sensing system that detects an overload situation, 2) a relay that conveys the information to the switching arrangement, and 3) the combination of contacts and the contact actuator. This arrangement is usually found in power transmission and distribution networks dealing with very high voltages, currents, and powers. Low power circuit breakers have the sensing and actuation mechanisms collocated or even built into a single unit.
Types of Sensing Systems
There are three types of sensing systems: namely, magnetic, thermal, or electronic. That said, a combination of any of the three types may also be used.
- Magnetic: the current to be sensed, or a known fraction of it, is passed through a coil that has a magnetic effect that can be used to actuate the contacts and open them.
- Thermal: the current passes through a heater that deforms a bimetallic strip and releases the spring-loaded actuating mechanism.
- Electronic: the current level is monitored through a calibrated resistance and operates an FET or another electronic switch.
Circuit Breakers in PV Systems
Recommendations for use of protection of devices including circuit breakers in PV modules against current conditions are given in IEC 60269-6 (volume 6 of IEC 60269-6) and also UL 248-19. For circuit breaks as well, IEC recommends a 20% and 25% margin in voltage and overload current, respectively. Again, UL recommendations are nearly the same. Ratings are specified by the manufacturers at given ambient conditions. Where temperatures exceed this value, derating must be applied according to the manufacturer’s specifications. As for fuses, calculations follow the same lines.
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