Circuit breaker and fuse sizing rules and overcurrent protection requirements tested on electrician licensing exams.
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- What is the primary purpose of overcurrent protection?
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To protect conductors and equipment from damage due to excessive current flow.- Where must overcurrent protection devices be located?
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At the point where the conductor to be protected receives its supply.- Name three types of overcurrent protection devices.
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Circuit breakers, fuses, and thermal-magnetic devices.- What is the maximum overcurrent protection allowed for copper conductors?
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The amperage rating must not exceed the conductor's ampacity.- What is the maximum overcurrent protection allowed for aluminum conductors?
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The amperage rating must not exceed the conductor's ampacity, which is lower than copper of the same size.- List five standard overcurrent protection device ratings.
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15A, 20A, 30A, 40A, and 50A (among others like 60A, 70A, 90A, 100A, etc.).- What does the 125% rule apply to?
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The sizing of branch circuit overload protection for motors and other continuous loads.- Define continuous load.
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A load where the maximum current is expected to continue for three hours or more.- What is a thermal-magnetic circuit breaker?
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A circuit breaker that uses both a bimetallic strip (thermal element) for overload protection and an electromagnet for short-circuit protection.- What is an inverse time circuit breaker?
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A circuit breaker where the trip time decreases as the overcurrent magnitude increases.- What is selective coordination?
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The condition where an overcurrent protective device operates to clear a fault without causing unnecessary disconnection of other circuits.- When must a time-delay fuse be used?
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When temporary overloads occur, such as during motor starting, to allow the circuit to function without nuisance opens.- What is Ground Fault Circuit Interrupter (GFCI) protection?
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A device that detects an imbalance between current flowing in the ungrounded and grounded conductors and de-energizes the circuit to prevent shock hazards.- What is Arc Fault Circuit Interrupter (AFCI) protection?
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A device that detects dangerous arcing conditions and de-energizes the circuit to prevent fire hazards.- What is the purpose of motor overload protection?
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To protect the motor and its branch circuit from sustained overcurrents due to motor overloading.
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