OHST Practice Exam

Which scenario is most likely to cause immediate activation of a magnetic circuit breaker?

1. Undersize wiring

2. A line to line short

3. A 20% overload for any period of time

4. Ground fault

The correct answer is: A line to line short

The scenario that is most likely to cause immediate activation of a magnetic circuit breaker is a line-to-line short. In this situation, the electrical current bypasses the intended load and flows directly from one live conductor to another, creating an excessive current flow. Magnetic circuit breakers are designed to respond quickly to short circuit conditions because these scenarios can lead to damage, fires, or other hazardous situations. As soon as the current exceeds the breaker's rated capacity, the magnetic coil inside the breaker generates a force that trips the mechanism, interrupting the circuit almost instantaneously. While undersized wiring can lead to overheating and eventually trip the breaker, this process typically takes longer and is influenced by thermal conditions. A 20% overload, while potentially significant, may not immediately exceed the breaker's tripping threshold, allowing for some tolerance before activation. Ground faults can cause tripping, but they may not trigger the breaker as immediately as a direct short circuit. Therefore, in terms of immediate response, a line-to-line short is the definitive scenario for activating a magnetic circuit breaker.