Basic Electricity

Overcurrent Protection

13 questions By Tony R. Kuphaldt

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Question 10 of 13

Magnetic circuit breakers trip by the action of an electromagnet coil, through which all the load current passes. When the attractive force of the magnetic field is strong enough, a mechanism triggers to snap the breaker contacts open, thus interrupting the circuit current.

What would the time-current curve for such a circuit breaker look like?

Reveal answer

Notes:
Magnetic-trip circuit breakers are very common in industry, but they are not appropriate for all applications. Discuss this with your students, and try to think of applications where such an overcurrent protection device would not be appropriate.
Question 11 of 13

Shown here is the time-current curve for a dual-element fuse. Thermal-magnetic circuit breakers exhibit similar time-current curves:

Based on this curve, what do you think the purpose of a “dual-element” fuse or “thermal-magnetic” circuit breaker is? Why would this style of overcurrent protection device be chosen over a “normal” fuse or circuit breaker?

Reveal answer

Such overcurrent protection devices combine the properties of “time-overcurrent” and “instantaneous overcurrent” protection in a single device, for protection against different types of damage resulting from different types of overcurrent conditions.

Notes:
Obviously, not all overcurrent conditions are the same. Ask your students to describe circuit faults that would cause slight overcurrent, versus faults that would cause extreme overcurrent in a circuit, and discuss the destructive consequences of each type of condition.
Question 12 of 13

In addition to possessing a primary current rating, fuses and circuit breakers also possess an interruption current rating, usually far in excess of their primary ratings. For example, a typical 15 amp circuit breaker for 120 volt residential use may have an interruption rating of 10,000 amps (10 kA)! Under what conditions could such a circuit ever bear so much current, and why is this rating different than the breaker’s primary current rating of 15 amps?

Reveal answer

Short-circuit conditions in electrical power systems may create “transient” currents of extraordinary magnitude. Overcurrent protection devices will, of course, either “blow” or “trip” under such conditions, but their interruption ratings have nothing to do with the current at which they open. Rather, a device with an interruption rating of 10,000 amps is certified to be able to stop a fault current of that magnitude once having opened.

Notes:
Students will naturally wonder how a blown fuse or a tripped circuit breaker could not stop a fault current, since there is no longer a condition of electrical continuity between its terminals once it has opened. Or is there? Discuss with your students how an electric current of enormous magnitude could possibly continue to conduct through a blown fuse or tripped circuit breaker.