A breaker’s primary function and electromagnetic forces

There’s a crucial link between circuit breakers and electromagnetics. There is good and bad . . . .

The circuit breaker’s primary function is to protect cables from damage during a short circuit, an overload, or a ground fault. Thermal damage has already been discussed in previous articles on voltage drop and ampacity. Under short-circuit conditions, this process is accelerated and can happen in a matter of seconds — or less.

The current magnitudes that are generated during a fault in just fractions of a second are enormous. Their most destructive effects are produced by the excessive heat and electromagnetic force they generate.

To illustrate with a basic example the large currents that are generated in such a short time, consider a typical 120-volt circuit that powers a 60-watt light bulb. The circuit resistance is approximately 120 volts x 120 volts ÷ 60 watts = 240 ohms. A well-bonded ground connection should have a resistance of less than 0.25 ohm. If you imagine a scenario in which the conductors in our hypothetical circuit short to ground (perhaps the vibration from a nearby fan has rubbed the conductor’s insulation raw), the resistance will drop instantaneously from 240 ohms to 0.25 ohm. This results in a current of 120 ÷ 0.25 = 480 amps. It won’t take long at that intensity for a #12 AWG cable, whose standard ampacity is between 25 and 30 amps, to burn through its insulation.

If the circuit isn’t opened so that the fault is cut short, that hot cable can cause fires. This situation is much more severe for the higher-voltage installations that electrical engineers deal with in nuclear power plants. They don’t want their cables popping like a fuse or causing fires that can have disastrous effects.


Not only that, but instantaneous faults produce something akin to a current step-function in the time-domain. Time-varying currents generate electromagnetic fields. The electromagnetic fields produce enormous forces within the conductors carrying the fault current. Rail guns intentionally harness this force to accelerate projectiles to 5,600 mph almost instantly (see “Electromagnetic Railgun,” at the Office of Naval Research).

Those same instantaneous forces can blow bus bars off of their supports and damage transformer windings. Arc flash is also a dangerous consequence of high-current faults combined with electromagnetic forces. If a circuit is faulted, and someone tries to close the tripped circuit breaker while the fault is still present, an arc flash can develop in the switching contacts in that panel. If enough energy is available, an arc flash can develop and vaporize the metal and pose grave danger to anyone who happens to be present at the fault location. These are the kinds of problems that our designs intend to avoid, and they are the kind of disasters poor designs can create. This is why the design of circuits and their protective devices must be approached with solemnity.