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Circuit Breakers - How do they work?


One of the more frequent questions ACB-N I am constantly being asked is about the different types of automotive circuit breakers.

Now when I say "type" I’m not referring to the size of the mounting.

SAE (The Society of Automotive Engineers) has determined that there are three basic types of low voltage circuit breakers...aptly named: Type I, Type II, and Type III.

It is important to understand how automotive circuit breakers work.

Inside of each automotive circuit breaker is a bi-metal strip attached to a stud or terminal. The other side of the bi-metal strip rests on second stud completing the circuit.

When there is too much current circuit breakerflowing in the circuit, due to perhaps an overload or a motor that is unexpectedly blocked from rotating, heat is generated in the wiring.

The resulting heat will cause the bi-metal strip to bend, disconnecting the end that just rests upon the stud effectively opening the circuit from current flow.

All automotive circuit breakers open in the same fashion...it’s the resetting that is the differentiator.

In a Type I circuit breaker, once the bi-metal strip cools down, the bi-metal strips snaps back to once again rest upon the stud (reconnecting the circuit).

In a Type II circuit breaker, a heating coil is wrapped around the bi-metal strip and connected to each stud. When the circuit breaker is closed, all of the current goes through the bi-metal strip. When the circuit breaker opens, current is then forced to go through the heating coil wire now creating additional heat...and not allowing the bi-metal strip to cool and reset. To reset a Type II circuit breaker, the user must turn off all power to the system (usually by turning the power off at the ignition). This way the current is turned off to the heat coil so now the bi-metal strip will cool and reset.

In a Type III circuit breaker, a mechanical latch is activated when the bi-metal strip is flexed. In order to reset the system, one must physically push a button or flip a switch to release the latch and re-engage the circuit breaker (once the bi-metal strip has sufficiently cooled).

OptiFuse manufactures a wide variety of Type I, II, & III automotive circuit breakers.


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