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January 27, 2010
Hazards of Automatic Reset Circuit Breakers

Recently an OEM customer of OptiFuse had a major incidence while using an OptiFuse ACB2-PL circuit breaker in one of their buses.  Due to circumstances unknown, a short-circuit developed under the dashboard of a vehicle causing an overcurrent situation...

The circuit was protected by an automatic reset circuit breaker which opened quickly when the short-circuit occurred.  Unfortunately, an automatic reset circuit breaker is specifically designed to re-close the circuit once the circuit breaker has sufficiently cooled.

In this case, the circuit breaker opened and closed multiple times over the course or several minutes because the short circuit was still present and never cleared.  Finally, after several minutes of cycling on and off, the contacts of the circuit breaker finally welded shut due to the heat of the present short circuit.  With the circuit breaker now incapacitated, the wires underneath the dashboard eventually grew so hot that the insulation on the wire caught fire causing lots of smoke and a bit of panic from the bus riders.

This incident illustrates the need for proper component selection when designing any type of circuit protection scheme.  The use of automatic reset circuit breakers are convenient for an overloaded circuit but offer little long-term protection when a short-circuit condition is present. 

The problem in the above example could have been avoided if a manual reset or modified reset circuit breaker was used in place of the auto-reset breaker.  This would have prevented the auto-reset breaker from cycling on and off and eventually failing.

An additional solution might have been to use a higher amperage fuse along with the automatic reset circuit breaker.  A "selective coordination" analysis  would allow the circuit breaker to open first and if the problem continues, the fuse is there as a back-up to provide additional circuit protection.  This type of protection scheme is used in most electrical systems found in buildings and utility systems.

All circuit protective devices are simply an insurance policy against potential dangerous and/or expensive problems from occurring.  If misapplied, there is no real protection being offered by the protection components and your insurance policy becomes null and void. 

It is fortunate that no one was hurt in the above example.  We can use the expirence to learn in order to prevent an event like this from happening again in the future.

Thanks to everyone for you continued support of OptiFuse where we continue our mission of educating and informing for the good our community.

Jim Kalb

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