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November 11, 2009
Using Circuit Protectors
 
 


I was having a discussion with an engineer yesterday when the subject arose regarding thermal circuit protectors.
A thermal circuit protectors is sometimes called a thermal only circuit breaker (as shown below). 

CBW57  
There are a few misconceptions regarding these breakers that I would like to clear up.

First, these circuit breakers are for supplementary circuit protection only (UL standard 1077).  That means that there needs to be a fuse or branch rated circuit breaker upstream from this type of circuit protector.  Otherwise, you can damage the circuit protector or even worse, cause the part to rupture or even explode.
CBW28

Typically these type of circuit protectors are used in appliances or industrial equipment which are plugged into a wall.  The wall socket is always protected with a branch-rated circuit breaker at the panel so the usage meets the requirements for all plug-in devices.

Secondly, these type of circuit breakers offer only thermal overload protection and do very little to protect against dangerous short-circuits within the piece of equipment.  A fuse, to protect against high short-circuit currents, is highly recommended in almost all designs. 

So you might ask, if I need to use a fuse anyway, why am I using a circuit breakers as well...?

The answer is simple, the fuse should be properly sized to provide short circuit protection only.  If there is an overload situation (for example, something causes the fan in the circuit to stop spinning), then you want the thermal circuit protector to open up instead of the fuse.  This way the circuit can easily be reset once the overload is cleared.

I suppose OptiFuse has one less opportunity to sell a replacement fuse...but using a thermal circuit breaker makes for a convenient and practical design.

Thank you very much for your continued support of OptiFuse, where we try to help you design better products.


Jim Kalb
President
OptiFuse

 

 
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