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December 2, 2009
Bi-Metal Strips

 
 


One of the primary reasons I became an engineer was due to my innate curiosity of learning how things are made and more importantly why things work like they do.

One of the more interesting products that OptiFuse sells is our thermal circuit breakers.  These devices are incredibly simply devices to operate but are very fairly complex in design and manufacturing. 

All thermal circuit breakers, be it low voltage automotive breakers or higher voltage line breakers, use a bi-metal strip that creates mechanical displacement (bending) of the strip.  The bi-metal strip is an "electrical bridge" between two contacts posts.  The bi-metal strip is solidly connected to one of these contact posts either by riveting or welding.  The other end of the bi-metal strip rests firmly against the other contact post but is not physically connected.

Bi-Metal Strip

The bi-metal strip is created by sandwiching two different types of metal (usually steel and brass) on top of one another.  The different metals expand at a different rate when heat is applied (one of the properties of metal is that it expands when it gets hot).  By placing two dis-similar metals on top of one another, the top metal of the bi-metal strip will expand faster than the bottom metal of the bi-metal strip.  This phenomenon causes the bi-metal strip to bend or flex when it gets hot.

Bi-Metal Bending

When an overload occurs in a circuit, the bi-metal strip gets hot.  The result is that the bi-metal strips will bend and lift the non-fastened end off the second contact post creating a gap between the bi-metal strip and the contact post, effectively opening the circuit.

After a short amount of time, the bi-metal strip cools (now that the circuit has been opened).  Once the bi-metal strip cools, it will unbend and resituate itself back on the contact post thus re-connecting the circuit.  If the overload condition is still present, the bi-metal strip will once again heat up and re-open the circuit.

Eventually, the constant opening and closing of the circuit will cause the contact point to wear out due to the sparking that is created each time the circuit breaker operates.

Now you know the physics behind the part...

Thank you very much for your support of OptiFuse as we do our part to make the world a bit smarter.

Jim Kalb
President
OptiFuse
jimk@optifuse.com

 
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