Monday, November 12, 2012

Circuit Breakers

Last week I was asked to get a circuit breaker for a customer.  When I asked about the application, I was shortly chastised and then asked "does it matter"?  Well, of course it does or I wouldn't have asked.  So lets have a short discussion about breakers....

Just to put it simply, there are two basic types of circuit breakers and they are:

- Magnetic Trip Only

- Thermal Magnetic

Each have their own place and here is why:

Magnetic Trip Breakers are commonly referred to as "Instantaneous" breakers.  They provide short circuit protection but do not have the built-in characteristics to provide overload protection.  They typically have an adjustment feature for adjusting the instantaneous trip anywhere from four to eight times the rating of the breaker itself.  There is no standard or regulation for this setting and it is usually adjusted in the field to meet the specific application.
These types of circuit breakers are typically provided on motor starters where they work in conjunction with the overload relay to provide complete protection for the application.  These breaker are not intended to be used as a feeder as they provide no overload protection for the circuit.

Thermal Magnetic Breakers give you the short circuit protection of a magnetic only breaker but also have time delayed overload protection which can protect in the event of a thermal problem.  This is typically detected via a bimetallic strip that is made up of two dissimilar metals that are bonded together in the breaker.  When excess current flows through the device, it causes the metal strip to bend causing the breaker to open if the current is in excess of the rating of the device. 

Magnetic breakers can be reset after a trip where a thermal magnetic breaker will not immediately reset after a thermal or overload trip until it has had time to cool back down from the trip. 

This is a very basic explanation regarding these devices but maybe next time when I ask about the application, I won't get a ear-full about my asking as to the application.


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