120VAC GFCI Breaker - Thermal Magnetic and Electronics

[Howard McCraney]

Does a 120VAC GFCI circuit breaker have a thermal magnetic trip unit that will trip on a fault, even if the GFCI electronic circuitry is faulty? Or, is the breaker totally dependent on the electronic circuitry.

 

[peter d]

Yes, the breaker does contain a TM mechanism.

 

[hurk27]

Does a 120VAC GFCI circuit breaker have a thermal magnetic trip unit that will trip on a fault, even if the GFCI electronic circuitry is faulty? Or, is the breaker totally dependent on the electronic circuitry.

The GFCI part of the breaker provides no over current protection just like in a receptacle, I believe they do use the same coil to trip the breaker as the magnetic trip unit does saving money so only one trip coil has to be used, but the over current (thermal) and the magnetic trip (short circuit protection) is independent of the GFCI part of the breaker, many times the GFCI electronics will fail but the breaker will still trip on over current or short circuit.

 

[jim dungar]

The GFCI part of the breaker provides no over current protection just like in a receptacle, I believe they do use the same coil to trip the breaker as the magnetic trip unit does saving money so only one trip coil has to be used, but the over current (thermal) and the magnetic trip (short circuit protection) is independent of the GFCI part of the breaker, many times the GFCI electronics will fail but the breaker will still trip on over current or short circuit.

A GFCI has a shunt trip coil (a coil that moves the breaker to the off position, tripping is not a requirement) and a thermal-magnetic mechanism. The thermal and magnetic portions usually use the same spring charged tripping mechanism (they have different actuating methods).

 

[hurk27]

A GFCI has a shunt trip coil (a coil that moves the breaker to the off position, tripping is not a requirement) and a thermal-magnetic mechanism. The thermal and magnetic portions usually use the same spring charged tripping mechanism (they have different actuating methods).

H'mm every GFCI breaker I have seen trip on a GF trips to the middle trip position not all the way to the off position, I saw a cut open GFCI breaker on display at a supply house and while I didn't really look that close so maybe I missed the second coil, if I remember right it was an ITE single pole, but your probably right that it doesn't use the same coil, I'm sure that none of the manufactures will have the designs of their breakers published online because of company secrets.

 

[Howard McCraney]

The reason I asked the question is this: Assume that you use a GFCI to protect a lake dock. At some point, if the GFCI electronic circuitry fails, it certainly would be a concern if the breaker would not trip at all on a fault. Therefore, its seems reasonable that the GFCI electronic trip circuitry and the fault trip (EM) would be independent. But, I can find no verification that this is the fact.

 

[gar]

130706-0839 EDT

Buy one, open it up, see how it is built.

In some sense the Sq-D standard QO breaker is thermal magnetic. But the high current magnetic part of the function is really the shape of the thermal element. Based on opening one a very long time ago. The QO thermal element is U shaped so high current will tend to magnetically push the legs apart.

Make a hairpin loop of wire. Pass a high current thru the loop and the wires push apart.
This is because there are like fields opposing each other. If the two end points are constrained in position, then the loop moves toward the shape of a circle.

.

 

[jim dungar]

H'mm every GFCI breaker I have seen trip on a GF trips to the middle trip position not all the way to the off position...

I agree that is what most breakers actually do, but it is not a requirement.