Recently one of our end-user customers began having problems with our automotive circuit breakers in one of their their applications. The exact nature of their problem was that the circuit breaker was opening at rated current (circuit breakers are intended to remained closed indefinitely at rated current) .
Needless to say, this concerned us greatly and the problem prompted us to do some additional internal testing of our circuit breakers as per the applicable SAE standard.
After some extensive testing in our lab, we confirmed that there were no problems with our circuit breakers that we could find. But this exercise led me to think about some of the possibilities of why the circuit breakers periodically failed in our customer’s installations.
This list is not exhaustive and could probably be added to by more knowledgeable people out there (please drop me a note if you can think of additional potential reasons that a circuit breaker might prematurely open in the field but not the lab at firstname.lastname@example.org).
All thermal circuit breakers work using a bi-metal strip technology. The bi-metal strip bends when heat is applied. This bending acts like a switch and effectively opens the circuit. The key to finding the problem lies in finding the additional heat source and eliminate the problem.
The SAE Standard (SAE J553) requires that all tests be performed in a 20º C (68º F) environment. All of the bi-metal strips used in the construction of thermal circuit breakers are calibrated at this temperature.
When the ambient temperature (also known as the outside air) in a field application rises above 20º C, then the circuit breaker will need less heat from the circuit to open and will consequently open with less applied current.
The higher ambient temperature can come from the circuit breaker location. For instance, the circuit breaker could be mounted under the hood of a vehicle where summer temperatures could easily exceed 60º C.
Smaller Wire Sizes
Trying to push through currents higher than what the wire is rated for, can cause significant heating problems. And remember, most often wire is made of copper, which is not only an excellent electrical conductor but also an excellent heat conductor as well.
As an example, a 50 amp circuit should use at least a #10 AWG wire size (or in many instances, a #8 AWG wire). If the wire being used is only a #12 AWG wire, then quite a bit of heat is being generated in the wire itself and most likely being transferred to the circuit breaker.
"Bundling" of wires can also create additional wire heat as the heat from near-by wires can add to the overall ambient temperature and also transferred to the circuit breaker.
Another cause of circuit heat is contact resistance. Contact resistance is created when any of the connectors are loose or improperly crimped. Current will continue flow in the wires and through the connectors, but additional heat is also created due to the higher resistance and many times transferred through the wiring to the circuit breaker.
Care must be taken to make sure that all of the connections within a circuit are properly fastened and tight.
Many times, excess vibration can cause connections to loosen which then can create a great amount of heat to be produced within the circuit.
Good Design Practices
An engineer must take into consideration all potential external heat sources before selecting the circuit components.
Wires and circuit breakers should be selected as to only carry 80% of their full-load capabilities.
All connectors should be selected with the potential of a high vibration environment in mind. All connectors should be properly crimped or tightened to manufacturers specifications. Anti-vibration hardware, such as washers and anti-loosening nuts should be employed where the potential for vibration exists.
I encourage others to share their own ideas regarding good design practices. OptiFuse will soon have a forum page in its website to allow collaboration and the sharing of good ideas among all of the great minds out there.
Also, a big "thank you" to those who take the time to write and comment on what they like and dislike about this blog and share ideas how we can continue to improve and provide valuable information.
Thanks to everyone for you continued support of OptiFuse as we continue to learn from each other.