AFCI Aluminum Protection

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Old 01-14-06, 05:22 PM
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AFCI Aluminum Protection

Will AFCI breakers protect a home from the potential hazards of aluminum branch circuit wiring?
 
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Old 01-14-06, 07:47 PM
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Thats an excellent question .With the exception of main circuit breakers all of the breakers I deal with are for use with copper conductors only. I assume the decision to make the AFCI breakers dual {Cu/Al}rated is up to the manufacturer.I am very interested to see what others have to say about the subject on this forum.
 
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Old 01-15-06, 04:45 AM
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In _theory_, AFCI breakers should help reduce the danger posed by aluminium wiring, but won't help prevent damage to the wiring itself by much.

The failure mode of aluminium wiring is at the splices and terminations, where as the conductors heat and cool during use, small (temporary) gaps form, oxygen gets into the gaps and forms insulating aluminium oxide, which increases the resistance of the connection causing more heating....eventually you have a very bad connection that gets very hot during use. The connection either breaks free, or the insulation gets very hot and melts, or the heating causes some other failure.

The AFCI will not help any of the issues that cause this problem. All of the heating and connection degradation issues leading up to the problem simply look like normal electrical loads.

Once the connection fails, if there is load on the circuit, you can get what is known as a 'series arc'. This is a spark of electricity jumping across the broken connection, and it can generate lots of heat.

It is called a series arc because it is electrically in series with the load. The current in this arc is limited by the load resistance, and is thus lower than the normal trip current of the breaker. When AFCIs were first proposed, they were supposedly going to detect series arcs. However an AFCI that detects series arcs is also prone to detecting normal arcs caused by normal operation of the electrical system, eg. switches opening or light bulbs burning out. So AFCIs as sold do not detect series arcs.

Eventually, because of this heating, you will get an insulation failure between the hot and the neutral wire, or between hot and ground. At this point you can either get a solid short circuit, or a hot to ground arc. If you have a solid short circuit, than the circuit breaker will trip in the normal fashion. But if you get a hot to ground (or neutral) arc, then you can have a very ugly situation. This is a 'parallel arc', where the load does not limit the current. But the arc itself limits the current, and you can have a situation where you see lots of heating but not enough current to quickly trip the circuit breaker.

Once you get to the parallel arc situation the AFCIs will make a difference in safety. The AFCI will recognize the arc, and because the current is higher than normal operating current, the AFCI will trip very quickly, probably much more rapidly than a normal breaker in this situation. This will help prevent the arc from causing a fire outside of the electrical system. But once you've reached the point of a parallel arc, there will already be pretty severe damage to the circuit itself.

ampz brings up another important point. If you choose to use an AFCI, make sure that the breaker terminal itself is actually rated for aluminium wire!

-Jon
 
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