Short in pullout box

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Old 08-25-08, 05:04 AM
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Short in pullout box

A few months ago I detected the distinct smell of burning electrical insulation coming from my basement. Several hours of searching failed to find the source as the smell was only intermittent. A few days ago I noticed that the l.e.d. on my water heater control box was off. In checking I found that one of the contacts in the pull out box had overheated melting the insulation off the wires and literally fried the pull out on that contact. The source of the previous odor had been found. My question is: Why didn't the breaker on that circuit trip? It seems that the heat generated could have easily started a fire under the right conditions. I intend to replace the pull out box but what other precautions should I take to ensure this won't happen again?
 
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Old 08-25-08, 05:15 AM
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Most likely this was caused by a loose connection. A too tight connection can also cause this.

The breaker did not trip because the current flow was not high enough. This could have occurred due to the thermostat cycling on and off but the heat was still generated.
 
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Old 08-25-08, 07:08 AM
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pcboss, Thanks for the quick input. I kind of suspected the loose connection but the breaker not tripping had me stumped. Your explanation seems right on.
 
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Old 08-25-08, 10:04 AM
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Question

Wonder if a AFCI might be usefull? Do they perhaps extend themselves to the arcing that may occur in the appliance itself. Hmm???
 
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Old 08-26-08, 03:27 AM
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The offered suggestions and a bit of analytical thinking brought me to a conclusion regarding the short itself. I failed to mention that this pull out had been in service for more than 3 years prior to the problem. It was intermittent - with just two of us in the house we had never noticed it but... during the period it appeared a sister in law was staying with us. She was a bather, not a shower taker. That meant a full tub of water for her alone. Add to that showers for my wife and I and the problem started. My theory is that because of the increased demand for hot water the water heater control until called for the second unit to kick in for quick recovery after those baths. The short was apparent then (smell) because of the higher power flow through the pull out. Of course, as pcboss said, when the power flow decreased because the second heating unit cycled off the smell was gone. That could explain why the breaker didn't trip - because the second heating unit didn't stay on long enough to trigger it. Any thoughts on this?
Sidecutter - the WH has an Energy Star control so a problem within the appliance itself should be detected by the control shouldn't it? There was no evidence of any arcing anywhere but in the pull out.
 
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Old 08-26-08, 06:05 AM
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The device that Sidecutter was talking about is an arc fault circuit interrupter. They are designed to detect arcs in the circuit wiring. This is currently provided by breakers, not appliances.

From the CPSC
http://www.cpsc.gov/CPSCPUB/PUBS/afci.html
 
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Old 08-26-08, 08:48 AM
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Common circuit protection (not arc fault) has no knowledge of what is drawing current only that if the current it is designed to protect is exceeded.

In your case the arcing would never have caused a circuit breaker to trip because the load that it was intermittently turning on and off (the heater coil) was a proper load for the breaker.

Think of it as you turning a light switch on and off at a high rate. It would never trip the breaker but each time you did it a small spark is created within the switch. Each time this happens heat is created. Do this enough times and you have enough heat to cause damage. This essentially creates a small arc welder but at no time can the total current exceed that which you are switching, the light.

People have the misconception that simple over current circuit protection will work in this situation but it will not. This is why arc fault current interrupters (AFCI) were created. Unfortunately they are expensive and will not work on all circuits such as those where there is naturally sparking, like a motor circuit. They are also primarily intended to protect against faults outside of the outlets, like an extension cord that is arcing but have the added benefit of protecting the entire circuit.

Properly installed I would not worry about your switch causing a fire. Installed in an approved box any problem would be contained. These things happen. It could have been a loose screw connection or the switch itself making poor contact. I would not consider installing an AFCI on a water heater circuit. It is just not necessary and would be prohibitively expensive.
 
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Old 08-27-08, 03:13 AM
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Thanks all. The replies were very informative and have eased my mind regarding a repeat occurence or a problem with the breaker. I purchased a new box to install and will be absolutely certain the connections are secure before I finish.
This web site is a true winner.
 
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