Fuse Box Stumper

jeff43222 · Jul 21, 2005

Re: Fuse Box Stumper

Originally posted by charlie b:

As for the "What happened?" issue, the homeowner told me the problem began when someone plugged in an air conditioner in a bedroom.

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That's a good start, but it begs a number of additional questions. Was this a brand new air conditioner being plugged in for the first time? If not, did it ever work before (i.e., in this same location and plugged into this same outlet)? Was anything else ever plugged into this same outlet, and did it work properly?

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The A/C didn't look all that new. Not sure if it ever worked in that same outlet previously. Also not sure if the outlet itself has worked OK previously. Unfortunately, the HO left me alone in the house with people who don't speak much English (HO's English wasn't all that great, either), so there were communication issues.

Does that outlet work now (i.e., plug in a lamp, and see if the lamp works)? I am guessing not.

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You are correct. Nothing works on brown even if the fuse is put back in.

Your first post said that if you pull the black fuse and put in the brown fuse, you get no voltage on brown. But that brings me back to my question on your voltage detector. When you did this test (i.e., pull the black fuse and put in the brown fuse), did you use that same voltage detector to conclude that you had no voltage on the brown hot? This may be a big, big clue!

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I also checked the receptacles when I put the brown fuse back in. Everything was dead.

Here is what I think:


Your "voltage detector" cannot detect the presence of voltage unless that voltage is causing current to flow. It is giving you a misleading indication that there is no voltage on brown or on red, in the two situations you described.

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The detectors I use definitely do not depend on current flow to detect voltage. I use these nifty devices all the time in the field; life would be much harder without them. I don't know the physics behind how they work (perhaps the sinusoidal nature of the voltage causes some kind of induction?), but they do indeed detect voltage without having to have any physical contact with the wires. I used one just yesterday to determine which K&T wire (on an open circuit) was hot and which was the neutral. My favorite detectors (and meters) are made by Fluke. There's a spirited discussion going on in another thread in this topic area where lots of us have weighed in on our favorite voltage detectors and their pros and cons. They are generally the size and shape of a thick magic marker and cost between $10-$20. Flukes cost a little more than other brands, and I've only seen them sold at the supply house, as they tend to be a brand that is marketed only to pros.





The neutral is open on the red/black circuit. See my earlier discussion.



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I also think this is worth investigating.



The a/c unit overloaded the #14 wire. It not only blew the fuse, it also "fused" the brown wire (i.e., melted some of the insulation and some of the copper, so brown is now an open circuit).



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I also think this is a strong possibility.



The fault on the brown wire caused it to also damage insulation on black, resulting in a black-to-brown short circuit. I think you are right in suggesting that this happened within the panel, or perhaps it happened at the point the wires leave the panel.



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The black and brown don't touch each other anywhere inside the panel, and they leave the panel through separate runs of Greenfield. Given the strange design of the panel, I could very well believe the short is inside the panel under the fuse cartridges.



Black and brown are on opposite poles. That is why if you put in both black and brown fuses, both fuses will blow.

To test this theory, you need to use a voltmeter, and you need to have access to wiring downstream of the fuses. Try this: Put in red and black fuses. Go to the split outlet in the kitchen. Using a voltmeter, check for voltage from hot to neutral on both sides of the duplex. Now remove the black fuse. See if you have lost voltage on one half of the duplex, but still have voltage on the other.

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I think this is also a good idea. I ran out of time the other day, so I wasn't able to test everything I wanted to. I told the HO that the problem was likely in the panel itself, and it should be upgraded anyway. If he calls me back for that job and the panel upgrade doesn't solve the problem, I'll troubleshoot the circuits with my VOM and get more details.

Thanks for all the useful suggestions!

Fuse Box Stumper

jeff43222

Senior Member