Monday, April 01, 2013

Circuit Testers Have a Blind Spot!

If you're like me, you keep a set of tools and can perform many home repairs on your own. And one of those tools is probably a 110 volt circuit tester, for use when repairing electrical outlets.

However, I just noticed there's a serious, even possibly fatal wiring misconfiguration that circuit checkers won't detect! So listen up and be on the lookout for this one.

Some time back I noticed that the metal frame of my refrigerator was "hot" and you would get zapped by touching both the exposed metal of the fridge back and the nearby breaker box. Clearly not good.

Upon further investigation I found that the ground pin of the outlet was also hot, which is not what you'd expect. Yet when I inserted a circuit tester in the outlet, it read A-Okay! Something was rotten in Denmark!

How could the circuit tester read OK when the ground was hot?? Very easily, it turns out.

If you're in an old house, maybe the outlets don't have grounds, and the wires are so old and blackened you can't tell which one is white or black. Nor can you easily check. So you mistakenly reverse the hot and neutral when rewiring the  outlet, and to boot you tie the "neutral" to ground, which normally could be an okay idea, since neutral will be tied to ground back at the breaker box.

Now you've got a hot ground. However, due to AC current, in the absence of a true ground, the circuit checker can't tell white from black, the two sides are completely equivalent, and thus it can't tell that the ground is tied to black, creating a dangerous situation!

As long as you've got a true ground, you're okay. The circuit checker can tell white from black, plus tying black to a true ground would immediately short out. But if you have no true ground, and the wires are grubby, don't guess. Do some serious external checking to figure out which one is hot, because the circuit tester won't flag it for you.


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