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Confusion: continuity across an open light switch. Doesn't seem right.

Confusion: continuity across an open light switch. Doesn't seem right.


Old 05-31-09, 09:38 PM
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Confusion: continuity across an open light switch. Doesn't seem right.

Roommate was using electric lawnmower plugged into same circuit as a window air-conditioning unit and the TV/VCR/computer complex, which unsurprisingly threw the circuit breaker. Okay, no big deal; we reset the circuit breaker and everything's back to normal, right?

Well, except that the air conditioner and entertainment center outlets still aren't powered. So only half the circuit is reset? That's not a good sign! Having only the vaguest of notions of how the place was wired (hint: cheaply, and probably more than a little wrong), I start poking around at the various outlets, slap the ceiling fan dimmer switch, and Viola, the other outlets suddenly spring to life. I'm an electrical savant, obviously!

But it's not so reassuring, is it? Since I don't like the thought of a connection so loose that slapping the wall can turn it off and on so close to where I sleep, I decide to pull off the plates and look at the wall switches apparently connected to this handy wall-slapping ability. Nothing's obviously loose, but what do I know - I'm no electrician. But I do have a couple cheap multimeters that I use to play around with some DC circuits for fun, so I start poking around doing some rough-and-ready continuity testing. Breaker's off, and everything that's operated by switches in the box is out like a light (well, they're mostly lights, so that makes sense), but I'm still checking for voltage first because some kinds of dumb make way less sense to me than others, and I prefer to waste actuarial years off my life doing stuff that's more fun.Beer 4U2

Anyway, out of curiosity I measure the resistance across one of the light switches; in the On position it reads 0 ohms, or near enough as makes no difference. But in the Off position it's reading something like 200 ohms, a damn sight less than the roughly infinity ohms I would expect for an open switch, if the little bit of theory I absorbed in college corresponds at all to reality. (And when I make the same measurement across the neighboring switch, I get the results I would have expected: 0 and sideways-eight-infinity ohms for the On and Off positions respectively.)

As I say, me not being an electrician, my first thought was, must be a bad switch. Switches are what, two bucks? So I replaced it. But while replacing it I notice that there's nothing wrong with the switch; out of the circuit it measures exactly like the other, zero and infinite resistance. Turns out it's the wires it's switching that are electrically connected somewhere behind the plaster, to the tune of 200 or so ohms. This switch controls a pair of outdoor porch lights, which (I'm told) have been experiencing unexpectedly short bulb lives for CFLs, but otherwise have been performing unexceptionally.

Nothing seems to be catching fire, or malfunctioning. The light fixtures switch on and off as they should. The people who installed the switches seem to have a very peculiar notion of quality craftsmanship (there are a couple of clusters of six wires each all jammed into a wire nut that shouldn't be able to hold half that many, and there's enough plaster caked up inside the box to do some minor statuary), but it's not acting up like I would expect, if what I seem to be observing is correct.

Anyone who understands AC care to enlighten me here? I'm all ears...

Thanks in advance.
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Old 05-31-09, 10:03 PM
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or am I OK with current wire and just change breakers (10 year old house in Arkansas)... thanks
The surge in the circuit probably fried the dimmer electronics. Standard CFLs are not compatible with most dimmers. Switches can only be checked disconnected from the circuit. Any load such as a light will provide a false reading because its resistance is additive to the circuit. AC should be on a dedicated circuit. Ideally the lawn mower should be on a separate circuit also.
Old 06-01-09, 09:24 AM
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I'll presume you performed the resistance test with the CB in the panel in the "Off" position.

The Branch-Circuit that extends from the panel can be perceived as a circuit that supplies power to loads on the circuit that are essentaily connected "in parallel" to the circuit. One side of a load connects to the Black wire, and the other side to the White wire. With say six loads so connected , there are six paths between the Black / White wires.

When you appilied the meter , one test-lead is one the Black wire , and the other test-lead is on the White wire thru the resistance of the load controlled by the switch.

Question; if you were testing across the Black & White wires, and there were other loads across the same wires, how would that effect the resistance reading ?
Old 06-02-09, 08:34 PM
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Point taken. Still not clear why the switch right next door in the same box switches between 0 and infinite, though.

Anyway, thanks for your time. (And yes, I know better than to try to measure resistance in an energized component, or to play around with live circuits generally. Although I understand why you can't take that for granted.)

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