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A puzzle


Jim Katen
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Here's a puzzle. This happened to me this weekend. If you instantly know the answer, please hold back and let some of the folks who are less secure with electricity take a stab at it.

My shop is about 100' away from my house. Power to it runs underground through three wires: two hots & a neutral. There are no metallic pathways between the house and the garage. At the shop's panel, the feeder neutral connects to a neutral terminal bar that holds all of the branch circuit neutrals and equipment grounding wires as well as a grounding electrode conductor.

On Saturday afternoon, I walked up there intending to rip a board on my tablesaw. As I entered the building, I switched on the lights. Nothing happened. "Oh heck," I said, "I must have left the lights on and they've all burned out." Windows still let in plenty of afternoon light, so I figured I'd just rip the board for now and replace the lights later.

When I switched on the tablesaw, I heard a low hum and the shop lights turned on. When I switched the saw off, the hum stopped and the lights went back off again.

Why?

- Jim Katen, Oregon

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Originally posted by AHI

Open neutral on the lighting circuit? Rodent chewed wire?

If there were an open neutral, that would explain why the lights didn't work when I tried to switch them on and why they came on when I turned on the tablesaw. But it wouldn't explain why the saw hummed.

Another guess?

- Jim Katen, Oregon

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You're all basically correct. Steven & Brandon also e-mailed me correct answers early on.

One of the hot legs opened and that stopped the lights from working. When I turned on the saw, it only got 120v instead of 240, so it hummed. Also, when it was in the "on" mode, the saw motor energized the dead bus in the panel and allowed the lights to come on.

Sadly no breakers have tripped. (No fuses. Sorry Bob.) This means that one of the three underground wires has failed. A gopher might have chewed it, but it's more likely that a plumbing crew nicked it last year and didn't tell me (the grass-combing buggers). When you nick the insulation on an underground aluminum wire, the aluminum corrodes away into nothing pretty quickly.

- Jim Katen, Oregon

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I'm not sure if a GFCI would really help in this situation....

My understanding of this "Strange Situation" is this( and I'm sorry I lack proper terminology):

When the saw was turned on, it back fed 110V through the dormant wire back to the panel in the shop, which energized everything on the other side of the panel. Can't really see a ground fault there.

Now, since the hot lead was open between the house (And the main panel) this voltage just remained in the panel, turning it into a straight up single phase circuit, (Again sorry about the lack of proper terminology) protected from one side of the double main disconnect in the main panel in the house.

When the saw was turned off, it just turned everything back an open hot situation. 1/2 of a panel.

I like the idea of gophers, and I think you should go Bill Murray Caddy shack Crazy...

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