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When I run the microwave oven in my rented apartment, the incandescent lights nearby glow much brighter.

What sort of mis-wiring could cause this?

EDIT: Suspecting a bad neutral connection, I measured the voltage between hot and neutral at a nearby electrical outlet. The voltage was 113 VAC. When running the microwave, that same voltage goes up to 129 VAC.

The voltage at that outlet measured between hot and ground is the same as between hot and neutral (i.e. it also increases when the microwave is running).

On the same circuit as the microwave, the voltage is 126 VAC without the microwave and 108 with it running.

I assume this means that the neutral and/or ground connections to the house may be faulty?

Aarthi
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nibot
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4 Answers4

11

The problem was a bad neutral connection from the transformer on a pole.

I called the power company--they had trucks out to replace the power line within 30 minutes.

Now the power variation is very small. Without the microwave running, I measure 117.5 volts on the lamp circuit and 118.2 volts on the microwave circuit. With the microwave running, the voltages go to 118.2 and 114.0 VAC.

nibot
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4

Have you confirmed that the lights and microwave are wired correctly? live-live, neutral-neutral, ground-ground?

You could test the receptacle with one of these:

Recep tester

http://www.amazon.com/50957-Tester-Installation-Operation-110-125V/dp/B002LZTKIU/

Jay Bazuzi
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First, some background: Inside of a breaker panel for your home are three connections, two for each phase of the power coming in, and one for a neutral/ground. Each phase is ~120v, which when added together (they are opposite phases) add together to ~240v. For the ~120v breakers, you connect one phase and the neutral which shouldn't have much current. Note, this is all approximate, 110-120v is pretty standard.

So based on your measurements, here's my theory: while the neutral should be close to 0v, when you turn on a high wattage device (the microwave is in that list) you're pushing a lot in and out of that ground, and out of phase with devices that are on half of the circuits in the home. I'm guessing that rather than going to ground, it's easier for that extra power to go to those other circuits where you see a net power gain, and for the same reason you see a small loss on the same circuit.

Edit: Thinking about this a bit more and I can come up with one problem, a broken ground on the neutral bar. You can check the ohms between the ground in an outlet and something else that should be grounded (metal stake hammered into the earth, or try a plumbing fixture like your sink). Then do the same with the neutral plug (check for voltage from neutral to the ground in the outlet first in case you are testing the hot by accident). If you see a solid connection with the ground/neutral, no voltage is passing through them, and the outlet tester didn't indicate any issues, I personally wouldn't worry.

BMitch
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More headaches are caused by floating/open, high resistance/loose, corroded,bad connections anywhere along the neutral in any 120VAC phase to neutral system! Neutals are current carrying conductors in single phase 120VAC systems which need to be properly grounded and have a clean low impedance connection all the way back to their source of power;.. the utility Transformer! The Utility Transformer neutral connection as well needs to be properly grounded!

The same headaches are encountered in a 12VDC automotive electrical system when the common negative is impaired in any way all the way back to the Vehicle Battery Negative terminal!

Liberty Patriot
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