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Re: [Phys-l] home wiring catastrophe



This is a second posting of my experience of the return (neutral) failure. The washing machine motor wouldn't start (not fast enuf for the starter winding switch to cut out. I was about to call the repair people for a new motor, but, laudate deum, I checked the potential at the terminals; only about 90 V. So I checked elsewhere; fortunately the other leg and about 140. Then I knew! I went outside and saw the break in the wires coming from the pole. The power co. replaced all three later that day.

Something could happen at the breaker box if any of the breakers are GFCI breakers because those have solid-state components in them and that portion might fry.


I agree.

bc, wonders if PG&E is liable if something fried.


Michael Edmiston wrote:

I'm not sure about frying the circuit breaker box (although I describe one possibility at the end of this message), but you can certainly fry individual appliances.

In the example Carl gave, appliance A and appliance B might survive if their resistances are the same. But suppose appliance A has a resistance that is only one-ninth of appliance B. When the neutral is in place, appliance A and appliance B have a potential difference of 120 volts across each of them. Now cut the neutral. The applicances act like a voltage divider. The voltage across appliance A drops to one-tenth of 240 volts = 24 volts. The voltage across appliance B rises to nine-tenths of 240 = 216 volts. This could actually fry both appliances depending on what the are. The 216 V across a 120 device will certainly wipe out any solid-state devices unless the appliance has an internal fuse. On the other hand, low voltage on a device that has motors in it can cause the motor to overheat and burn up.

Most likely the two 120-volt phases are not "balanced" in the house. This changes depending upon what appliances are turned on at any particular time. This means the 240 volts will not divide as 120 and 120, but will divide in some other ratio that adds to 240. Currently-running appliances on either side (the higher voltage side or the lower voltage side when the neutral is cut) are in jeopardy.

Something could happen at the breaker box if any of the breakers are GFCI breakers because those have solid-state components in them and that portion might fry.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270
edmiston@bluffton.edu

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