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[Phys-l] home wiring catastrophe



Something happened in our subdivision recently and I can't explain it to my own satisfaction.

While working on one of those cubical boxes that distribute power into local houses (for underground wiring, as we have in our subdivision), the power company accidentally severed the neutral line. This fried the circuit panels (and possibly the entire 120-V wiring system) in 5 houses, actually lighting one of them on fire. Sadly, these 5 houses have been declared unsafe and the residents have been living since then in hotels.

My question is: Why did severing the neutral do this?

Here's my (evidently flawed) thinking. Can you correct me please?

I'm told the 120-V-rms is distributed in two phases, let's call them A and B. I assume they're 180 degrees out of phase. Now I assume phase A runs through circuit breaker A (let's just say one for simplicity) and then through appliance A (again, just one for simplicity and assume it's purely resistive) with resistance R_A and then to neutral. Similarly phase B runs through breaker B and then appliance B with resistance R_B and then to neutral.

If I cut the neutral, I am now connnecting the following in series: breaker A, appliance A, appliance B, and breaker B and putting 240-V-rms across that series. So what? The original rms currents were 120/R_A and 120/R_B, both presumably less than the typical 15 A breakers. Say R_A = R_B = 12 ohms, so each current is 10 A. After cutting the neutral we have a single rms current of 240/(R_A+R_B) which is still 10 A.

Even if it were too high, the breaker would simply trip. Why should the panel burn up? Some neighbors are trying to convince me that they're 120 V circuits and now carrying 240 V. I'm of course unhappy with the idea of voltage being carried and prefer to think about currents.

The only way I can figure it is if somehow the 120 V circuit box is sensitive to the absolute potential and doubling it therefore causes problems. I can't see why this should be true. For one thing, what would the absolute potential be relative to?

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Carl E Mungan, Assoc Prof of Physics 410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-5002
mailto:mungan@usna.edu http://usna.edu/Users/physics/mungan/