Re: O.T. but electricians will appreciate this


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Posted by Sherman in Idaho [72.47.153.29] on Thursday, December 04, 2014 at 17:27:03 :

In Reply to: Re: O.T. but electricians will appreciate this posted by Randy On Long Island [63.117.245.30] on Thursday, December 04, 2014 at 16:55:08 :

Split phase circuits on knob and tube can be a lot of fun if you accidentally cut the neutral -- turn on one light and another one comes on at the same time. Or turn on one high-powered bulb, and and it comes on dimly, but the low-powered one on the other phase blazes like the sun and then burns out. If electronics are involved, all kinds of mischief can happen. Lose the neutral in a 3-phase wye and things get even stranger. I remember one time when we had lots of trees falling on the wires due to snow, and for half an our or so, most of the single-phase parts of downtown (depending on which line your particular transformer was attached to) were getting half voltage. Strange thing was the CFL bulbs worked just fine on 60 volts. Of course the incandescents were dim, computers were very unhappy, even the ones on batter backups, and we unplugged all refrigeration units quickly lest the low voltage burn them out.

On the really old knob-and-tube, they fused the neutrals as well as the hot wires. Seems good at first -- all the more safety. But if the neutral fuse blows, now everything (hot and neutrals) is hot, and a short to ground from neutral, which should normally do nothing, will make plenty of sparks. The neutral fuse often blows because the hot fuse blue first so somebody replaced it with a higher-power fuse (I've seen 30 amp fuses on 15 amp 14 gauge branch circuits many times), so the next thing that happens is the neutral fuse blows. Then they replace that one with another 30 amp fuse, and if the problem is an overload rather than a dead short, the wiring burns up to protect the fuse. My building had a bad fire at one point in its history which started around the old asbestos-lined indoor fuse/meter box, and I've found remains of the overheated wire that probably started it.



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