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10 May 2010 @ 10:51 pm
Electrical question  
I'm working on a story in which someone who is an experienced electrician debugs and/or fixes an electrical problem (which has knocked out power to most of the building) by doing something, a real-world thing based on his experience, that seems like magic to a less-experienced person watching. In the current draft I have him identifying the one live wire in a bunch of dead ones by feeling for the warmth and 60-cycle hum. Is this plausible? Can you suggest anything better? It would be best if it were something that you had actually done.
 
 
 
jackwilliambelljackwilliambell on May 11th, 2010 06:18 am (UTC)
There is an electrical tester you can buy that, no kidding, is set off by simple proximity live wire even if it isn't part of a circuit. (In other words, there is power to the wire - but not flowing through it. Even a turned off light switch would set it off.) I have a cheap model, but a professional would probably have something like this: http://www.texsoinstruments.com/tic-tracer-300hv

Unless there is a short circuit or a massive overamp (which would normally trip a circuit breaker), the wire should *never* be warm. A hot wire would be a *very* bad thing and electrical systems are designed to avoid them. The 60-cycle hum is plausible, but not from a wire. I have a circuit breaker that hums when the dryer is running (i.e. drawing a lot of amps), but that is because it has a solenoid (based on an electro-magnet, similar to how speakers work if not exactly the same thing).
David D. Levinedavidlevine on May 11th, 2010 06:27 am (UTC)
Thanks!
David D. Levinedavidlevine on May 11th, 2010 06:28 am (UTC)
I'm looking for something he can do with his knowledge, not a special piece of equipment.
jackwilliambelljackwilliambell on May 11th, 2010 06:48 am (UTC)
Well, if I went to an electrical panel, found a tripped circuit breaker, reset it and watched it trip again, I would assume I had a short circuit somewhere. I would then need trace the circuit to find the short, which could be a problem if the panel wasn't marked well or was in an old building.

If I had a good idea what part of the building the short was, but not which wire out of N, then having someone keep resetting the breaker while checking wires for warmth might work. But I am willing to bet that no professional would ever do that. It is way too likely to start a fire or burn something else out.

I have found bad wires by smell in cars, aircraft, and appliances (if not in a building). When a wire shorts or overheats the insulation melts and you get a 'burning rubber' kind of odor, with an extra sharp shot of ozone. Not pleasant, but it does linger for a little while after the event.

You can make a lo-fi electrical tester with a small lightbulb, some wire, and an ice pick (or something else sharp). Attach wire to a ground and to your light bulb. Attach more other wire to the other pole of the light bulb and wrap the other end (stripped) of it around the ice pick. Poke the ice pick through the insulation on a wire you think is hot and, if it is, hey bingo the light goes on. Make sure to insulate things with electrical tape or you will be testing the hard way.

Basically there is no magic or fast way to do this in my experience. That tester I mentioned in the previous post is the closest thing to magic you are going to find, mostly because it works even if the circuit is broken so long as there is power to the wire; which is very cool in my opinion. The cheap one I have is the size of a large felt-tip pen; maybe your guy carries one around just cuz he thinks they are cool. Or, if this is in the future, he has one that is part of a sophisticated future version of a Swiss Army Knife?
jackwilliambelljackwilliambell on May 11th, 2010 07:15 am (UTC)
Here is a true story from long ago: I was working in an office and I kept smelling something burning. Everyone else in the office thought I was imagining things, but my sense of smell is very acute and I started to get worried about it.

Finally I sniffed around and noticed it smelled stronger towards the ceiling. I got a step ladder, moved aside some ceiling tiles, and checked up in the suspended ceiling. There, almost directly over my desk, was a burnt patch in one of the ceiling joists, right next to one of the wires the ceiling framework was suspended from. (This was not an electrical wire, but rather was structural.)

This metal wire was attached at one end to the metal framework of the suspended ceiling and to a nail in the joist at the other. The nail was touching a metal conduit that ran along the joist. I figured that there must be a wire grounding out inside the conduit or something and we called an electrician.

The electrician came out and determined that there was no power to the wires in the conduit. They were actually part of an emergency power system that ran to a generator in another part of the building and were only live when the generator was running. However, there was power to the metal ceiling grid! It was grounding out through the conduit with enough amperage to scorch the wooden joist.

So we pulled every ceiling panel and looked for bare electrical wires touching the ceiling grid or a light fixture with a problem. Everything was OK. The electrician and I kept poking around and finally found the problem. It took us nearly a week.

Outside the building was a transformer on a power pole. The transform had a major fault and was sending 230 volts into its thick copper grounding wire, which ran down the pole and into the real ground. This was happening all the time, even when power in the building was completely shut off (which was how we finally found it).

Now, electricity is funny stuff. It always moves from high potential to low potential over the lowest resistance path. Even when it is AC. Even when the potentials are across widely different materials and hundreds of feet of distance. In this case the high potential was the dirt near the power pole. The low potential was at the other end of the building where the conduit ended at the generator and was itself grounded to dirt.

In between you had the grounding wire for the lightning spike on the roof of the building, which ran up the side of the building within ten feet of the power pole and over the metal roof. The metal roof was attached with metal screws and some of them were also attached to metal straps, one of which touched the metal grid of the suspended ceiling three offices away from where I worked. You can work the rest of the path out yourself.

The fix? The power company replaced the transformer and paid our electrician's bill. Afterwards a mysterious computer fault that required me to reboot our mini computer (I said this was a long time ago) every week or so went away and never came back again.
dd-bdd_b on May 11th, 2010 01:07 pm (UTC)
Yikes; that's a fine story! And definitely encourages running down potentially serious symptoms (anything smelling like it's burning!).

Electricity is weird stuff (I've redone about half the wiring in 3 of the 4 houses I've owned; just added some to the fourth, it was more modern).
David D. Levinedavidlevine on May 13th, 2010 06:11 am (UTC)
The sniff test is exactly what I was looking for. Thanks!
martianmooncrabmartianmooncrab on May 11th, 2010 06:41 am (UTC)
you can touch the wire to another wire... insulated of course.. it should spark and arc.
Tomvoidampersand on May 11th, 2010 06:50 am (UTC)
When I took electric shop there was one kid who was just a loser. We all had to do a project and he decided to do a really big one, just to show everyone (including himself) that he could. It was a 3-way light switch, with the boxes connected by about 40' of conduit bent around corners, all up to code. At the end when we showed off our projects, he screwed in a light bulb, plugged in the power, flipped a switch, and it didn't work. You could just see the kid melting as he told himself what a loser he was. "Pa", the shop teacher said brightly, "hold on, maybe it's just a bad bulb." He unscrews the light bulb, licks his fingers, sticks them in the socket, and asks the kid to flip the switch again. Pa said "yes, there's current in there, you can turn it off", waits for the kid to turn it off, and then takes his fingers out of the socket. He goes in the back room and gets a new bulb, the light goes on, the kid gets an "A", and everybody is happy. Pa explained to the class that he knew what he was doing and was used to 110V AC, but we should not be sticking our fingers into live sockets until we knew what we were doing, and we should never mess with 220V or 440V. "That stuff is really dangerous."

By licking his fingers he made his skin more conductive than the interior of his body. He put two fingers in the socket tightly together, with his middle finger on the hot terminal in the bottom of the socket, and his forefinger up against the neutral terminal around the side. That way the electricity flowed across his skin from one finger to the other and out. What you don't want to do is touch just the hot terminal, because then the electricity's easiest path to ground is up your arm, into your chest, through your heart, and down your legs to the floor. That's how you can die. There may have been some risk of the kid getting hot and neutral mixed up, or worse, wiring up the socket with hot and hot, but Pa was pretty sure he got it right, and if not, I think he could have pulled those fingers out real fast.

If you have a wiring bundle, and you don't know if a line is hot, you could take apart a join by unscrewing the plastic nut that holds the wires together. Separate the wires, lick you finger, put them between the two wires, and see if you get that powerful tingling feeling. But you'd better be sure that one end goes to neutral or ground.

Now if you want real magic, try debugging a ground loop. I haven't done that myself, but ground loops are a common problem in real world analog electronics, and fixing them is an art. There are brute force solutions such as driving 20' long 2" diameter copper stakes into the ground in a hexagon and wiring them together. Doing it with more finesse requires understanding how current can flow, usually in the direction you don't want it to, through just about anything metallic.
jackwilliambelljackwilliambell on May 11th, 2010 07:25 am (UTC)
One thing: If you do the wet fingers trick make certain that your other hand is not grounded to *anything*! DC burns you, but AC kills most often by messing with your heart. If you ground out in such a way that the electricity flows across your chest, you get a quick quick trip to fibrillation city.

I would say that 'Pa' wasn't doing a good thing by showing you guys that little trick when he could have simply tried a different light bulb. Even if he was smart enough to put his other hand behind his back when he did it, it doesn't mean anyone watching him picked up on that and kids will try anything to look cool.
Tomvoidampersand on May 11th, 2010 07:34 am (UTC)
The kids in the class were pretty smart. I think there was some discussion about how maybe Pa had done that trick a few more times than was good for him. All I can say for myself is that I have yet to be tempted to try it. But I think of him every time I change a light bulb.
dd-bdd_b on May 11th, 2010 01:10 pm (UTC)
I hear railroad people used to us a variant of that trick (two fingers into the two large plug holes) to determine if a particular signal needed 220V or 440V lamps.

Me, I feel like I'm living dangerously if I replace a light switch in a hot circuit, standing on dry wooden floors in rubber-soled shoes, using tools with insulated handles, held in leather gloves. With no obvious large grounds nearby.
Assassinmarsgov on May 11th, 2010 10:33 am (UTC)
Essentially, if you can read a wiring diagram, you're performing magic. The lay person does not see an orderly representation of switches, fuses, wires, transformers, and the like; he sees a jumble of lines and squiggles. A brief glace leads to overall understanding ("Yeah, I see that you're powering...") and two minutes of study gives an experienced electrician a place to start the troubleshooting process.

I don't know if you read any languages that are written non-European characters. I realized that my ability to read Hebrew appears fairly magical, which led me to understand this about circuit diagrams.

Other professions have similar magical abilities ("this shadow here shows osteopersosis...").
the laughing leaping waterminnehaha on May 11th, 2010 06:35 pm (UTC)
lakeboy_55 is your go-to guy here. He's an EE.

K.
markensmarkens on May 11th, 2010 08:20 pm (UTC)
Dunno whether this will help you or not, but here's a true story from my experience: One day I was walking out the gate at home and smelled natural gas. Investigating showed the smell concentrated at the gas meter. Not good. Called gas company, who came out and determined that the gas pipe had corroded under ground to the point where it was starting to leak. Replaced the gas pipe, but the question remained why the pipe corroded. Kind of looked like electrical corrosion, but why would there be voltage on the gas pipe??

I used an electric meter to check it out, and determined that there was, in fact, an electrical potential present on the pipe. Yow! There was only one place where it could come from, a gas fireplace insert in the bedroom. Further measurements showed a consistent potential, consistent with "leakage" of an ungrounded circuit. It was supposed to be grounded, so where had the ground connection failed?

It turned out that circuit was supplied via a metal conduit over the roof. The hot and neutral were carried on wires in the conduit, and the conduit itself provided the ground. But the conduit had broken along the way, and ground connection compromised. Presto, the gas pipe became the new (less than perfect) ground connection.

Fixing the conduit also fixed the floating voltage problem.
Wendy S. Delmatersafewrite on May 12th, 2010 03:04 am (UTC)
Per my husband (30+ year Honeywell technician) this is NOT plausible. There is, however, a device called commonly called a "hot stick." It glows (and possibly beeps) in the presence of live wire without physically touching a live circuit or conducting any electricity.

My son, an electrician, calls the same tool a "non-contact voltage tester," and produced one for me to play with. Here's a link to that model: http://www.amazon.com/Sperry-Instruments-VH-602A-Non-Contact-Detector/dp/B000650T28
Wendy S. Delmatersafewrite on May 12th, 2010 03:08 am (UTC)
If, as other comments you have suggest, you need a no-device solution, and you have a desperate or idiotic character, he can always "feel for the tingle" in a wire - but as a a professional safety manager I'd rather we not suggest people do that: there's enough juice in you average home socket to theoretically kill 200 people! Electrical shocks can stops the heart; please do not go there. (Maybe he could hide the device or rig it to vibrate and look like a magician, okay?)
David D. Levinedavidlevine on May 13th, 2010 06:12 am (UTC)
I have him sniffing for the smell of burnt insulation now. Thanks for your concern!