How can you tell the hot from the nuetral in “knob and tube wiring”?
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Don't know if it would work, but since the two wires are spread +/- 6" apart...maybe one of those nifty "voltage sensors" that you just encircle the wire with (no need to expose the conductor)???
Can you get to en earth ground (cold water pipe etc)? f so, you'd have 110 between hot and there, zero otherwise...
A Fluke tester is great for that.
Don't know how I got along without it.
You mean a sniffer BH? I was figgering that a sniffer would likely give him a false hot reading... maybe not.
PaulB
I'm not sure what you mean by "sniffer".This is what I was talking about:http://www.tequipment.net/FlukeVoltAlert.html
I got one of those talking scales for my birthday. When I stepped on it this morning it said: "One at a time, please"
Yupp, that's a voltage sniffer...
I have that same fluke tester, and it's great.Touch it to the insulation on a hot wire, and it lights up (don't have to touch bare wire). No light on neutral or ground wires.********************************************************
"It is what we learn after we think we know it all, that counts."
John Wooden 1910-
Re: "Touch it to the insulation on a hot wire, and it lights up (don't have to touch bare wire). No light on neutral or ground wires"Be careful. Those testers, I have used four different models, all have weaknesses. They are a good rough test but they can get you hurt if they fail to read a live condition and they can cause a lot of confusion with false positives.On the false negative side they don't read through shielding. I had a large pair of cable cutters with a hole blown in the blades, even held closed you could drop a pencil through it, when a helper tried to read a cable with the stranded ground surrounding the live conductors. It didn't read hot and so he was sure he was chopping out the right cable. It sounded like a cannon going off and the soot made him look like a coal miner when he got out. Luckily, other than a hell of a sound and flash, there was no fire and he didn't get shocked and, after a while, his hearing returned.Water and mud on the conductors can cause these devices to fail to read consistently and the electrical mechanism itself is subject to rain and sweat. Not very sensitive but I have had to dry them out to get them to work properly after a heavy rain. Thing is it doesn't tell you it isn't working. It just doesn't. A potential safety hazard. They also will show false positives. A neutral won't trigger it if it is in solid electrical contact. A little corrosion at the connections or a slightly loose screw can cause the neutral to ring just like the hot. Given even a slight ground fault and similar corrosion or loose connection and the ground too will ring like the hot. I have seen situations where all three were ringing. If who don't believe these devices can be easily fooled needs to just reach up with it and run it vigorously through your hair. Most, particularly in dry conditions, will ring loudly. Non-contact voltage detectors are not bad, I recommend them to my carpenter friends when they need a rough and ready way of determining if a wire is live. As I pointed out false positives are more common than false negatives so, because the carpenters aren't there to troubleshooting, it ends up they are just a little extra safe. I also recommend that they get a simple neon test light, about $5, as a second test.Like any tool they have limitations. I typically have one on my tool belt. Their inexact and frequent false readings is excused because they are small, light, easy to use and can give fairly reliable readings without damaging the insulation. If the worse case is that there might be a small flash and you get a new strip hole in your cutters it may be good enough. Small enough risk for a false negative. On the other hand if you have to work in a situation where your grounded, like when working in a crawl space on damp ground, where you can't get away these sorts of testers are not good enough. Know their limitations.
Very good advice...............I'm not an electrician, but the electrician I work with has given me the same warning -- but as you said, it is a very handy device for those of us in the carpenter crowd. I have carried it for about a year, and so far, false positives have been an occasional problem, but I can't remember getting a false negative. Mostly, I have used it to map circuits, and for that purpose it seems reliable. You've never met anyone more chicken than I when it comes to working with electricity; I will even trudge all the way to the basement to shut off a circuit before I change a simple switch.So I hear what you're saying. ********************************************************
"It is what we learn after we think we know it all, that counts."
John Wooden 1910-
> If who don't believe these devices can be easily fooled needs to just reach up with it and run it vigorously through your hair. Most, particularly in dry conditions, will ring loudly.
Works with dry hair. But per my professional hairdresser stepdaughter, if your hair's that dry, you need a conditioner. ;-)
I tend to rub it a few times on my sleve, (the tester, not may hair....) then touch it to the sleve. It's a test worth doing each time you use it, since the ability to detect static electricity verifies that it's working. Forgetting to turn it on, or a dead battery, are the main causes of false negatives. The static beep test eliminates those. It'll give you one quick beep for static, and a continuous tone for AC power.
-- J.S.
Good points.Conditioner may be a just what's called for. My boyish good looks have largely worn off and I can use all the help I can get. I was thinking fishnet ...Assuming the unit is functional, had a helper drop one in a puddle and then say, about feeders we were set to chop out, 'all those wires are dead'. Of course they were still live as a firecracker. Cutting them with uninsulated cable cutters, in the rain, while kneeling in a muddy ditch, could have been interesting. I get by on a little skill, some care and a whole lot of luck.False negatives are pretty rare. Failed units, dead batteries or wet mostly, and shielding being the main causes. Once had a carpenter borrow one and then say it hadn't worked when the line sparked when he cut the conduit with a sawsall. That it couldn't 'read' the voltage through the conduit hadn't crossed his mind. False positives can be confusing. Hot, tired, confused and frustrated people tend to make more mistakes so even though it is less a direct threat it can be a concern. Which, even though it can be a pain to rig, is why a long-line ground reference is so good. It makes clear what is going on in a pretty unambiguous way.
> Which, even though it can be a pain to rig, is why a long-line ground reference is so good. It makes clear what is going on in a pretty unambiguous way.
Yup. Back in the olden days when I was doing production sound, I made up some 25 ft. and 100 ft. clip leads. If bad ground is the problem, a known good ground is the only solution that'll work.
-- J.S.
Sometimes, with good lighting and a little luck, you can tell which is which if the insulation is relatively intact. The wires are most commonly tinned copper with natural rubber as insulation with a woven cotton outer cover.
This cotton cover is typically black, or very deep brown, with medium brown woven together for the hot side. The neutral being medium brown and off-white woven together. When in good condition and under good light it is easy to tell them apart. With some wear, dirt and lousy light they can be difficult to tell apart.
In my experience if the K&T wiring has not been messed with or modified by the local 'handy-hack' the color code is usually pretty consistent. Hot being dark and the neutral return lighter. If the system has been messed with parts, or even all, of the system may be reversed. If the insulation is damaged or missing your going to need other methods.
Don't panic if the insulation is in tatters in a K&T system. It is designed to operate safely without insulation on the wires. Then K&T was installed insulation technology was pretty poor and it was assumed that the insulation on the wires would fail. The structure being isolated from the voltage by the porcelain knobs and tubes. The bare wires held in open air between insulators. Just like on power poles.
You can use a non-contact voltage detector. Sometimes these will give you repeatable results. Often not. But with experience this is often enough.
Using a meter and reading between the two conductors and any nearby grounded surfaces, sometimes even a damp wall or soil will read enough to give a clue can be useful but you have to be careful and have enough experience to know when a reading is throwing you for a loop.
Instrument issues can cause confusion. High impedance meters and non-contact voltage detectors can read ghost voltages as usable current. Low-impedance meters, usually analog, solenoid voltage detectors, a Wiggy being the best of these, and neon test lights are largely immune to these influences.
Problem is that with only two conductors, even things you might assume to be grounded, your not sure which is live and which is just the return path. Using a metal pipe is usually good unless the water heater, pool pump water softener or something defective is using the pipe as a ground path. Metal ducts are good unless the air handler motor is defective and even the earth itself can be live if a line underground goes bad.
Best, surest, method is to disconnect the loads, turn off lights, unscrew them if necessary. Unplug everything. Return current on a neutral can throw you off so remove the loads. Be aware that doorbell transformers and thermostatically, or humidity, controlled exhaust fans might not be accessible so they still can throw you off.
Rig a 'long-line', a long length of insulated copper wire, into the neutral-ground point at the main panel. Take this with you, easier if it is a 500 foot reel with the stripped end exposed at the center and strung on a large screwdriver as axle or a short length of rope looped through the center, as a known good reference. Just snake it around as you move. This can be reused many times. Having a known good ground reference brings a lot of clarity to what can be a confusing situation.
With a long-line reference you can often tell the real hot from the neutral return because of the voltage drop of the load. Anything that reads significantly below line voltage might be a neutral. But a hot line with a loose, corroded or damaged connection or three might have cause a similar reading. Experience and context, as always, will have to be your guide.
Thanks----You confirmed my thoughts on what I have to do. I was hoping that I wouldn't have to run a ground up from the basement to the fourth floor ( attic ) of this civil war era house.
Thanks again!!!!!
Just grab the stripped part of one of the wires. You'll know fast. It's like life, a 50-50 shot.