OT - Voltage Sniffer

BillConnerFASTC

Well-Known Member
I'm trying to replace some - not all - of the knob and tube - basically where walls are open. There isn't much left and this may actually be the end but my immediate pondering is determining which is actually in use. The apparent total lack of logic or organization of knob and tube wiring not withstanding, you can only trace it till it goes out of site, which is anywhere the original plaster and lath is still good. Will a voltage sniffer indicate wire in service, even if no load on it?
 
Yup. That's why you can use them on receptacles, despite no load being connected. Not bad for finding a break in a long cable either. You can plug one end in and sniff down the length of the jacket until the detector stops beeping. That's where your busted conductor is.
 
It will detect hot wires, but not neutrals. That may not matter, as they generally run near each other. Before you go hunting, turn on all of the light switches, so that a switch leg isn't thought to be unused.
 
I'm trying to replace some - not all - of the knob and tube - basically where walls are open. There isn't much left and this may actually be the end but my immediate pondering is determining which is actually in use. The apparent total lack of logic or organization of knob and tube wiring not withstanding, you can only trace it till it goes out of site, which is anywhere the original plaster and lath is still good. Will a voltage sniffer indicate wire in service, even if no load on it?

A voltage sniffer will indicate voltage and at times my Fluke has indicated down to lower voltage even if not rated so. Could be important given resistances of old circuits in wiring. A good plot of all circuit breakers in where they relate to all outlets of what ever type in the building would be the better start for this and long term use. Dependant on the building in outlets not known about or etc., a good voltage sniffer, some various screw or plug type adaptors etc. for it to plug into etc also for connecting CB sniffers also. Study and plot of the power panels and where they by radio with an assistant at times if large enough a building.... "it's out,.. no go back" type stuff is the best starting point in plotting out the system in what breaker controls what. Than as you have walls exposed as you express, certainly if in the area this might be going to you have control, and use the sniffer to check if these wires are what you think they are. Possible they are for some outlet not found yet also so good to check.

At least from my last 1999 reading of the code, Knob and Tube while normally 14ga thus only 15A I would downrate, was still viable though not recommended. I believe next year comes out the next code book and I am already approved of work buying for me the Handbook expanded verseion for me to study. Grew up in a very old house with exposed knob and tube running all thru the basement but I think much of the rest run thru Sch 40 gas pipe at least to some areas to replace the gas lights.

I would map/plot out the building first before I did any work - simply because I/you can, and work from there.. What's in use you now have a map to what it might be connected to or what it might be going to in circuit breakers that now this voltage sniffer indicate in turning offonce the mapping is done and if not related to known outlets perhaps indicate something not known about to trace.

Were this circuit not something known as to where it goes or what breaker controls it, you than have a problem in all ways. Map out first is my recommendation and I on site normally wear the voltage sniffer on my shirt collar in just to remind me to check when ever I do something, everywhere and often. If something does go wrong though... I made a mistake in my research of the curcut which can happen. Checking often and aways can save you from just like three months ago for me... thought I completely analized this wiring, nope it was power coming in from here instead of there. That's odd for even a 1972 house how they wired it.
 
OK. Anyone have specific favorite brands? I see some have flashlights built in which seem like a good idea. Recollecting first Makita I used with the led array and how wonderful I thought that was.
 
My favorite is the old Fluke VoltAlert with no switches, no flashlight, nothing but the sensor and light-up tip. It looks like that model is no longer available. I have a newer Fluke with a flashlight that I hardly ever use, because the flashlight button is so easy to press, it gets turned on in my tool bag and the batteries are always dead. The original VoltAlert lights up with every bump of my toolbag, but batteries last years.
 
A note on the voltage ratings - some are rated 90V - 250V, some are rated as "low voltage". The only difference is sensitivity, and therefore the distance from the source they sense. A 90V+ model will likely light up on a wire with 60V passing through it, you just have to be close. Conversely, a low-voltage model will be undamaged and light up when placed near a higher-voltage line, however it may start sensing from so far away that you can't localize what's hot and what isn't.
 
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One thing to keep in mind is that these, like anything, can fail. If you use one where a lack of indication could expose you to hazardous voltages, always test it before use. Just swipe it across your shirt. The static charge should cause it to signal the presence of voltage. If not, check the battery or throw it away. Don't get me wrong, these are good devices and using one might prevent a shock, just don't totally rely on them.
 
I'll just test and make sure my index finger is on ground before my middle finger touches the hot leg - though hard to know off hand in a 110 year old house with k&t which is hot and which isn't. Rubber soles too.
 
Sorry I'm late to the party on this one, as I've replaced knob & tube in I don't even know how many buildings.
Personally I don't like those "sniffers" for something like tracing knob & tube sometimes they'll show something as "live" when not, or not "live" when it is. Especially with 3-way switches. As mentioned before they also are no good in tracing identifieds or "neutrals". And yes I always have mine on me and use it all the time, you just can't always trust them.

Knob and Tube wiring, generally DOES make sense if you think like the electrician who installed it and remembering that your wires don't need to run together. Actually saved a lot of wire.

I generally use my "sniffer" in conjunction with my clamp-meter (if you know the draw on a circuit you can trace it easy ;)), and "tone-tracer". Most of the time just good sense does most of the tracing for me.

I remove ALL exposed knob and tube as much as possible, and remove anything not exposed as much as I can without too much damage (unless specified to remove ALL knob and tube, which has been the norm more often now). Often I use the old wiring to pull in the new wiring as much as possible (obviously not where there is a knob in the way, or a tube facing the wrong way).
Generally houses wired with knob and tube were built in such a way that little superficial damage need be done to do a rewire to a lot of the building (especially if a one floor). The hardest is the stuff like the lights on the first floor in a home with 2 or more floors.

Do a general trace. Turn EVERYTHING OFF, save one circuit. Trace it out, use your clamp meter to verify the correct neutral. Repeat with the other circuits to be removed.

Turn off a circuit to be removed. and simply follow the wires in exposed areas, cutting off as you go. Leave tails where you need to re-connect or fish.

As for recommending "sniffers" I've had many different brands. I've liked my FLUKE best. The Klein ones USED to be terrible (the buttons always broke), but got better however there is still an issue with the button (I find now its too easy to press).
 
"The hardest is the stuff like the lights on the first floor in a home with 2 or more floors."

Yup. Basement, two floors, partially finished attic. It's the circuits show up on two or three floors that are killers.
 

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