Non contact tester

[ptonsparky]

On e of our local suppliers sells a non contact tester to indicate the presence of voltage. We like them.

Hand Ditch digging today around a service with multiple direct bury feeders in the area. Trying to make sure the SE conductors didn’t make an unexpected turn we used the tester. No power, test on live circuit, ok. Repeat with power applied to feeder we are looking at. Indicator works at energized wire in panel box but not in ditch, where it indicates no power.

The three of us had never seen that before. New batteries didn’t help. We only had the one tester with us. Toes up tester or some other explanation?

 

[sameguy]

Won't read through the insulation.
Cables have a ground around them 5K?

 

[zbang]

Concentric neutral? I'd expect that to shield the electric field from the hots.

 

[ptonsparky]

No concentric neutrals. Service is single phase 120/240 volt. Nothing fancy. Direct bury AL Feeders consisted of two hots, no neutral, no EG. (Why I’m there).

 

[ATSman]

Yeah, my take on this is these testers have their place in certain circumstances but almost have to be backed up always by a contact meter. What is strange here is they usually error on the side of indicating voltage when there is none. As I understand it did not indicate voltage when the wires were energized? May have something to do with the conductors being buried and below the ground plane which cancels out the energized signal.

 

[ptonsparky]

Yeah, my take on this is these testers have their place in certain circumstances but almost have to be backed up always by a contact meter. What is strange here is they usually error on the side of indicating voltage when there is none. As I understand it did not indicate voltage when the wires were energized? May have something to do with the conductors being buried and below the ground plane which cancels out the energized signal.

Yes, it indicated voltage above ground, in the panel but once in the trench near the exposed conductor it showed none. Didn’t even blink.

 

[gar]

180914-2151 EDT

A non-contact voltage sensor works on a capacitive voltage divider from the hot wire to the sensor, capacitive coupling possibly to you or could be conductive, and capacitive coupling from you thru your shoes to earth or equivalent.

Buried wires in earth with a very conductive earth may shunt (shield) much of the electric field from the wires to earth before every getting to your sensor. Learn to do electric field mapping using curvilinear squares.

Instead try running current thru the wire, at least 10 A, and use a very sensitive milligauss meter, like a resolution of 0.01 milligauss, to sense the magnetic field. But you want only one conductor as the current conductor in your underground cable. Use some other wire spaced 10 to 20 ft away for the return path.

.

 

[ptonsparky]

180914-2151 EDT

A non-contact voltage sensor works on a capacitive voltage divider from the hot wire to the sensor, capacitive coupling possibly to you or could be conductive, and capacitive coupling from you thru your shoes to earth or equivalent.

Buried wires in earth with a very conductive earth may shunt (shield) much of the electric field from the wires to earth before every getting to your sensor. Learn to do electric field mapping using curvilinear squares.

Instead try running current thru the wire, at least 10 A, and use a very sensitive milligauss meter, like a resolution of 0.01 milligauss, to sense the magnetic field. But you want only one conductor as the current conductor in your underground cable. Use some other wire spaced 10 to 20 ft away for the return path.

.

We had 15 1000 watt metal halides on one feeder and 16 on the other so I think the current was high enough. The old feeders have been replaced already so we won’t be doing further tests. I do have a milligaus meter and have used it you suggest, just not the 10 to 20’.

 

[gar]

180913-1154 EDT

ptonsparky:

When you have two conductors tight together with the same current flowing thru them, then the external magnetic fields a short distance away pretty much cancel each other.

If you spread these wires apart, then there is a strong magnetic field anywhere within the loop created by the separated wires. This is an effective way to find wires within a wall or fairly deep in the ground.

You make relatively low inductance wire wound resistors by forming the wire in a hairpin loop and then winding it on the insulating core.

When you have a tight hairpin loop (a Romex cable), then the inductance is lower than when the loop is expanded out.

.

 

[Cow]

I have a Fluke glowstick that I've used many times in ditches on DB wire, I can't think of a time it didn't light up when it should of. But most of the wiring we're checking is 480v.

I do know it doesn't read well through SO cord or Romex that's gotten wet. But I don't recall it ever having an issue testing individual conductor.

 

[ptonsparky]

I have a Fluke glowstick that I've used many times in ditches on DB wire, I can't think of a time it didn't light up when it should of. But most of the wiring we're checking is 480v.

I do know it doesn't read well through SO cord or Romex that's gotten wet. But I don't recall it ever having an issue testing individual conductor.

I’ve had the Flukes but IIRC, the low voltage detect level was around 90 volt.

 

[kwired]

Capacitive coupling (as Gar mentioned) is what these detectors work off of. If working in average moist soil such coupling is going to be reduced.

 

[Cow]

Capacitive coupling (as Gar mentioned) is what these detectors work off of. If working in average moist soil such coupling is going to be reduced.

That makes sense.

We're in a sandy desert like environment over here, so it's usually pretty dry when we dig down to the DB wires we're working on. It would explain why we haven't had an issue like Tom is getting our glowsticks to light up.

 

[ptonsparky]

We are very damp in this area. Not mud but not dry sand by any stretch of the imagination.

 

[don_resqcapt19]

On e of our local suppliers sells a non contact tester to indicate the presence of voltage. We like them.

Hand Ditch digging today around a service with multiple direct bury feeders in the area. Trying to make sure the SE conductors didn’t make an unexpected turn we used the tester. No power, test on live circuit, ok. Repeat with power applied to feeder we are looking at. Indicator works at energized wire in panel box but not in ditch, where it indicates no power.

The three of us had never seen that before. New batteries didn’t help. We only had the one tester with us. Toes up tester or some other explanation?

They won't work if the earth is damp. They won't work if the paper in NM is wet.

They should never be used to prove the absence of voltage. Actually I don't see then as having any real use.

 

[kwired]

They won't work if the earth is damp. They won't work if the paper in NM is wet.

They should never be used to prove the absence of voltage. Actually I don't see then as having any real use.

Me either, I almost never use them.

 

[zbang]

They should never be used to prove the absence of voltage. Actually I don't see then as having any real use.

They show the likely presence of voltage, makes them useful for troubleshooting ("looks hot here, and here, and here. whoops, not there").

 

[kwired]

They show the likely presence of voltage, makes them useful for troubleshooting ("looks hot here, and here, and here. whoops, not there").

Yes a little usefulness for troubleshooting, but still not a definite confirmation of voltage or absence. Seen them indicate voltage when there isn't any as well.

Used to have inspectors use them to check device yokes, luminaires, etc. for equipment grounding. Turn on switch, hold non contact tester up to cover screws, if it doesn't light up the device yoke is likely bonded to EGC, but I suppose can give false indication under right conditions. But if it does light up, they are pulling cover and checking things out. Only had one time where something was bonded but couldn't figure out why the NCVT was indicating voltage, but when they give unexpected results one must understand they work off capacitance and for some reason there was something present to make it light up.

 

[don_resqcapt19]

They show the likely presence of voltage, makes them useful for troubleshooting ("looks hot here, and here, and here. whoops, not there").

I will not even say that they show the likely presence of voltage. They indicate voltage where a conductor is not connected to a load and just run in close proximity to a conductor that is actually energized.

 

[ptonsparky]

They are just another tool. We now know they don’t work reliably in ditches and reliably is used loosely.