Martindale V7 false positives?

[Heyesy1981]

Hello All,

I keep a Martindale VT7 voltage indicator in my kit bag as a rough an ready check for live then check for dead before I work on a circuit.

Today I've had a situation where the VT7 indicates > 50 V on a light switch fascia plate (Schneider Lisse Deco) but when I check this with my multimeter the fascia voltage reads 0V to ground / neutral.

I've checked continuit between the fascia plate and earth in a ligthswitch in a separate room and it is earthed okay. Same for the fascia and adjacent metal lad socket fascia plate 0V.

Has anyone come across this before? I'm trying to think of modes by which it could pick up a > 50 volt reading.

 

[pc1966]

It is one of the common problems of the no-contact indicators. Basically they only need microamps at most to signal volts being present, so any sort of capacitance to nearby live conductors will do it.

Get a proper voltage tester that draws a mA or so, that way you have a reasonable indication that there really is power!

 

[Pretty Mouth]

This is a single pole contact tester? I believe (though stand to be corrected) that it uses the user as earth reference, and it may be that you yourself are not grounded. Try carrying out the test again, while touching some earthed metalwork

 

[marcuswareham]

I would never trust the non-contact testers to prove anything, iv had 2 of them and both couldn't give consistent information

I use a Martindale Drummond to check for live or dead, Its brilliant, very simple, can see the indication lights from all angles and it doesnt anoyingly beep at your like some of the other voltage indicators, it has once before tipped an RCD testing L - E so perhaps the LED/neons inside it are just over 15mA or there was already alot of earth leakage somewhere, it was annoying on that one occasion

 

[brianmoooore]

Had a case once when a non contact voltage detector lit when it was brought near to earthed metal or an earth wire. Turned out that the concrete wall and floor were live as a result of a pierced cable in the wall, in turn making me live.
I get similar on a car forum that I'm a moderator on and on which I post electrical help. Frequently get told "I measured 12 volts at this point" or similar, with the poster not realizing that an unloaded multimeter will still read 12 volts through a series resistance of a few thousand ohms. I tell them to put the meter away, and use a 12 volt lamp of at least 5W instead.

 

[Lucien Nunes]

The non-contact indicator probably isn't giving a false positive reading as such. Its purpose in life is to detect electric fields and it is probably detecting one; your task is to identify the cause and nature of the field and convert that into a diagnosis about the wiring. Are you sure the fascia is earthed, as in you get a valid, low Zs at the point? Could it be that the back box is earthed but the fascia is not? Could it be connected to the CPC but the CPC is not earthed? This is a separate consideration to whether something is 'live' or not as measured by a multimeter.

Contrary to popular opinion, so long as it is of good quality, a non-contact indicator can be a versatile, reliable, accurate troubleshooting aid. But to get the most from it, and to avoid jumping to the conclusion that it gives all sorts of random indications, one needs to understand the physics of electric fields and how it works, and get a feel for some of its parameters. In 25 years I can't think of a time when any of my voltsticks actually lied to me. Certainly there are times when what they can tell isn't what I want to know, but they are just one of the tools in the box; not every tool is suitable for every purpose.

The situation described by @brianmoooore - the whole environment leaking to line - is a good example. I had exactly that in Russia, and was able to detect it in seconds with the voltstick, but you would never know about it using a multimeter or Martindale. Some would just say 'Stupid voltstick thinks earth is live' and chuck it in the bin, but if you understand the physics it's obvious why it would behave that way.

 

[static zap]

Do make sure , there is not an indication for the portion of floor you are stood on .
ESD conductive soles stood on a floor board nail thru a cable under old floor boards ...
(this is unlikely --was more likely static pickup )

If leaning on a damp wall ,, thats leaked live a bit ... Earths win show up live!
Everything is relative to you ..and you may not actually be at earth potential..
Problems
a) Oversensitive
b) Applies no load to banish THOSE Darn Phantoms.

Un trustworthy .. unsafe ..and Dam confusing.

 

[telectrix]

i got a voltstick with a forked end. useful for ID'ing a live cable in a cluttered switch assy. or trunking as it only indicates on the conductor within the fork. only 1 hand required. obviously use an approved tester to confirm.

SAGAB Volt Stick PRO230 Non-contact Voltage Tester | ISSWWW.CO.UK

The Volt Stick Pro 230 professional voltage tester is designed to safely check for the presence of an AC voltage, without making any disconnections in cables, wall sockets, fuses, circuit breakers, junction boxes, etc | Available now at ISSWWW.CO.UK

isswww.co.uk

 

[pc1966]

The non-contact testers have their uses, but you have to be very aware of their imitations. Specifically they can be wrong both with:

• Showing volts on a "dead" but floating line
• Failing to show a live cable if the surroundings are also in a high electric field

The first is an annoyance, leading to potentially wasted effort to track down the non-existent power, while the 2nd can be fatal!

 

[Lucien Nunes]

Specifically they can be wrong both with:

• Showing volts on a "dead" but floating line

In this case I would not consider the Voltstick indication 'wrong' because the voltage it is detecting is actually present on the line. The source impedance of that voltage could be anywhere between zero and sky-high, but the Voltstick does not make any claims to indicating the source impedance. Arguably, it is more correct than a multimeter that pulls the line down to a few volts or a low impedance tester that pulls it below its own detection threshold. These two are disrupting the very thing they are trying to measure, while the Voltstick is not. If someone does not know how to interpret that indication, that's their lookout. A machinist would not try to measure a ball-bearing with a tape measure, nor a 20-foot length of metal stock with a micrometer. One has to use the correct measuring device for the application and interpret the reading appropriately.

The high ambient potential situation is extremely rare but is a valid reason not to rely on the Voltstick to confirm isolation.

 

[telectrix]

The high ambient potential situation is extremely rare but is a valid reason not to rely on the Voltstick to confirm isolation.

voltstick readings can always be confirmed with a fast moving wet finger. ?

 

[timhoward]

On a sort of related note - years ago I had a small not contact volt stick with a sliding adjuster for sensitivity. I found the adjustable nature of it really handy especially when multiple cables are clipped side by side to a wall. It was not expensive either. I just wondered if anyone had seen one like this anywhere as I've failed to find one.

 

[pc1966]

In this case I would not consider the Voltstick indication 'wrong' because the voltage it is detecting is actually present on the line.

True, but that is not an answer to the question that is usually asked of one!

That is the problem, the assumption the answer to "is for a low impedance poser source present?" that an electrician normally wants to know, versus the actual result which is "is significant electrical field is present?".

 

[telectrix]

True, but that is not an answer to the question that is usually asked of one!

That is the problem, the assumption the answer to "is for a low impedance poser source present?" that an electrician normally wants to know, versus the actual result which is "is significant electrical field is present?".

can yous put that in plain inglish? love the poser source. are you in essex?

 

[Heyesy1981]

Hello All,

I measured the fascia voltage with respect to the incoming gas pipe and the incoming earth of the main supply and it was measured at 0.005 Vrms.

It seems that the VT7 is picking up > 50 V via induction, leakage or some other means. I think I will buy a propper unit to avoid the ambiguity in future!

Thanks for your replies.