Apparent voltage on plastic service head

Well measuring surface voltages on an insulating surface is always a challenge - any real meter has to draw some current to operate, and depending how good the insulator is,  as it is in effect in series with a very high value resistor, the voltage will drop away more or less as the current flows. This is not normally a problem - measured with really high impedance equipment, in fact there are very high voltages all over the place - tens of thousands of volts appear on the surface of sellotape as you peel  it off the reel in a dry room, and the voltages that make cling film work and protect your sandwiches are also in the high kV. Actually having very dry skin I often find touch screens don't work for me and cling film gets in a terrible tangle, but then I'm less likely to be electrocuted !!

There is a very real problem with some early insulating plastics slowly changing their chemistry as they decay and becoming rather better conductors than when they were new, This is mostly reported as affecting the black phenolic resins, especially some DNO equipment  but I'd not like to say it can't happen with others.

The voltstick  is more complex as it is a non contact sensor and is therefore sort of measuring capacitive coupling rather than resistive (internal resistance of the thing will be gigohms), and then completing the circuit through the user, - so you can't really be sure of the state of the insulation from a volt stick reading alone, just know that there is something live on the other side of it - which in this case we should not be too surprised. After all it finds wires inside perfectly good insulation ;-)

If I had to verify the insulation I'd be looking at pressing a pad of something conductive (if it was a lower voltage a salt water wetted cloth, but not in this case I think, perhaps a conductive rubber would be better !!) on each face in turn. It would be interesting to know what they used and what the effective input impedance was.

I don't see it as anything to worry about, just a limitation of voltsticks as a means of testing.

Mike.

Thanks. Though you say it finds wires in good insulation, why do you think the tester is activating on the plastic head, but not the insulated tails, etc - if it's finding the live wires, you'd think it would activate on the tails as would a non contact volt stick, but it doesn't?

coupling into that probe is essentially capacitive -  unlike a traditional meter it does not need a pair of low resistance connections to complete the circuit. 

One 'contact'  is a mixture of your hand and body capacitance, and the capacitance of the body of the probe itself to free space.

This will be a few hundred pF in series with a few k ohms, for you, in parallel with  a few tens of pF for the body of the probe itself.

The other contact is inside the probe tip, when not touching directly, this will need to be close enough to a large enough body of live metal to get a significant displacement current flowing to operate the internal electronics. (significant here is probably  <~ 1pF and equivalent to about 3000 megohms input impedance so a few tens of nano-amps from the mains )  The reason it works in some places and not others will simply be down to the shape of the electric field lines, as that will determine the capacitance. So it might not work alongside a thickly covered meter tail and yet may trigger inside a bundle of insulated wires.

Do you have the exact spec or a link to the data for the one you are using?

Mike.

See link below for information on the TIS810:

• www.testinstrumentsolutions.co.uk/.../tis-810-contact-single-pole-voltage-detector

Note that unlike many voltage detectors of similar appearance, it is described as a Contact Single Pole Voltage Detector and has appears to have a conductive tip.

Operating principle will be the same as you have described above though, voltage detection via capacitive coupling to to earth.

Have you tried putting your other hand on [*] the service head, by the neutral, to simulate the final stages where instead of pointing the stick at the surface/insulator, you reach around (~0.23) so the knuckles of the holding hand are that much nearer the neutral greatly increasing the capacitive coupling of your body to the 'earthed/neutral' and cross bonded metalwork?

[*] perhaps with a piece of cling film or thin poly bag for extra safety, in case that case is actually partially conductive ;-)

Aside: for an electronics folks (frequencies >75Hz ;-) this stray capacitance is why the old fashioned idea of disconnection of an oscilloscope earth for those metal cased Tektronix 'scope didn't work - the case to ground capacitance was already sufficiently high that it still provide a ground loop return path at any reasonable HF frequency.

It's a fun calculation to consider [just] two 1cm^2 plates separated by a piece of sellotape, in picoFarads, then scale it to the human body, then scale it to impedance at operating frequency - it's how most noise get's into electronics, from the PSU switch and the metalwork! Try 230V through 1pf into a 10kohm sensor input impedance, on e of those high sensitivity ones..

Thanks for the link. Delightfully short on useful technical detail isn't it !.

Do you know what the tip is made of ? I must admit I had  assumed from the video some sort of insulator material, but if it is a metal probe under some sleeving then perhaps its closer to the old neon screwdriver  of old in terms of performance and needs hundreds of microamps to operate.

The idea of a retest with a layer of known insulator on the service head  - such as a piece of sellotape or cling film is a good call actually, also as a way to see if there is a significant resistive effect or not as the effect  pf a few hundred microns on any capacitive coupling will be 9/10 of sod all but the additional resistance even of a thin film layer will be enormous.

Mike.

Yes, it  does appear to be a capacitively coupled relation of the neon screwdriver!

As the voltage detector is decribed as 'contact' I am not certain that is is being used in its intended mode here and it would be interesting to see the video repeated with a non-contact voltage detector, such as a Fluke 1AC-E II.

(On first watching the video, I thought the installation was isolated at the service head as the meter screen appeared to be blank.

My mind therefore turned to a voltage difference between local earth and the TN-C-S earth or diverted neutral currents causing the detection over the neutral / earth part of the service head. 

However, on watching the video again, there does appear to be activity on the meter screen and the installation is not isolated.)

- Ross

yes, I had  wondered about how much was energized but I think all of it, but then it also looks like I mis understood the operation  of the probe.

Mike

Here you go: 

The first video is of my using the contact volt stick (with the metal probe) with the suggested cling film.

The second video is of the DNO using a 'non' contact volt stick.

The service fuse is in, and the installation energised / in use

So, the question is, given they state that it should activate between 50-600v, should it be returned as faulty?

Thanks

Did you do a test in the case where you used the cling film to insulate your left hand while 'touching' (flat hand/fingers) the service head? (my internet is slow for watching the videos..)

In a sense it's "you" that is the return connection for the glow stick, so placing your other hand onto the insulated (cling film) service head (esp near the neutral / earth / armour) and thus having a good capacitive contact will enhance the ease of the glow stick to activate. It looked as if that happened with your knuckle when testing the service head insulation.

If you are standing well back and only reaching with one hand and pointing the stick to the potentially live contact, then your body capacitance back to earth/neutral is much lower creating less leakage (the situation in the earlier video). 

The glow sticks and older neon screwdrivers can give false positives, as my father in law often mentioned (CEGB electrician, with stories;-). He liked his test lamp!