Why wouldn't a volt pen pick up voltage?

[captaincaveman]

Well I'm confused !

Just went to isolate some kind of antique shower pump thingy in a bathroom for a plumber mate but the switch to it in the bathroom was painted over several times so I couldn't get my voltage tester with probes on it. So I used my non touch volt pen on the switch and the cable going into the switch which showed 230v power.

I isolated what I suspected was the power supply in the basement - an old wylex isolator. Tested the bathroom switch again with my volt pen and 0 volts - Fair enough. That would suggest the isolator was the right call.

Thing is though, When I tried my volt pen at the cable at the isolator (as a whole and also each individual conductor in the isolator) I got no voltage displaying on my pen - with the isolator energised or not. I checked the fuse in the isolator which is fine.
I retried my voltage pen on the switch in the bathroom with the isolator energised / not energised, with the same results.

SO, why am getting a reading with my voltage pen from the bathroom switch and cable at the bathroom switch, but not getting a reading at the isolator outgoing cable??

I'm going to have to return when I have more time to smash open the switch and do more substantial tests cos I like this plumber but I'm still at a loss on why this be...

 

[DPG]

Put that fuse carrier in the right way up I can't be dealing with that.

WestOCDward

 

[Dave OCD]

Steady on, I got the exclusive OCD tag.

 

[Lucien Nunes]

It's a Memspan switchfuse, they were very popular in the 50s & 60s. IIRC Memspan was the metal version and Memspel was the all-insulated one. Or was it the other way round? As above the switch is interlocked with the cover. The slot in the cover for the switch lever is narrower towards the on position which prevents the user removing it with the switch on. Then, once the cover is off, the interlock plunger springs out and should prevent the switch being turned on unless it is manually held in.

Volt sticks detect electric fields. The probe contains a piece of metal that forms one plate of a capacitor, the other plate is the target you are sensing and / or anything else conductive around it. The same thing happens at the body of the stick, the innards are one plate and whatever surrounds it (typically your hand) is another. Between the two is a voltage sensing circuit with a such a high input resistance that just a few picofarads of capacitance between the parts under test and the probe tip will pass enough current at 50Hz to trigger it.

The art and skill of using a stick comes from understanding how electric fields and potential gradients behave in space, and how they are modified by the proximity of other conductors and conductive objects. If you can visualise these, you can get much more out of a stick. People often voice an opinion that they are unreliable and unpredictable, but most of the time what that means is that the stick is behaving in a way they cannot account for, because they don't have a clear mental picture of the electric field that the stick is detecting.

Naturally these extra variables make the stick an unsuitable tool for testing for dead. Not because the stick is unreliable or unpredictable in itself, rather that due to its complex behaviour there are too many ways to accidentally misread its indications, as with a multimeter. I have been using the same volt stick for the best part of 20 years and it has never done anything unexpected or that didn't agree with other tests. Even when on one occasion (in Russia) it was lighting when probing a good neutral, more so than when probing an energised line, there was a perfectly rational explanation. Answers to that one on a postcard.

 

[Pretty Mouth]

Even when on one occasion (in Russia) it was lighting when probing a good neutral, more so than when probing an energised line, there was a perfectly rational explanation. Answers to that one on a postcard.

My best guess: You, as the user, were somehow more closely coupled to line potential than earth potential? Perhaps standing over cables such as those to light switches, whilst a long way from anything 'earthy'?

 

[Lucien Nunes]

Correct in principle but the cause was rather more blatant. I was standing next to the metal cabinet of a display fridge that was fully live at 220V to earth. It had been temporarily installed in a conference centre in a room with a wooden floor, far enough away from any radiators etc that no-one had got a shock off it yet. This was nearly 30 years ago and TBH I cannot now remember the exact cause of it being live. It was something like being plugged into an extension lead with two white cores that had been TN-C'd i.e. appliance earth and neutral commoned to one of the cores, but then connected to the supply the wrong way round so that the appliance earth was actually connected to the line.

Needless to say the behaviour of the voltstick made me immediately wary of my surroundings and the problem with the fridge was not far to seek.

 

[Petej999]

Well I'm confused !

Just went to isolate some kind of antique shower pump thingy in a bathroom for a plumber mate but the switch to it in the bathroom was painted over several times so I couldn't get my voltage tester with probes on it. So I used my non touch volt pen on the switch and the cable going into the switch which showed 230v power.

I isolated what I suspected was the power supply in the basement - an old wylex isolator. Tested the bathroom switch again with my volt pen and 0 volts - Fair enough. That would suggest the isolator was the right call.

Thing is though, When I tried my volt pen at the cable at the isolator (as a whole and also each individual conductor in the isolator) I got no voltage displaying on my pen - with the isolator energised or not. I checked the fuse in the isolator which is fine.
I retried my voltage pen on the switch in the bathroom with the isolator energised / not energised, with the same results.

SO, why am getting a reading with my voltage pen from the bathroom switch and cable at the bathroom switch, but not getting a reading at the isolator outgoing cable??

I'm going to have to return when I have more time to smash open the switch and do more substantial tests cos I like this plumber but I'm still at a loss on why this be...

View attachment 98641

The short answer is , don't use a volt stick/pen when testing for safe isolation purposes,

 

[captaincaveman]

UPDATE:
I went back, Isolated both at the Memspan switchfuse (thank you Lucien) and also at the main CCU (as an additional percaution), used my volt pen on the painted over switch in the bathroom, smashed the bloody thing open with my hammer, and did dead/live testing for real with my voltage tester before ripping it out. It was indeed the Memspan fuse. I still can't believe I haven't knowingly come across one in the past. Learn something new every day

Cheers guys

 

[littlespark]

UPDATE:
I went back, Isolated both at the Memspan switchfuse (thank you Lucien) and also at the main CCU (as an additional percaution), used my volt pen on the painted over switch in the bathroom, smashed the bloody thing open with my hammer, and did dead/live testing for real with my voltage tester before ripping it out. It was indeed the Memspan fuse. I still can't believe I haven't knowingly come across one in the past. Learn something new every day

Cheers guys

You opened a switch up with a hammer?

(Best Obi-Wan Kenobi voice)

“Young Padawan, you are now the Master”

 

[captaincaveman]

You opened a switch up with a hammer?

(Best Obi-Wan Kenobi voice)

“Young Padawan, you are now the Master”

Read the post - It had been painted over several times (including the actual toggle). I could have tried breathing on it to melt the paint away but a hammer seemed quicker

I ought to have used my club though - Caaaaaaaaaptain Caaaaaaaavemaaaaaan

 

[pc1966]

I could have tried breathing on it to melt the paint away but a hammer seemed quicker

Yes, a hammer and chisel job for heavy paint, or just skip the chisel part altogether!

 

[telectrix]

who needs a chisel?

 

[Baddegg]

How else do you open switches?