PT stray voltage?

[BrassElephant]

Hello all, first thread here. Love this community, thanks to all.

I have a little PT on the bench, it comes out of an old tube CB radio. Schematic here: http://www.bunkerofdoom.com/lit/cb/sonarfs23/sonar_FS23_schematic.gif

The schematic is sorta hard to make out at first, but that's not really part of the issue.

Whenever I energize the primary, I get the expected voltage readings on each set of secondary windings.

I first did this test with the PT still installed in the chassis. All of the windings were disconnected and floating (gator-clipped and insulated and safe), but the PT was still physically installed in the chassis. No earth ground connection. I rested my hand on the chassis for a second, and felt a slight vibration. DMM said 52V between chassis and earth ground (outlet on bench).

So I took the PT out of the chassis, and tested again. Same result, 52V on screw-posts thru the PT.

Gator-clipped screw-posts to earth ground on bench. Voltage on screw-post disappears, no change in secondary voltage readings, no current draw.

So my gut tells me this is normal. Just wanted to be sure. What's the word?

[sluckey]

It's called a phantom or ghost voltage. Put a 1K resistor across your meter leads and it should disappear.

[PRR]

> No earth ground connection.

If the transformer stray capacities are exactly balanced, you expect an ungrounded transformer to float to half the 120V supply. A little less counting meter-loading. 52V is not unreasonable.

You also see this when checking "3-way" switches on stair lighting. The traveler lead can be floating on both ends and on a hi-Z meter will read 50V or so, because it runs right with a 120V and a 0V wire.

1k loading will make it 'go away'. 10k and 100k are more typical *initial* skin resistances; but if you don't let-go a continued electric shock will break-down cell-walls leaking cell-juice and skin resistance quickly drops to 1k or less.

[BrassElephant]

Ah, I see. That did the trick indeed.

Time to go read up on phantom voltage.

Thanks a ton!

[sluckey]

I recently bought a Klein MM770 dmm that actually has a Lo-Z mode especially for dealing with phantom voltages. Lo-Z mode actually puts a 3K resistor across the probes. Occasionally useful to bench techs. More useful to an electrician that deals with long runs of electrical cables.

[BrassElephant]

Oh interesting.

To be honest, I'm a 2nd year electrician apprentice. So I really should've known about this phenomenon already haha. I seem to remember an occurrence like PRR mentioned, on a service call, phantom voltage on the travelers in a 3-way switch. But this phantom voltage on a high voltage PT didn't ring that bell.

Now I know!

[thetragichero]

If the transformer stray capacities are exactly balanced, you expect an ungrounded transformer to float to half the 120V supply. A little less counting meter-loading. 52V is not unreasonable.

You also see this when checking "3-way" switches on stair lighting. The traveler lead can be floating on both ends and on a hi-Z meter will read 50V or so, because it runs right with a 120V and a 0V wire.

this solves a conundrum i had a few weeks back. thank you

[PRR]

> electrician apprentice. So I really should've known

Electricians are very practical people. And electric utilization is not as simple as it may seem. So even experienced electricians rarely get into the "why". Just say "stray voltage!" a lot.

Here the key lesson is: if you can touch it, ground it!

[BrassElephant]

> electrician apprentice. So I really should've known

...

Here the key lesson is: if you can touch it, ground it!

Now THAT is something I've always known, way before I ever started my adventure into electricity.

This was just the first time I had ever noticed potential on a xfmr housing. I've tested PTs out on the bench before and never noticed it. Is this something that doesn't occur with every PT? Or just something I didn't catch before?

If the chassis was earth-bonded, I wouldn't have noticed it this time. But since it wasn't, and because I saw it, I just wanted to be sure this PT wasn't malfunctioning in some way. When I gator-clipped to earth and didn't get any sparks or smoke, or glow on the dim-bulb, I assumed it was a common phenomena. Just wanted to check here with the experts.

[pdf64]

When I did my apprenticeship, filament lamp test probes were the preferred kit to check for mains power, as they wouldn’t respond to such stray voltages. eg https://docs.rs-online.com/def9/0900766b80494f46.pdf

[Williamblake]

So does the winding in old power transformers that is grounded on one side and floating on the other solve this?

[PRR]

When I was young, the sparkies used a Wiggy. Cheap, rugged, never showed phantoms.


https://electricalconnection.com.au/david-herres-all-about-wiggy/
https://en.wikipedia.org/wiki/Solenoid_voltmeter
https://www.radiomuseum.org/r/square_d_wigginton_voltage_tester_5008.html
https://patents.google.com/patent/US2075860A
They have become hard to find.
https://forums.mikeholt.com/threads/wiggies-solenoid-testers-where-did-they-go.2555692/
Similar toys have many unwanted features.
https://www.grainger.com/product/IDEAL-Voltage-Tester-3T887
https://www.grainger.com/product/1CWL3

[BrassElephant]

So I was bored last night and hooked up this test again. This time put the DMM between outlet hot and xfmr primary, looking for current.

It shows 192ma of current thru the primary, both with earth-bond and without. No load on the secondary. Is this expected?

Also, is this phantom voltage occurrence common to every PT, or just some?

Thanks for educating me

[PRR]

....DMM between outlet hot and xfmr primary, looking for current. It shows 192ma of current thru the primary,

Exciting current.

Antek has no-load specs:
50VA  https://cdn2.bigcommerce.com/server6000/2b6qq/products/178/images/895/AS_0524__36611.1387480532.1280.1280.jpg
200VA: https://cdn2.bigcommerce.com/server6000/2b6qq/products/384/images/1346/AS-2234__64381.1510892638.1280.1280.jpg
These are toroids which may tend to lower exciting current than E-I types.

[BrassElephant]

Exciting current. Thank you. Once I learn the common terms for the phenomena, I can finally type in the right stuff to do the proper research.

Thanks everyone!