Sparks from oscilloscope ground

[TomS_]

Hi everyone, Im not sure what is going on here.

Lately Ive been noticing that when attaching my oscilloscope ground to parts of my circuit I sometimes get little sparks flying off.

Also, if I touch, say, the ground of a USB lead coming from my computer and the oscilloscope ground I can feel a 50hz tingle much like mains frequency, just not as harsh (I have been shocked a couple of times before).

As I was writing this I had an idea... Using my multimeter I measured AC voltage between the USB ground and the oscilloscope ground I see about 60VAC?? 

Wondering if anyone knows what would be causing this? Its definitely not right...

[GeorgeOfTheJungle]

ISTM that's a floating GND, the voltage you see might be coming from the mains' input filter crapacitors.


https://uk.tdk-lambda.com/faq/question.aspx?id=99

[TomS_]

This got me thinking...

My scope is plugged in to a surge protected power board/extension lead. Those things have caps and MOVs and other bits and pieces in them...

Its very subtle, and you have to look close, but it seems the neon which indicates "surge protection active" is lit. From a distance it doesnt look like anything, maybe just some light reflecting from the glass bulb of the neon through the coloured window, but upon closer inspection it is definitely putting out some light.

I moved my scope to another power board which doesnt have this neon lit, and Im not seeing the same thing. Meavuring voltage between USB ground and scope ground I get millivolts if anything. So maybe the surge protection of this other one is busted or leaking or something.

I could buy a new one, or I could try to fix this one I guess, I'll just have to buy some parts as I dont tend to work with higher voltages like mains.

[Fungus]

It may be that you have a bad ground connection somewhere, check your plugs and mains cables.

If your USB is from a laptop then it will probably be floating and it might simply be floating very far from mains ground.

nb. The entire reason you attach a ground clip to get rid of these differences in GND while you take a measurement.

Using my multimeter I measured AC voltage between the USB ground and the oscilloscope ground I see about 60VAC?? 

This is consistent with a floating power supply. You can have a voltage difference but there should be no current flow (at least, not after the capacitors in the power supply have charged to the same level as 'scope GND).

If the sparks annoy you then you could attach the USB GND to mains GND with a separate wire and leave it there while you're working.