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Old 22nd February 2013, 06:07 PM   #1
6L6 is online now 6L6  United States
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Default Ground loops through the AC earth

Is there a simple (yet safe) way to stop the possibility of ground loops made from scope - function generator - DUT through the AC earth?

I am going to get a cheater mains plug for the scope and gnerator to lift the AC ground - is that OK? There must be a better way.

Thanks in advance to those more experienced in this.
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Old 22nd February 2013, 08:01 PM   #2
DF96 is offline DF96  England
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Isolation transformer.
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Old 22nd February 2013, 08:07 PM   #3
6L6 is online now 6L6  United States
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Ok, that's a logical answer, but only if I know what device to place it on... )

'Scope? Function generator? DUT? All of them?
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Old 22nd February 2013, 09:51 PM   #4
tvrgeek is offline tvrgeek  United States
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Back in the lab (out in your neck of the woods actually) once when I had to trace a highly intermittent issue (glitch on ground) I stole every UPS off the factory floor and floated my scope and analyzer on batteries.

By ground loop, you are concerned that safety and neutral are bonded in the panel, but the are not quite the same by the time they get to the outlet. First, the question is why? Is your problem line harmonics or RF? Is it coming from the lines, or is it coming from something on your bench? Things like PC's and many appliances can do really ugly things to the power lines.

Correctly designed line filters work quite well. It is not unheard of to lift safety by 10 Ohms or so. Just ton't tell UL.

Trip-Lite iso-bars have RF filters on each set of outlets. That's all I use on my bench. Seems good enough. My measurement noise floor for 60 Hz harmonics is about -130. Second harmonic is usually the worst due to full wave rectifiers.
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Old 9th March 2013, 09:15 AM   #5
warrjon is offline warrjon  Australia
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In the lab I use a power board with the earth pin removed plug all gear except one into the power board and (I usually use the Fluke 5720A ) plugged into the power so all grounds are through the 5720. This is the procedure for measuring uV when the tolerance a couple of nV.
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Old 9th March 2013, 09:32 AM   #6
AndrewT is offline AndrewT  Scotland
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Don't float the scope.

If you do disconnect the scope ground and clip the probe ground to a Live part the whole scope is now at Live voltage.
Can you be sure that there is no exposed conductive part on the scope that is just waiting for you to touch !!!!!!!
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Old 10th March 2013, 08:50 AM   #7
warrjon is offline warrjon  Australia
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Quote:
Originally Posted by AndrewT View Post
Don't float the scope.

If you do disconnect the scope ground and clip the probe ground to a Live part the whole scope is now at Live voltage.
Can you be sure that there is no exposed conductive part on the scope that is just waiting for you to touch !!!!!!!
Yes you are correct there, I don't use a scope often and when I do it's mostly for setting up a pulse train before a test.
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