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Measuring Mains with Rigol DS1052E

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ciccio:
Thanks PetrosA for your response:
I've verified this years ago with the Electric Power Company's technicians, after one of my many calls for service.
-The neutral to hearth bond is at more than 400 meters from my house (in the company's big MV to LV transformer). Lots of new building between.
-I use a single phase of a 3-phase network (the other 2 are used only occasionally by the condo's  common system (stair and external  lights, etc) and most of the time they are  unloaded. The other buildings may be connected to the same phase than mine, or to  one of the others. Nobody knows...
-The ground is connected to hearth  at my house, with 3  earthed poles  (verified with a Megger type instrument) so my RCDs work reliably.
-The neutral is connected to ground at the transformer.
-The neutral (and also the live) ables that supply my house are actually undersized, but the power dropped is acceptable to the company's standards.
The voltage between neutral and ground varies with the day of the week (different loads), with the hour of the day and with the season (summer: more air conditioners: more current draw).
Early this morning I has less than 5 volts, but in no case, even at it's maximum,  it could light a neon voltage tester..... If this happened,  the Power Company would surely intervene (or at least I was told so by one of the technicians)

This situation is "normal": when I was younger and was a touring "sound  engineer's helper", we had similar problems with power supplied to our stage's systems, and we never trusted both live and neutral.

IanB:
It reminds me of a time many years ago when I was delivering and setting up some equipment on an industrial site on the other side of the world. We had some low voltage power supplies with earthed metal cages and when we happened to rest them on metal strips under the floor there were sparks flying. We measured voltages and the "earth" wire was at a potential of about 100 V. As I recall the situation, someone had wired up the supply wrongly at the distribution panel and at least two of the live, neutral and earth wires were swapped.

vk6zgo:
Another way of looking at this is to say:-

"One side of my Oscilloscope goes to the mains earth,& the neutral line is ultimately connected to the earth,so I can get a fair idea of the 50/60 Hz signal by not connecting the probe earth to anything,or by connecting the probe earth to the earthed case of the device,& just probing the mains active lead."

Your Oscilloscope safety earth still protects you in this case,plus you can determine if your neutral & active connections are transposed.

The best way is, of course to use an isolation transformer so you can get a better idea of the mains waveform,as some HF stuff may be lost using the method referred to above.
In many cases,all you need is a rough idea,though.

On another point,I'm not sure why everybody freaks out about measuring mains voltages.
OK,the mains has a very low internal resistance/impedance,& can source huge amounts of current,but the
peak voltage of say,a 240v supply is less than 400v.

You can definitely look at voltages of this level with a Tektronix, HP,Siemens,Philips or BWD x10 probe!
Maybe the "El Cheapo" probes aren't as well rated! ;D

VK6ZGO

logicnibble:
So, the best way to make those measurements is to use an isolation transformer. Got it.

But if I had to measure it directly, can I use the Rigol DS1052E not connecting its mains plug earth connection?

If the oscilloscope is battery powered, I should no problem, right?

Kiriakos-GR:

--- Quote from: vk6zgo on August 03, 2011, 08:30:09 am ---On another point,I'm not sure why everybody freaks out about measuring mains voltages.

--- End quote ---

I will agree that fear its not a good adviser, you are less concentrated and you can cause a mess due your own panic.
Please exclude the electricians from your list,  we love Mains ... LOL   :)

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