Safety and Instrument Ground

azeem_33@hotmail.com · Jan 31, 2014

I am experiencing a strange situation. The safety and instrument grounds are connected to separate earthing pits, the land has high moisture and salt contents, therefore the resistance between safety and instrument ground is 60 ohms approx.

The problem is when i connect multi-meter, it beeps when connected in one direction, if the multi-meter terminals are reversed there is NO beep.

Any suggestions, what might be causing this ???

osmanmomin · Feb 1, 2014

azeem_33@hotmail.com · Feb 1, 2014

Lolz, i am talking about the connectivity test with multimeter, which causes a beep when there is a short circuit.

ktmtragic · Feb 1, 2014

Start with another meter

Tharon · Feb 1, 2014

I never liked using the beep test on a multimeter.

milldrone · Feb 1, 2014

Here is what the folks over at Mike Holt's electrical forum have to say about separate ground(ing)
http://www.youtube.com/watch?v=qNZC782SzAQ

Tharon · Feb 1, 2014

Sorry to hijack this thread, but I've got a question about his comments at the end about the welder.

Our shops use large steel tables to do all their welding work. They always put the "Work" lead onto the table, and weld with the steel component sitting on the table.

We've always permanently bonded these tables to the steel structure of the building (which is also bonded back to the main distribution source). Is that still recommended? We've always been afraid that if the work piece lead became disconnected from the table that the welder might use the person as a path back to the source.

azeem_33@hotmail.com · Feb 2, 2014

I have already tried changing the multimeters, serval of them infact, but the result is the same.

ktmtragic · Feb 2, 2014

High moisture low resistance 60 ohms is too high anyway. How far are the earths apart?

osmanmomin · Feb 2, 2014

1.) Multi meter type?
a.) Earth Tester?
b.) Instrument Tester?
c.) Standard multi-meter?

2.) Standard multi-meter -> selection range kohm/ohm during “beep”?.
3.) The red probe on what side and black probe on what side during ”beep”.
4.) Recall the principle of diode + and - polarity . Test with diode with ur multimeter forward and reverse probe.result?
5.) Check/measuret instrument to ground with multi meter selection small range AC VOLT.
Any value?
6.) Safety Earth/Ground come from MAIN TRANSFORMER.
7.) Instrument Ground come from ???.
Nowdays earth and instrument already mess up,he,he.

danw · Feb 2, 2014

5 stars for Milldrone's link

shooter · Feb 3, 2014

60 ohms is too high, to avoid ground loops, all grounds should be connected on 1 rail or bar anywhere.
And yes instrument ground should be possible to disconnect, to check for these ground loops.
The main ground wire should be same size as the feeding wires are (or 0.5 above thumbthickness(hihi)
with a multimeter it is impossible to measure as the voltage is too small to detect.
about 1 micro Amp is enough for the modern meters.

Tharon · Feb 3, 2014

shooter said:
The main ground wire should be same size as the feeding wires are (or 0.5 above thumbthickness(hihi)

Depends on your local codes. Like here in the US, that is not the NEC regulation.

Lancie1 · Feb 5, 2014

..the land has high moisture and salt contents, therefore the resistance between safety and instrument ground is 60 ohms approx.

For wet salty soil, you should be able to get 5 ohms or less between two separate grounding systems, IF you use concrete-encased electrodes installed according to the US National Electrical Code, Article 250.52.

A higher voltage on one grounding system relative to the 2nd one will cause a meter conductivity check to beep in one direction but not the other.

Safety and Instrument Ground

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