Floating Earth Faults

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  • #26
dlgoff
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There do exist devices called "Electrostatic Voltmeters" that measure the electric field surrounding a charged object.
One of those could be used to measure the unknown potential difference posed in fonz's original post....
How neat and simple. I had no idea this existed. :approve:
 
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"Somebody else" would be me? To refresh your memory:

If you connect just ONE classical voltmeter between one voltage source terminal and ground the voltmeter will quickly show reading 0 in most of the cases.
If you connect TWO voltmeters, first one between one terminal and ground, second one between other terminal and ground, and the voltage source is ideally balanced wrt ground, you'll have ±V/2 readings.
Thanks again zoki. I'm struggling with this though could you briefly explain why this is the case? If not at least refer to some text that might explain.
 
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Thanks again zoki. I'm struggling with this though could you briefly explain why this is the case? If not at least refer to some text that might explain.
I think your questions are answered and exemplified fine. As for the references, a typical textebook that teaches fundamentals of electrical engineering would suffice.
 
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jim hardy
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I'm struggling with this though
A traditional voltmeter has resistance.
Two voltmeters in series make a resistive voltage divider.
If they're of equal resistance, half the voltage will appear across each.
It's really that simple.
If the junction of those two meters happens to be grounded; well, that's what zoki is telling you.

Most ground detectors work on that principle.

Real world example: (Oh No, here comes another boring anecdote !)

In the power plant we had 125 volt battery power system for stuff that's really important like the emergency lube oil pump for Mr Turbine.
That battery itself was un-earthed so that a ground fault wouldn't trip something important.

To tell us when an accidental ground appeared we connected two 120 volt (25 watt) light bulbs in series across the battery, so each glowed a kinda dim orange. The junction of those lights we earthed.
Draw that on a piece of paper for yourself. It's just a voltage divider with middle grounded.
Now, should a ground fault come along on either the + or - side of battery , it'll put low resistance in parallel with one of the light bulbs.
Add that to your sketch.
So, one light goes out and the other goes to full brightness.
Those "DC Ground Lamps" were located prominently on the board so the operators knew right away we had a DC ground and could dispatch electricians to find it.
In the nuke plant we put similar lamps on our 120 VAC instrument power system which was also floating(and quadruple redundant).
Clamp-on ammeters were the basic instrument for chasing ground faults. Since lamp current returns through the faulted wire a sensitive meter around both wires of a circuit with a fault on it will report that lamp current.

I wanted to invent GFCI-type circuit breakers for such applications that'd illuminate a LED instead of tripping. I think somebody has done that but they aren't widely used. They'd be a godsend for maintenance men.

old jim
 
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Averagesupernova
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I always like your boring anecdotes Jim.
 
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You can make the same with 3 Ph AC -- resistors and light bulbs. Still used today in older factories with 3 Ph ungrounded or high impedance grounded systems.
 

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