Single Pole Ground Fault?

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MD88

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Hello,

I was doing some research on switchboards vs. switchgear, and I came across a discussion about ground faults, and that the MCCBs in switchboards have a very low single-pole ground fault, despite the fact that their short-circuit rating can be similar to that of switchgear. Because of this, switchboards should only be used on solidly grounded systems... I kinda realized that I don't really understand ground faults, and I definitely don't see how a single pole fault would affect an ungrounded or resistance-grounded system differently than a solidly grounded. Does anyone know of a very basic and practical explanation of what happens in a ground fault, with the various configurations? Thanks!!

Here's a link to the original document - see the section near the bottom of page 2
http://www.geindustrial.com/Newsletter/ind_nov01_left3.pdf
 
MD88 said:
Hello,

I was doing some research on switchboards vs. switchgear, and I came across a discussion about ground faults, and that the MCCBs in switchboards have a very low single-pole ground fault, despite the fact that their short-circuit rating can be similar to that of switchgear. Because of this, switchboards should only be used on solidly grounded systems... I kinda realized that I don't really understand ground faults, and I definitely don't see how a single pole fault would affect an ungrounded or resistance-grounded system differently than a solidly grounded. Does anyone know of a very basic and practical explanation of what happens in a ground fault, with the various configurations? Thanks!!

Here's a link to the original document - see the section near the bottom of page 2
http://www.geindustrial.com/Newsletter/ind_nov01_left3.pdf
Click to expand...
The diagram at the end explains what they are saying. For an ungrounded system, one breaker (one pole) will have to interrupt a line-line fault which can be close to a 3-phase bolted fault value. The current flows line-line.

For a system with a neutral, the current flows line to ground and the one pole will interrupt line-ground current. In the grounded neutral case, a double-line to ground will be interrupted by two breakers (two poles).
 
Hmm.. I'm not really understanding how, in the 2 fault scenario, having a solidly ground neutral would a) make the fault current going through the first pole any less and b) trip the second pole.
 
MD88 said:
Hmm.. I'm not really understanding how, in the 2 fault scenario, having a solidly ground neutral would a) make the fault current going through the first pole any less and b) trip the second pole.
Click to expand...
The impedance is less for a line to line fault so you have more fault current.

Think of the tripping like this:
In the first case, only the fact that the second line went to ground (i.e. was connected to the other line) made a fault condition. When any one breaker clears, the fault is over.

In the second case, either line contacting the grounded neutral creates a fault and both breakers will have to clear for all fault current to stop.
 
Solidly grounded, the breaker pole only has to open an arc across phase-ground voltage. Two poles in series will interrupt phase-phase faults.

On an ungrounded system the single pole could have the full phase-phase voltage impressed across its contacts, making arc interruption more difficult. Some MCCB's are rated for this and some are not.

If it says 480/277V it's not.

(Shoot me down if I have it incorrect.)
 
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