Arcing Ground Fault

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George Stolz

George Stolz

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I have always been under the impression that an arcing ground fault is generally of short duration, and the primary concern that they present is the immediate and sudden superheating around the fault, resulting in burns of personnel.

Is the voltage potential on bonded parts a serious concern for shock potential?

Are some arcing ground faults sustained for more than a few seconds?

Thanks in advance.
 
To clarify what I'm getting at, there's currently another thread in progress where someone appears to have a lot of concern for the voltage present on exposed metal on the electrical system during an arcing fault. I never have studied much about the phenomenon, and was inclined to believe that the concern had little basis - but I have no solid research or experience to substantiate (or refute) that belief, so I'm asking. :)

So, how long can a long-lasting arcing fault last? What kind of voltage would be seen on the exposed metal during the event?
 
George Stolz said:
So, how long can a long-lasting arcing fault last? What kind of voltage would be seen on the exposed metal during the event?
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Any arcing fault 240V or greater supplied by a 125kVA or larger transformer can arc forever, or at least until something gives out.

Usually a protective device clears the fault, but if the arcing current is not high enough to initiate a procective device then it will just arc and arc.
 
George Stolz said:
I have always been under the impression that an arcing ground fault is generally of short duration, and the primary concern that they present is the immediate and sudden superheating around the fault, resulting in burns of personnel. ...
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George -
Here is some background, a couple of quotes from IEEE Buff book 242. They cover both solidly grounded which is what I think you were discussing, and ungrounded, which is an over voltage issue:

Ieee 242 8.2.2 Solid Grounding
One disadvantage of the solidly grounded 480 V system involves the high magnitude of destructive, arcing ground-fault currents that can occur. However, if these currents are promptly interrupted, the equipment damage is kept to acceptable levels. While low-voltage systems can be resistance-grounded, resistance grounding restricts the use of line-to-neutral loads.

Ieee 242 8.2.5 Ungrounded
Ungrounded low-voltage systems (see Figure 8-5a and Figure 8-5b) employ ground detectors (e.g., lamps or voltmeters connected from each phase to ground) to indicate a ground fault. These detectors show the existence of a ground on the system and identify the faulted phase, but do not locate the ground, which could be anywhere on the entire system. The system operates with the ground fault acting as the system ground point. The ground-fault current that flows is the capacitive charging current of the system, generally only a few amperes. If this ground fault is intermittent or allowed to continue, the system could be subjected to possible severe overvoltages to ground, which can be as high as six or eight times phase voltage. Such overvoltages can puncture insulation and result in additional ground faults. These overvoltages are caused by repetitive charging of the system capacitance or by resonance between the system capacitance and the inductances of equipment in the system.


George Stolz said:
...Is the voltage potential on bonded parts a serious concern for shock potential? ...
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I don't think so. My thinking is that if all of the non-current carrying metal parts, building steel are grounded, then little shock potential exists.

George Stolz said:
...Are some arcing ground faults sustained for more than a few seconds? ...
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Shouldn't. Relays are available to take these off-line quickly. Like a few cycles or less - (I think).

carl
 
Carl, I appreciate the response. :cool:

coulter said:
I don't think so. My thinking is that if all of the non-current carrying metal parts, building steel are grounded, then little shock potential exists.
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Now, do you mean grounded or bonded? This is kind of a crucial aspect of the other discussion.

It has been mentioned in other discussions regarding lightning that just the aspect of having all the equipment riding out the event at the same (even elevated) potential keeps damage from occurring.
 
George I have pictures of an arcing GF that burnt right past the main fuses intact (fried on the outside) to the utility finally oopened. I have seen this time and again.

Do they last long? what's long when the arc is so hot it vaproizes all metal along it's path.
 
George Stolz said:
I have always been under the impression that an arcing ground fault is generally of short duration, and the primary concern that they present is the immediate and sudden superheating around the fault, resulting in burns of personnel.

Is the voltage potential on bonded parts a serious concern for shock potential?

Are some arcing ground faults sustained for more than a few seconds?

Thanks in advance.
Click to expand...

The main issue with the so-called arcing faults is that thye don't last long enough to operate the ordinary OCPD. They exhaust themselves, BUT they establish a restrike path and re-ignite repeatedly. Eventually the repetead strikes generate enough heat that they result in a fire.

One of the most striking example I have seen was a 150' section of heat tracing with metallic grounded braiding over the tracing material. Out of the 300'+ long circuit the ground fault burned up more than half of it. The breaker newer 'saw' it. We discovered when our caustic line frooze up.

Arc fault protective devices employ a microrocessor that stores the I2T value of low level faults, computes the sum of them and compares it to the stored models. When the preset model quantity of the I2T value is reached it trips the device. It needs to be able to distinguish from many other overcurrents such as inductive or resistive inrush, motor starts, normal switching arcs - try to repeatedly switch ON/OFF a load on an AFCI to see if it is able to distinguish from a fault, etc.
 
weressl said:
One of the most striking example I have seen was a 150' section of heat tracing with metallic grounded braiding over the tracing material. Out of the 300'+ long circuit the ground fault burned up more than half of it. The breaker newer 'saw' it. We discovered when our caustic line frooze up.
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Why no GFP?
 
George Stolz said:
Carl, I appreciate the response. :cool:


Now, do you mean grounded or bonded? This is kind of a crucial aspect of the other discussion.

It has been mentioned in other discussions regarding lightning that just the aspect of having all the equipment riding out the event at the same (even elevated) potential keeps damage from occurring.
Click to expand...

Well slap me around the block twice - what was I thinking, especially after listening to bobby 0's diatribe. Thank you for the correction:confused:

Yes, Bonded.

carl
 
IMHO all of these discussions of grounding, bonding, earthing, etc. should probably be split into about 10 separate well defined threads...and on top of the confusion caused by multiple different discussions in a single thread, we are also dealing with some language barriers as well.

As has been mentioned above, the big problem with arcing ground faults is that they can continue for long periods of time without opening the breaker. Laszlo's description of 150' of heat trace getting eaten away is a good example of how long this can go on.

Such a fault will cause current to flow on the EGC/bonded metal system, and voltage drop in this system could cause a touch potential between one part of the system and another, causing a shock hazard. However the same impedance that keeps the fault current low enough that the OCPD doesn't respond will also mean that the voltage drop in the EGC/bonded metal system is lower than might be expected in a bolted fault.

-Jon
 
winnie said:
all of these discussions of grounding, bonding, earthing, etc. should probably be split into about 10 separate well defined threads...
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Jon I could not agree more, with 'well defined' being the key.

Those two threads Bobby started are a rats nest of various topics and IMO because of that the threads are useless.

Roger closed one already and I think the other is sure to follow.
 
I once saw over 350" of busway blown, 1st 100' burnt beyond recognition last 250' every 90 burnt up, Main switch toasted.

The GFP did trip, BUT the building personnel adjusted the GFP settings and kept trying to close the Main Line Switch, till it Blew.
 
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