Arcing Grounds - Ungrounded Systems

[NickParker]

How does arcing ground occur as opposed to the bolted fault through the charging currents of the unfaulted phases? Is it similar to the repetitive faults (momentary) due to winds?
How and why does the repetitive restrikes occur?

 

[waross]

Google "Spark gap transmitters". If the arcing ground is in a transformer of motor winding, it may act as an auto-transformer and superimpose a high frequency. high voltage potential on the wiring system.
This high voltage will be "trapped" between the high impedance to high frequencies of the supply transformer and the high impedance to high frequencies of the motors.
In my school days, I was privileged to overhear two instructors comparing notes of actual failures when they had worked in industry in the days of ungrounded delta systems.
They would experience a large number of motor failures until the arcing fault was located and repaired.
Due to the high impedance to high frequencies of the motor windings, the failures would be on the first few turns from the line connections.
One instructor related an incident where a 1000 Volt meter was destroyed by over-voltage on a 440 Volt circuit.
The fault was repaired without ever determining the actual maximum voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Even in "ungrounded" systems, there is an unavoidable capacitive coupling between the phase conductors and ground. When there is an intermittent fault, the unfaulted phases initially have phase to ground voltage equal to the normal phase voltage (e.g. 480 V). If the arcing fault stops, this voltage discharges through the capacitive coupling to ground based on the X/R time constant. But if another arc occurs before the voltage goes fully down, the voltage can increase to much greater than normal phase voltage. Similar to voltage doubler circuits in electronics. This can be quite nasty for motor windings.

If you search for "repetitive restrike" you'll find more information and a much better explanation than I've given you.

Cheers,

Dave

 

[NickParker]

When there is an intermittent fault, the unfaulted phases initially have phase to ground voltage equal to the normal phase voltage (e.g. 480 V). If the arcing fault stops, this voltage discharges through the capacitive coupling to ground based on the X/R time constant.

It seems, first I would need to understand the meaning of intermittent fault/arcing faults. Is my below understanding correct?
Arcing Fault:
An arcing fault is the flow of current through the air between phase conductors or phase conductors and neutral or ground. It can be caused by Loose connection, Dust, water or other contamination accumulate and cause insulation breakdown.
(for example, continuous arc between a terminal and the cubicle enclosure - for visualization)

Intermittent Fault:
Intermittent fault is the one which could happen only incase of accidents, or weather conditions forcing a phase conductor to touch another phase or ground.

 

[dpc]

Any fault other than a direct "bolted" fault is probably an arcing fault. An arc is current flowing through the air (or plasma). Many situations can cause intermittent arcing faults. On ungrounded systems, arcs often self-extinguish only to re-strike again.

https://www.eaton.com/content/dam/e...d-systems-from-intermittent-ground-faults.pdf