Elevated Phase-to-Ground Voltage, Ungrounded System
I have heard that one of the dangers of an ungrounded system is that the phase-to-ground voltage could
elevate due to a ground fault. Example: A phase-to-ground voltage for a 480 volt ungrounded system
could exceed 600 volts. Where can I get more information on this subject?
From: Neeser, Dan
IEEE Std. 242-1986 (Buff Book) addresses
this issue. Below is text from reference standard:
7.2.5 Ungrounded Systems (p. 278, 279)
"If this ground fault is intermittent
or allowed to continue, the system could be subjected to possible severe overvoltage to ground, which
can be as high as six or eight times phase voltage. This can puncture insulation and result in additional
ground faults. These overvoltage are caused by repetitive charging of the system capacitance, or by
resonance between the system capacitance and the inductances of equipment in the system."
"Ungrounded systems offer no advantage
over high-resistance grounded systems in terms of continuity of service, and have the disadvantages
of transient overvoltage, locating the first ground fault and burndowns from a second ground fault."
From: Welshons, Kenny KK SNORC
From the IEEE Green Book 142-1972:
1.2.14 Restriking Ground Faults. Field
experience and theoretical studies have shown that arcing, restriking, or vibrating ground faults
on ungrounded systems can, under certain conditions, produce surge voltages as high as six times normal.
The conditions necessary for producing
overvoltage require that the dielectric strength of the arc path build up at a higher rate after each
extinction of the arc than it did after the preceding extinction. This phenomenon is unlikely to take
place in open air between stationary contacts because such an arc path is not likely to develop sufficient
dielectric recovery strength. It may occur in confined areas where the pressure may increase after
each conduction period.
Neutral grounding is effective in reducing
transient voltage buildup from such intermittent ground faults by reducing neutral displacement from
ground potential and reducing destructive effectiveness of any high-frequency voltage oscillations
following each arc initiation or restrike.
Look at Pages 32 and 33 of the seventh edition of the
IAEI Soares Book on Grounding I edited. I cover the effects of arcing or sputtering faults on a 480
volt ungrounded system. Here, the actual voltage on the system can reach several times the normal
480 volts. This is due to transient overvoltage. These high voltages can wipe out motor windings.
As I recall, EC&M ran a series of excellent
articles on the subject of transient overvoltage on electrical systems a few years ago. The articles
may be available on their Web site.