480 volt ungrounded delta

[peetey]

When we have a garden variety 480 volt ungrounded delta service (secondary side) how is it defined by the NEC or in more technical terms? I want to have it defined technically as to distinguish it from other variations of 480 volt transformer arrangements. I like to be very precise about that which I am speaking about, and I need this in the most technical and precise manner that it can be defined. This is for an apprentice so I need to impress him!!

peetey

 

[dpc]

I think "480 V ungrounded delta" is pretty definitive.

If you consider this a "garden variety" system, your plant is in need of some upgrading. IEEE has advised against use of ungrounded 480 V systems for at least 40 years.

 

[peetey]

How much current flows to put any one phase in phase with ground by grounding it?

 

[davidbeach]

Charging current, depends on system size and configuration, usually very small. High resistance grounded systems provide all of the benefits while avoiding the worst problems with ungrounded delta.

 

[apowerengr]

dpc is there a specific IEEE reference? I have a customer requesting a 480 ungrounded service for some painting equipment they intend to purchase from an European equipment supplier.

 

[dpc]

I'd check the Green Book (Grounding) or the Red Book (Industrial Power Systems). I don't have them in front of me right now.

Ungrounded 480 V systems have issues with transient overvoltages due to repetitive re-strike of arcing faults charging up the capacitance to ground.

At present, the most favored system is probably high-resistance grounding if there are qualified electrician on site.

 

[7anoter4]

I agree with dpc.
See:
i-Gard Ground Fault Protection Ungrounded Systems to High Resistance Grounding

http://www.i-gard.com/pdf/application guides/Conversion Guide.pdf

"Ungrounded systems employ ground detectors 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.
If this ground fault is intermittent or allowed to continue, the system could be
subjected to possible severe over-voltages to ground, which can be as high as six or
eight times phase voltage. This can puncture insulation and result in additional ground faults.
A second ground fault occurring before the first fault is cleared will result in a phaseto- ground-to-phase fault, usually arcing, with a current magnitude large enough to do damage, but sometimes too small to activate over-current devices in time to
prevent or minimize damage.
Ungrounded systems offer no advantage over high-resistance grounded systems in terms of continuity of service and have the disadvantages of transient over-voltages, locating the first fault and burn downs from a second ground fault.[ IEEE 242-1986 7.2.5]"
see also:
Draft IEEE Guide of Protecting Power Transformers para.7.4.1

http://www.pes-psrc.org/k/k1/C37-91Draft05.pdf