LV ungrounded system - HVAC power supply 2

[cherryg222]

We have a project in Mexico, where the client has insisted on ungrounded (delta-delta connection of Transformer) system for Low voltage (480V). From the design basis, it is clear that the purpose is to maintain power to process loads, which cannot be shut down without proper supervision and safety.
Now comes the fun stuff. We are also asked to supply HVAC loads from the same process MCC which is a part of the ungrounded system.
Now my question is: Does NEC (Article 250.21) allow ungrounded system for HVAC? Article 250.21 of NEC is pretty specific about ungrounded systems and where they are permitted.
Specifically, in my opinion, HVAC systems are far more susceptible to manual intervention. Many a times, even a nearby safety switch may not be operated to investigate a problem in a HVAC unit. If a L-G fault exists, and the person/persons touch the equipment, even though the system is ungrounded, leakage current in the tune of 1 to 2A per MVA can flow due to capacitive ground of several kms of lv cables. In my opinion, all non-process loads like HVAC should be supplied separately from solidly grounded system (separate Transformers + LV MCCs).

One more piece of info: Mexico country code is NOM-001-SEDE-2012 which is an adopted version of NEC.

Let me hear what is your opinion on this.

 

[dpc]

Ungrounded 480 V systems used to be the standard. We've moved away from them for a lot of good reasons, but there are still many in service. Last time I checked, still allowed by NEC under 250.20. A 480 V delta system cannot be grounded such that voltage to ground does not exceed 150 V, so it is not **required** to be grounded. (looks like 250.20.B(1)

I wouldn't recommend it, but it's still allowed, AFAIK.

 

[waross]

Locally ground all equipment. Consider gradient control mats near grounded equipment.

We have a project in Mexico, where the client has insisted on ungrounded (delta-delta connection of Transformer) system for Low voltage (480V).

I suspect your client has been reading some very old text books.
"With a delta system, in the event of the loss of a transformer, the system may still be powered at 57% capacity."
A couple of things.
That was the norm when transformer windings were insulated by cotton threads. SCC and DCC, Single Cotton Covered and Double Cotton Covered wire.
Yes the system could be powered, open delta, AFTER the OUTAGE caused by the failed transformer.
You may wish to make your client aware of the dangers of high frequency overvoltages caused by arcing ground faults.

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

 

[Parchie]

@cherryg222 (Electrical)

Yes, you can power your HVAC in an ungrounded system! The requirement for uninterrupted processes, even with some minor faults on the system is possible. They are decided by economic reasons and has nothing to do with whether the owners there are old school or what!

I have been involved in a similar project, in my other life which relies on technologies that protect the electrical system from the usual downside of having an ungrounded system! A GPT is installed on those low voltage switchgear and a 67N protection is functional, even better than that of a solidly grounded system. A pulsing high-resistance ground fault location system is installed that allows for the operations to continue, while locating the faulted feeder is being done by the electrical crew.

 

[waross]

Trivia, dpc. I have never worked on an ungrounded delta 480 Volt system.
The last ungrounded delta system that I had the misfortune to encounter was so long ago that the standard voltage was 440 Volts. grin
Not the standard voltage in Canada when I worked on it about 50 years ago, but the standard voltage when it had been installed new many years before that.


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

 

[jghrist]

Yes, you can power your HVAC in an ungrounded system! The requirement for uninterrupted processes, even with some minor faults on the system is possible. They are decided by economic reasons and has nothing to do with whether the owners there are old school or what!

A high resistance grounded system will give the advantages of ungrounded without the worries of high frequency overvoltages caused by arcing ground faults.

 

[cherryg222]

Thank you all for your valuable comments. Indeed, some old practices are in play here. I have used 5A HRG for 480V system, with gf detection both on Transformer neutral as well as on individual feeders. Usually, the neutral gf detector provides an annunciation for action. Once technician arrives in Substation, the front facia unit will identify the faulted unit. I have never worked with an ungrounded system. I believe companies like Bender have solutions for ungrounded systems also.