Corner Grounded Delta

[BenCrook]

My installation has several buildings that have ungrounded delta 480 volt 3-Phase systems. Has anyone had any experience in converting the ungrounded delta system to a corner grounded? Is it safe to do it? How is the best way to accomplish it?

 

[weh3]

Why do you want to do it?

William

 

[BenCrook]

I am wanting to convert it to a corner grounded delta system so that an overcurrent protective device can be persuaded to trip in the event of a phase to ground fault. We just recently experienced a situation where a 3-phase motor had a phase to ground fault occur and we had all sorts of wierd things happening, but tripping an overcurrent protective device wasn't one of them-it can't complete the circuit.

 

[rbulsara]

bencrook:

How big a system you have? What size service transformer(s)?
Are there individual transformer for each building?

You may want to know why was it installed as ungrounded system in first place? What kind of occupancy this buildings have?

Nonetheless, based on what you posted, I do not see any problem with grounding one corner of a delta system. It is not very common, but it is one of the recognized method sof grounding, even by NEC. It is safe.

Be sure to identify and educate the operators and maintenance persons of the same.

There is nothing special to it, you take one the phase at the service entrance equipment and bond it to the grounding electrode/water pipe/ buiding steel etc, as you would with a neutral of a 4 wire system.

Also make sure all of your equipment enclosures are firmly bonded to the ground (not to the grounded phase, except for the service entrance equipment)
If the a common transfomer/service feeds multiple buildings, make sure you ground the same 'phase' at every service entrace.

Ungrounded systems, should have had some kind of ground fault monitoring system, do you have that?

 

[BenCrook]

Thanks for your comments.
The buildings have a 12,470 volt 3-phase ungrounded delta overhead distribution (no neutral or static line) coming to a service pole (They each have their own service). The service poles have three single phase cans connected delta-delta and most of them have 75 kva per can or 225 kva for the 3-phase service and the service into the building is 480 volt 3-phase delta (no ground wire). The building function has both sensitive electronics and hazardous (explosive) materials. The buildings were built back in the 1940's so I don't think I will find out why the system was installed as ungrounded delta and at that point in time it may have been just to save the cost of the wire.
In answer to your last question-no there is not a ground fault monitoring system installed--I wish that there were, it would have saved me some grief on the initial round of troubleshooting the problem.

 

[davidbeach]

If you corner ground the system, and if you have any fuses you will need to remove the fuses on the grounded phase and replace with jumpers. NEC240-22.

 

[rbulsara]

OK..

The GF monitoring systems are not expensive.. you may want to look into that first. Specially keeping the 'explosives' in mind.

Also you may want to review NEC and consult your local inspector on this issue. Although not very clear in NEC, article 250.20 (B)1 "implies" that if the LV system is grounded, L-G voltage should not exceed 150V, in the USA.

It in fact says that if you can ground a system so that you do not exceed 150V L to G, then you must ground a system. But does not seem to prohibit having 480V to ground, I think.

 

[rbulsara]

good point, david.

 

[davidbeach]

rbulsara,

The requirements you reference are used to prohibit ungrounded 208Y systems. It could also prohibit a corner grounded 240V delta since using a grounding transformer a 240V system could be grounded with a L-G voltage of only 139V. There is no way of grounding a 480V system (or any over 260V L-L) to meet the 150V L-G standard.

 

[rbulsara]

i agree..

 

[dwftx34]

rublsara and davidbeach, I belive the refernce in 250.20(b)(1) is to insure 240/120v single-phase or split single-phase systems are grounded. Not to "imply" the voltage of a system to ground is limited to 150v L-G.

 

[resqcapt19]

If you change this to a grounded system, the the grounded conductor identification rules in Article 200 will apply. The grounded conductor will have to white or gray if #6 or smaller. If larger, it can be white or gray or identified white or gray using phase tape.
Don

 

[wfowfo]

Quote:
"The buildings were built back in the 1940's so I don't think I will find out why the system was installed as ungrounded delta and at that point in time it may have been just to save the cost of the wire."

A bit off topic....

3 wire ungrounded deltas were installed as a means of preserving power when only one phase went to ground. On older 12.5 kv distribution systems when lines were longer, more remote, and had fewer lineman, one phase could get grounded (ie-trees, broken pole, etc.) without interruption of service. Repairs could be effected when possible.

The same theory was used in plants to keep processes from shutting down and ruining product. Ground detect devices would notify personnel of a ground, and repairs were performed at the next scheduled shutdown.