2500KVA xfmr service entrance not grounded - ?

[francis.key19]

Hi,

At our site, we have two 2500KVA service transformers (12470v primary, 480v secondary). X0 terminal is not grounded and not brought inside the building.

On one of the transformer, the X0 terminal is connected to the frame of the transformer. This transformer is a wye-wye transformer.
On the other of the transformer, the X0 is not connected at all. This transformer is a delta-wye.

For many years, the folks here thought that these are delta ungrounded systems on secondary side. There is no ground fault detector in either of the transformers. My questions are:

1. Is this a violation of the NEC code.?
2. Should we have the X0 grounded and bonded to a ground electrode? What would be the impact to existing three phase systems.?
3. Do we need to install ground fault detector? There are no main CB on either transformer (secondary side), but there are several conductors leaving the secondary and wired to indivisual CBs (i.e. no bus or switchboard) mounted on walls. Is this allowed under NEC?
4. For many years, one phase could go to ground and noone would know it until the next phase go to ground. This created havoc with the equipment. I am looking for suggestions on how to improve safety of these systems. The site is over 60 years old and we just recently came upon this.
5. What would be the impact of not grounded the X0 as related to short circuit and arc flash.

Thank you in advance for your help and guidance.

Regards,

FK19

 

[augie47]

Hi,

At our site, we have two 2500KVA service transformers (12470v primary, 480v secondary). X0 terminal is not grounded and not brought inside the building.

On one of the transformer, the X0 terminal is connected to the frame of the transformer. This transformer is a wye-wye transformer.
On the other of the transformer, the X0 is not connected at all. This transformer is a delta-wye.

For many years, the folks here thought that these are delta ungrounded systems on secondary side. There is no ground fault detector in either of the transformers. My questions are:

1. Is this a violation of the NEC code.?FK19

See 250.24(C) and 250.20
Where your XO has been grounded at the transformer, 250.24(C) would require a grounded conductor. (or not have XO bonded)

2. Should we have the X0 grounded and bonded to a ground electrode? What would be the impact to existing three phase systems.?FK19

If XO is bonded, you would need a neuitral to each service disconnenct.
Many folks feel there are advantages to a grounded system. I can see little advantage to an ungrounded system unless you have ground detectors

3. Do we need to install ground fault detector? There are no main CB on either transformer (secondary side), but there are several conductors leaving the secondary and wired to indivisual CBs (i.e. no bus or switchboard) mounted on walls. Is this allowed under NEC?FK19

NEC 250.21(B) requires detectors. Also, the advantage of an ungrounded system relies on your bring able to detect that 1st fault to ground.
As far as feeding multiple disconnects, NEC 230.71 basically limits you to six mains and they need to be grouped.
230.2 makes allowances for large buildings and services over 2000 amps. More details would be needed.
(with that size transformer, you most likely meet one or more requirements for multiple locations)

4. For many years, one phase could go to ground and noone would know it until the next phase go to ground. This created havoc with the equipment. I am looking for suggestions on how to improve safety of these systems. The site is over 60 years old and we just recently came upon this. FK19

again..ground detectors

5. What would be the impact of not grounded the X0 as related to short circuit and arc flash.

FK19

Not knowledgeable enough to advise

Thank you in advance for your help and guidance.

Regards,

FK19

 

[Sahib]

5. What would be the impact of not grounded the X0 as related to short circuit and arc flash.

The advantage of ungrounded system in your case is that there is no short circuit or arc flash hazard for any one phase to ground fault unlike a solidly X0 grounded system where such problem can be to an extreme level. But for phase to phase short or arc flash, both ungrounded and grounded systems can have the same level of hazard.

 

[francis.key19]

The advantage of ungrounded system in your case is that there is no short circuit or arc flash hazard for any one phase to ground fault unlike a solidly X0 grounded system where such problem can be to an extreme level. But for phase to phase short or arc flash, both ungrounded and grounded systems can have the same level of hazard.

Thank you for help.