Converting from ungrounded to grounded 480 system 1

[myersro]

We presently have a 480vac Y ungrounded distribution system. We bring in 69kv from the utility and reduce it to 12470v in our yard substation. Then we distribut 12470 delta into the building and reduce it to 480 wye in 7 substations. When we have a ground fault in our system, we detect it with ground lights and we try to locate the problem at the next down time. This does not always happen in a timely matter and we have had many failures because of the ground fault. Sence we are not repairing the faults we are losing the advantage of the ungrounded system. I am looking at converting the system to a grounded system because our processes are not critical. (we make cereal and breakfast bars) Is there any problems that any of you have experienced when converting to a grounded system?

 

[alehman]

You may be requierd to or want to add ground fault protection on the 480V systems. You should perform short circuit calculations to make sure your ground fault current doesn't exceed any ratings.

 

[TestBeforeTouch]

I would install a high resistance grounded system with a pulsing circuit on the resistor instead of a solidly grounded system. The high resistance grounded system gives you all the benefits of the ungrounded system (continuity of service, reduced equipment damage and arc flash for single phase faults) plus the ability to locate a ground fault in a timely manner. One manufacturer of this equipment can be found at

http://www.startco.ca

 

[dpc]

A high-resistance grounded system has a lot of advantages, but it does require the same diligence in tracking down ground faults as an ungrounded system. Since that appears to be a concern, it may be prudent to go with a solidly-grounded 480V and use the brute-force approach.

Since you have ungrounded wye transformers now, converting to solid grounding is straightforward - just ground the neutral points. But you also have think about where the fault current is going to go when you have a ground fault. With a solidly-grounded system you need a good fault return path to ensure that the upstream protective device will see enough fault current to operate. If you are relying on an old steel conduit system for your equipment grounding conductors, you need to make sure connections are good and that the conduit system is a good ground conductor.

Also, I agree with alehman, you need to consider need for the addition of ground fault protection. This is required for larger circuits per the NEC.