ECN Forum Forums Code Related Discussion NEC & other Code issues Xformer Grounding

Since under the conditions of 250.20(B) we are not required to "Bond" the centerpoint of a why 480v 3ø system, could someone explain to me why we would not do it anyway?

Can’t cite code, but “general rules” are to solidly ground if voltage-to-ground can be limited to 150V, or solidly ground if ø-n {120, 277 or 347V} loads are served. Ungrounded low-voltage distribution {Y or ∆} is permitted, but can be unreliable and disastrous from insulation damage through transient overvoltages that develop across the inherent ø-g capacitance in each phase.

In non-utility-owned, 240-600V systems where service continuity is desired in non-solidly-grounded systems, high-resistance grounding and ground detection are recognized as a reasonable compromise. Typically, non-solidly-grounded 480, 600, 2400 or 4160V are amenable to hi-r grounding.

99NEC450-5(c) Ground Reference for Damping Transitory Overvoltages is a salient reference.




[This message has been edited by Bjarney (edited 07-23-2004).]

You may also leave the neutral and not take it as a circuit conductor and not ground it. In that case, you would have a 480-volt, ungrounded, delta, 3-wire feeder. You are permitted to "corner ground" the delta, put in ground detection equipment, or set it up just as you would any delta system. After the feeder leaves the confines of the transformer enclosure, you cannot determine that its configuration started out as a wye.

George are you questioning when to ground a system, or when to not ground a system? Or are you concerned with the location and placement of the main binding connection??

Cpal- I guess I could of worded the question better. The issue is not if I have to ground the system because Code has an exemption for a 480v system under some conditions. That being the case, and I have the option of not grounding the system, should I ground it anyway and why? Example: 480v welder circuit- no grounded conductor needed, Parking lot lights (480v) no grounded conductor needed. I know I need an equipment grounding conductor but I don't need a grounded conductor. Why or why shouldn't I ground the system?

George, if ø-n loads are not served, a very good tradeoff is using a resistor between transformer-secondary neutral and equipment-grounding conductors/grounding electrode.

Ungrounded low-voltage systems are trouble .




[This message has been edited by Bjarney (edited 07-25-2004).]

Thank you Bjarney, this is the kind of information I was looking for. Many thanks.

High-resistance grounding has the advantage of no overcurrent-device with the first phase-to-ground fault—a.k.a. “process continuity”. But, if ignored the second phase-to-ground fault will cause operating of two overcurrent devices. Ground detection MUST be installed, monitored an acted upon rapidly at first-fault inception, or the whole idea is worthless.

Generally, ungrounded systems can be retrofitted with high-resistance grounding and simple ground detection at reasonable expense. One online paper is www.neiengineering.com/papers/paper1JN.pdf




[This message has been edited by Bjarney (edited 07-25-2004).]

I don’t believe the parking lot lights could be connected to a 480 volt ungrounded transformer. I don’t have a copy of the NEC in front of me but doesn’t article 225 limit the voltage to ground to a maximum of 277 volts? Since a ground fault on one phase would be undetected your voltage to ground could easily be 480 volts.

From a 99NEC point of view, these sections seem to support outdoor amd highbay >277V lighting with limitations.

§225-7. Lighting Equipment Installed Outdoors. (d) 600 Volts Between Conductors. Circuits

§210-6. Branch-Circuit Voltage Limitations. (d) 600 Volts Between Conductors.

§250-21. Alternating-Current Systems of 50 Volts to 1000 Volts Not Required to Be Grounded.

§410 Part P Special Provisions…