ECN Forum Forums General Topics of the Trade General Discussion Area 480 Delta

I was talking to a guy from Massachusetts (electronic engineer) and he says 480 delta is common up there for industrial applications. How is that generally grounded? He didn't know but he says he thought it was ungrounded. That did not sound right.
I have seen plenty of 480/277 wye but I have never seen a 480 delta down here.

Never mind, we determined that it was corner grounded delta. He just did not realize it.

We have several mission critical facilities that use an ungrounded delta system.
In the event of a ground fault the system stays energized until it can be corrected.
The only time that the system goes down is when there is a second ground fault before the first one gets cleared.

An ungrounded delta is kinda capacitively coupled to the Earth, so a ground detector device is used.
There are 3 lights wired from phase to a ground rod and all glow continuously at the same brightness.
If one of the phases has a ground fault that light will go out and the other 2 get much brighter.

That tells you which phase the ground fault is on...but finding where it is becomes very challenging.
One of our facilities has had a ground fault on one of the phases for several years...and the maintenance folks are still trying to chase it down.
I think there may be a ground in 2 places on the same phase; which is like trying to find the 2 bad bulbs in an old string of Christmas lights.

are you sure that's not a high impedance grounded neutral system?

i was at a plant with a set-up like that and i can't remember the exact fault locator they had but we traced the ground fault with some type of frequency fault locator, even through metal conduit and enclosures, found it pretty quickly ( i think 1 hour)

Nope.
It's a true ungrounded 3 wire Delta.
Since the facilities are critical we can't shut them down unless we do a major upgrade so most of the existing Services are the original ones from the 1930s...or earlier.
One of them still has the oil insulated vertical drawout cast iron MV circuit breakers.
Major upgrades are designed with a conventional 3Ph4W system.

the reason i ask is you said this about the indicator lights



i don't think the light would light up if it was completely ungrounded,

this light set-up is what is used on high-impedance grounded systems, a first ground fault will only be low current like 5 amps(i don't remember for sure). the lights do the same thing you are describing.

I may have misspoke, but it's been a while since I've had my hands inside one of these units (for obvious reasons).

The '3 lights' style of ground detector works on a true ungrounded system such as the ones installed back in the 30's because the phase conductors are only capacitively coupled to ground.
There is no physical connection between any energized conductor and ground in our systems.

http://www.ece.mtu.edu/faculty/bamork/EE5223/EE5223TermProj_Ex3.pdf

BTW - Our new installations take advantage of high impedance grounding to maintain system integrity while reducing the potential for catastrophic failure if a second ground fault should occur before the first can be tracked down and cleared.

thanks for that link, looks like some good information. do the lights light up dimmly though without a ground fault on a true ungrounded system?

Yes. Under unfaulted conditions all 3 lights are dimly lit.
That also actually acts as a good safety check because if a light burns out you know it right away.
Generally they last forever because they are lit at only a fraction of the voltage that they are designed for.

I'm curious. What you done so far to find the fault(s)? Any plans for a powering down the critql systems for maintenance?

I ask since I'm may be transferring soon and be working on ungrounded deltas. Always eager to learn something new