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Old 01-14-2019, 11:35 PM   #1
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Default 480v Ungrounded Delta

Went on a service call today about fluctuating voltages to ground on a 200a 480 feed to a tenants unit in an industrial park. All L-L voltages in the units panel are 486v. Line to ground were around L1, 360v, L2, 270v, L3, 280v.

The 1600a feed in the electrical room measured the same.

Tenant has an electrical engineer employee that claims that its a corner grounded system and the ground goes bad. And only a few months ago it was reading to ground A=0v, B=480v, C=480v.

I did not see any sign of a bonding jumper in the electrical room. All disconnects tapped off of the gutter had all 3 lines fused. There was also a gutter with 3 bulb holders and 3 burned out 250v 15w bulbs on the wall in the electrical room.

That makes me believe that it is indeed designed as an ungrounded delta, as a corner grounded would not have the ground fault lights and wouldn't have fuses on all 3 conductors. Looking at the transformer bank that feeds this part of the building, there is no ground hooked anywhere to the conductors, just bonding the transformer cases together and to the slab rebar.

I have a feeling there was a long standing ground fault somewhere in the system, nobody knew due to burned out ground fault lights, the somebody fixed the fault or demoed out a circuit or feeder that had the fault.

The tenants engineer was not present today, and had just said that the grounding was "faulty" and and that the voltages now are "not referenced to ground properly - a very bad situation".

This 1600a drop feeds ~12 other units in this portion of the building. The place began as an engineered wood product manufacturer in the late 70s, sold mid 2000s, and is now leased industrial spaces. This tenant appears to have various wood sawing / planing /sanding machines and conveyors, so they obviously have VFDs.

What is the best method of remedying the situation if they won't use the ungrounded 480? Changing to corner grounded would not exactly be fun having to make solid grounded conductors at every fused disconnect in the place. I am thinking of putting in a ground fault detector with audible alarm at the service, and setting a 150kva 480 delta-wye xfmr in their unit to feed their panelboard.

There is a second 1200a service that feeds the other half of the facility, and part of their unit, that somebody had converted from ungrounded to corner grounded, and left all 3 phases fused.
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Old 01-15-2019, 12:02 AM   #2
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Ungrounded delta Services are RARE. The Pocos pretty much hate them.

Left unsaid: what exactly is being powered?

Paper mill/ lumber mill -- what?

THAT'S a HUGE signal as to whether the Service was and is 3-phase delta.

The average EUSERC firm is just not going to get an ungrounded Service.
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Old 01-15-2019, 12:22 AM   #3
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Originally Posted by telsa View Post
Ungrounded delta Services are RARE. The Pocos pretty much hate them.

Left unsaid: what exactly is being powered?

Paper mill/ lumber mill -- what?

THAT'S a HUGE signal as to whether the Service was and is 3-phase delta.

The average EUSERC firm is just not going to get an ungrounded Service.
Building was originally a sawmill / truss factory. Company sold / went under a few years back, and the facility got divided into units for lease. Tenant in this unit makes custom trim, siding, and such from exotic woods. Various sanding / planing lines, likely chock full of VFDs.

Power company supplies 3ph 13kv overhead, goes thru a CT metering setup at the road then to customer owned lines the rest of the facility.

I know for a fact it is a 3ph delta drop to this electrical room, I can see the the bussing on top of the transformers connected in delta. I just don't seem to believe that it is corner grounded like they are telling me, I think there was a long existing fault on one leg that nobody knew about, and made them think it was corner grounded.

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Old 01-15-2019, 01:09 AM   #4
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My hunch is that you've nailed it.

A corner grounded system would be obvious.

An ungrounded system ought to have indicator lamps.

What's the story with them?
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Old 01-15-2019, 01:47 AM   #5
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My hunch is that you've nailed it.

A corner grounded system would be obvious.

An ungrounded system ought to have indicator lamps.

What's the story with them?
Lamps are inside a locked electrical room where they are never seen, all 3 were burned out, disconnect that it was tapped off of didn't appear to have power either.

Planning on installing a premade 3r unit with a siren on the outside of the electrical room.
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Old 01-15-2019, 06:27 AM   #6
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I think your theory makes sense - they are mistaking a ground fault for a corner ground.

Would it possible the ground fault is intermittent, sometimes it's a dead short to ground and you see A-G = 0V, other times it's a high impedance fault and you see A-G = who knows what voltage?
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Old 01-15-2019, 07:52 AM   #7
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The utility transformer would likely also be grounded if this is a corner grounded system. Can you see any evidence for that?

Sounds a lot like an ungrounded system.
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Old 01-15-2019, 08:09 AM   #8
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The utility transformer would likely also be grounded if this is a corner grounded system. Can you see any evidence for that?

Sounds a lot like an ungrounded system.
The presence of the ground fault lights makes it a sure bet it was ungrounded at some point - I don't know what else they could be, you could verify that's what they are.

If someone intentionally corner grounded it, it sounds like it wasn't done by the numbers, is it possible that the utility transformer would remain ungrounded?
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Old 01-15-2019, 11:14 AM   #9
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Quote:
i think your theory makes sense - they are mistaking a ground fault for a corner ground.

Would it possible the ground fault is intermittent, sometimes it's a dead short to ground and you see a-g = 0v, other times it's a high impedance fault and you see a-g = who knows what voltage?

what he said
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Old 01-15-2019, 11:18 AM   #10
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Verify the service with the POCO.
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Old 05-30-2019, 12:00 AM   #11
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How was this resolved? I'm curious. Thanks.
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Old 05-30-2019, 07:46 AM   #12
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Ungrounded Delta services were once very common on industrial services. The failed indicator lights seem to prove that this is what you have. Ignoring a ground fault was also a common problem, until another phase had a ground fault. Then a big boom!

I really would not want to do a corner ground on a 1600 amp service due to the potential fault current that could due serious damage. A resistance ground system would be a better option.
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Old 05-30-2019, 09:49 AM   #13
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Ungrounded Delta services were once very common on industrial services. The failed indicator lights seem to prove that this is what you have. Ignoring a ground fault was also a common problem, until another phase had a ground fault. Then a big boom!

I really would not want to do a corner ground on a 1600 amp service due to the potential fault current that could due serious damage. A resistance ground system would be a better option.
I was once in plant where the maintenance shack was heated by the grounding resistor of the resistance grounded system. In the winter, guys would take their coffee breaks standing in proximity of the resistor! I realized that if the resistor was hot, it must be because of a ground fault somewhere. It took my young mind a bit to catch that.

I asked the department head why we didn't track down that ground fault. He said: "Kid, do you know just how cold it gets in this shop in the winter?"
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