corner grounded delta troubleshooting

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chaoakax

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Los Angeles
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Electrician
Hi everyone, I've been doing 99% residential electrical work, and recently decided to take upon a commercial job, which seemed like should have been easy enough but here comes:

So its an old office building that has 200A 1ph, 120/240v main for the offices, and a separate 100A 3ph 3 wire 240V meter with a square D 3 phase disconnect right below it. Before using it, i measure the voltage and get:
A to ground 240v
B to ground 240V
C to ground 0
A to B 240V
A to C 240v
B to C 240v
never before having heard about corner grounded delta systems, i automatically assumed the C phase is dead from utility and called for them to check on it and proceeded to installing a 3PH sub panel with breaker main to feed single phase AC units. Clients wanted all mini splits and they are all single phase 30A units. So to picture the overall system, it is a Meter, with the 3ph disconnect at the bottom, feeding a 3ph 100 subpanel with a straight shot of 30ft of EMT conduit. Then from subpanel we fed the AC units with 30A breakers. Edison rep calls tells me that its a normal system and that is how it is supposed to be and it works fine. AC guy turns on 2 of the units and tries firing them up. Nothing happens. Checks at the subpanel on the 100A main breaker at the sub, says gets absolutely no voltages there, and cant get to the Square D 3ph disconnect to check on the fuses because they are locked. So i have to go there to check on it but i am out of ideas what could have gone wrong. On a side note, when i first opened the old 3 ph disconnect, i noticed that only A and B phases had fuses installed and C had nothing. Now, what could have gone wrong? Could it be that the C phase was tapped as neutral at the utility transformer? How should i proceed when troubleshooting this?

I appreciate all the help i can get
 
Take your own load bank with you. A 240v baseboard heater or scavenge a range top element. Use a little Field Engineering and set up a test stand. Confirm proper operation with simultaneous voltage and current measurements.

Those units may have VFDs in them and those may not like the 240v to ground.
 
There is no neutral on your system. The C phase is the grounded conductor.( that’s why you get 0volts from it to ground but full system voltage from phase to phase) Do not fuse or run this conductor through the breaker as it is your fault current return path.
 
1) Are there any GFCI breakers involved? Any surge suppressors?

2) The 'grounded conductor' is the system conductor that is intentionally connected to ground to create a grounded system. 99.9% of the time the neutral gets grounded, but the physics of the situationis that you can ground _any_ single conductor in a system.

In a corner grounded system the line to line voltages are perfectly normal, but the line to ground voltages are surprising, because we aren't used to seeing 0V on a perfectly fine phase conductor. But in the same way that a neutral can zap you, a grounded phase conductor can deliver power to a load.

3) For the purpose of delivering power to a load, all that matters is the line to line voltage. But if any part of the system cares about line to ground voltage, corner grounding can be a problem. Noise filters and surge suppressors can be a problem here. VFDs are known to sometimes have a problem on corner grounded systems (but other VFDs work just fine with them).

Straight up across the line motors work just fine with corner grounded supplies.

4) Be very sure the minisplits are specified to run on a corner grounded system. They likely are, because they are international designs and the same hardware is probably used for the 240V line to neutral single phase common in other parts of the world. But it is by no means certain that they are suitable.

I think there are a couple of electricians who do lots of HVAC work on the forum. They may chime in about minisplits and corner grounded supplies.

5) You should never fuse a grounded conductor. In residential the grounded conductor is just tied to the neutral bus, but it is common in industrial to use a 3 phase disconnect with a blank in place of the fuse for the grounded conductor. Totally normal to see that. It is fine to use a multi pole breaker on the grounded conductor because it opens all poles at the same time.

6) So far I don't see any culprit in your description. There should have been the same 240V L-L at the subpanel, with one phase 0V to ground.

Jonathan
 
chaoakax said:
never before having heard about corner grounded delta systems, i automatically assumed the C phase is dead from utility and called for them to check on it and proceeded to installing a 3PH sub panel with breaker main to feed single phase AC units.
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chaoakax said:
Clients wanted all mini splits and they are all single phase 30A
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When you installed the sub panel did you wire it black,red, blue, and equipment ground.
Did you use 2 pole breakers to supply the mini splits

(Black, red) (red, blue) (black, blue)

You said when you installed everything you didn't know it was 240 V corner grounded.
 
chaoakax said:
proceeded to installing a 3PH sub panel with breaker main to feed single phase AC units.
Click to expand...
Is it typical for a 3-phase panel to be used with a corner grounded delta then the grounded phase has an OCPD in it?
240.22 (1) allows it. I would be inclined to use a single phase panel and land C-phase on the neutral bar.
Thats also interesting its a utility service, I have only encoutnered it in plants with customer owned transformers.
Make sure the grounded C-phase to all the minisplits white or gray, unless they allow Blue by special permission.
 
tortuga said:
Is it typical for a 3-phase panel to be used with a corner grounded delta then the grounded phase has an OCPD in it?
240.22 (1) allows it. I would be inclined to use a single phase panel and land C-phase on the neutral bar.
Thats also interesting its a utility service, I have only encoutnered it in plants with customer owned transformers.
Make sure the grounded C-phase to all the minisplits white or gray, unless they allow Blue by special permission.
Click to expand...
While the code would permit a single phase panel for a corner grounded system, I have never seen one used in the field.
 
ptonsparky said:
Years ago my local SED said 'don't even try' when I broached the subject. It wasn't worth the effort to discuss it further.
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However if we did do that, I think that it would be more likely that the grounded conductor would be correctly identified as such. Something that I also have never seen in the field.
 
Dsg319 said:
Do not fuse or run this conductor through the breaker as it is your fault current return path.
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You can run it through a 3 phase breaker, because it opens all three poles at the same time. You just can't use a fuse on that phase of a corner grounded 3 phase system.

don_resqcapt19 said:
While the code would permit a single phase panel for a corner grounded system, I have never seen one used in the field.
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I think that's because corner grounded 240V delta systems are usually NOT also 240/120 3 phase 4 wire, so no 120V to neutral available, and to use 240V single phase you would have to use "fully rated" breakers (as opposed to "slash rated") which are more expensive, in which case you may as well just use a 3 phase panel.
 
Jraef said:
You can run it through a 3 phase breaker, because it opens all three poles at the same time. You just can't use a fuse on that phase of a corner grounded 3 phase system.



I think that's because corner grounded 240V delta systems are usually NOT also 240/120 3 phase 4 wire, so no 120V to neutral available, and to use 240V single phase you would have to use "fully rated" breakers (as opposed to "slash rated") which are more expensive, in which case you may as well just use a 3 phase panel.
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Thank you. I have never personally ran into such a set up in the field. Rarely a high leg delta do I see.

But I’m curious even in the main/service disconnect can the grounded conductor be ran through the breaker? Or what is a typical installation?
 
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