Corner Ground Delta

kwired said:
Lost neutral when you have line to neutral loads and the variance of voltage to the connected 120 volt loads is different situation than losing the ground reference itself though. Remove bonding jumper and you still have 120/240. Voltage measured to earth may vary as loading conditions vary though, and capacitance is a factor in what voltage readings you will see to ground. Grounding the system gives your measured voltages to earth more stability as the system has a fixed reference.
Click to expand...
Yes the reference to earth does not really do anything other than give us the voltages to ground that we expect. All of the various systems that we ground, will still have the same voltages between the actual circuit conductors, with or without a connection to earth.
 
I worked for a school district with a lot of older schools that had corner delta grounded systems. Our electrical personnel were aware of the schools that had these systems. I worked with the elevator repair personnel; they were not part of the school district. Almost all of the elevator people had no contact with corner grounded delta systems, even though they were marked, they did not understand that the grounded phase, in our case always the “C” phase showed nothing to ground when it was energized. A huge safety issue.
When I first started at the district, we had a high school that was in the process of being remodeled, the electrical contractor used the corner delta grounded primary transformer to feed a secondary wye grounded transformer. After the smoke cleared, they brought generators in on low boys to power the school.
 
Rjryan said:
, even though they were marked, they did not understand that the grounded phase, in our case always the “C” phase showed nothing to ground when it was energized. A huge safety issue.
Click to expand...

Why is it a safety issue? The grounded conductor would be mostly safe to touch, just like a split phase system.

When I first started at the district, we had a high school that was in the process of being remodeled, the electrical contractor used the corner delta grounded primary transformer to feed a secondary wye grounded transformer. After the smoke cleared, they brought generators in on low boys to power the school.
Click to expand...

Could you elaborate on that? I don't really follow and understand what they did or did not do and what went wrong.
 
jim dungar said:
Then they were not qualified workers.
Per OSHA and NFPA 70E a qualified worker knows how to identify hazards and what steps are needed to protect themselves.
If all of your schools had this system it was not unique nor a secret.
Click to expand...
First of all, about 8 of older schools had grounded corner delta, which I thought was a lot for our older schools, (could have clarified that better) out of 140 schools.
Second the Technicians OTIS ELEVATOR, had never run across a grounded corner dealt distribution system before coming to the district and we had to alert them to the system. If you have never had contact to this distribution system it is hard to identify the hazards.
 
Rjryan said:
First of all, about 8 of older schools had grounded corner delta, which I thought was a lot for our older schools, (could have clarified that better) out of 140 schools.
Second the Technicians OTIS ELEVATOR, had never run across a grounded corner dealt distribution system before coming to the district and we had to alert them to the system. If you have never had contact to this distribution system it is hard to identify the hazards.
Click to expand...
What are the hazards?
 
Rjryan said:
First of all, about 8 of older schools had grounded corner delta, which I thought was a lot for our older schools, (could have clarified that better) out of 140 schools.
Second the Technicians OTIS ELEVATOR, had never run across a grounded corner dealt distribution system before coming to the district and we had to alert them to the system. If you have never had contact to this distribution system it is hard to identify the hazards.
Click to expand...
I would think any electrician worth his salt could tell you the type of bank from the voltages.
 
Rjryan said:
View attachment 2578671The biggest problem is that you don't recognize what you are working with, you have three phases of power, but only two phase showing voltage to ground.
Click to expand...
Why is it a hazard?
If you are a grunt pulling wires, you pull as instructed by someone who should know how to read prints that were made by someone that designed the project. The grunt should be nowhere near an energized anything, which equals No Hazard. Anyone above that grunt should already know how to recognize what they are working with. They should not be there if they can't. They are not qualified.

I did not like the 480 corner ground because it was 480. I had seen 700+ on a corner ground delta before I owned a pair of gloves. (I didn't like the spark that followed my meter probes as I disconnected.) Electrofelon commonly works with voltages well above that.
 
Rjryan said:
View attachment 2578671The biggest problem is that you don't recognize what you are working with, you have three phases of power, but only two phase showing voltage to ground.
Click to expand...
The biggest problem is SOME may not know what they are working with.
At that point they need to ask their J-man what’s going on for a teaching moment.

I’ve worked with voltages from 115kV down to 24V, in all kinds of banking configurations.

Check around. Look at the bank. Think.

We had a guy that wasn’t thinking and tried to check the voltage on a 4160 bank.
Once

Meter blew up in his gloved hands
 
IMHO the only significant problems with corner grounded delta are ones of degree and probability, not hard and fast 'bad' vs 'good'.

For the same L-L voltage (and thus the same level of power delivery at a particular current) you have a higher L-G voltage. In a safety sense, IMHO a corner grounded 480V system is a higher voltage than a neutral grounded 480/277V wye system. This is borne out by the requirement to use fully rated breakers for corner grounded systems, but only 'slash' rated breakers for grounded wye systems.

L-G faults are more common than a L-L faults. In a corner grounded system, a L-G fault _is_ a L-L fault.

IMHO it is fair to say that a corner grounded delta presents an increased hazard relative to the same voltage wye, but I don't think the increased hazard is sufficient to say that they should be prohibited. On the other hand, I'd agree that installing a corner grounded system is not a 'best practice' for new installations.
 
Rjryan said:
If you have never had contact to this distribution system it is hard to identify the hazards.
Click to expand...
I was pointing out that there is no one universal qualification for all tasks.
Qualification is determined by a company's Safe Work Practices program. It is possible for a person to be qualified for one task and not another.
 
Rjryan said:
View attachment 2578671The biggest problem is that you don't recognize what you are working with, you have three phases of power, but only two phase showing voltage to ground.
Click to expand...
I struggle to see how it is an increased hazard. I could agree with Winnie that it is an increased hazard in the sense of it just having a higher voltage to ground than some other systems. As far as it being odd or rare, it seems unlikely that would pose a problem. If someone is connecting something without measuring voltages, well you could say that about any system.

One slight possibility I could see is if someone assumes it was split phase and connected 120 L-N load to it. However that would be extremely unlikely as I bet very few corner grounded systems are fed from a panelboard that looks like split phase. Even if you had a smaller panelboard with 2 poles and a grounded bus, it would have to use slash rated 277/480 breakers (if 240 corner) , or higher breakers, so that's going to make it not look like 120/240. If that doesn't trigger a closer look, than that person really should not be working on a supervised on anything in my opinion.
 
And, as is clear from the above diagram, the grounded conductor will be white or grey, so it's function should be obvious. I don't see how this is any more hazardous than the neutral on any other system. As one of my old bosses used to say, "Beware of the noodle." His point was that the neutral is probably the most dangerous one as it shows no voltage until disconnected, but CAN be carrying significant current, which if it comes from an inductive load can push very hard against being disconnected.

Mark
 
busman said:
And, as is clear from the above diagram, the grounded conductor will be white or grey, so it's function should be obvious. I don't see how this is any more hazardous than the neutral on any other system. As one of my old bosses used to say, "Beware of the noodle." His point was that the neutral is probably the most dangerous one as it shows no voltage until disconnected, but CAN be carrying significant current, which if it comes from an inductive load can push very hard against being disconnected.

Mark
Click to expand...
This would show again, that someone was not qualified to do what they was doing.
 
busman said:
And, as is clear from the above diagram, the grounded conductor will be white or grey, so it's function should be obvious. I don't see how this is any more hazardous than the neutral on any other system. As one of my old bosses used to say, "Beware of the noodle." His point was that the neutral is probably the most dangerous one as it shows no voltage until disconnected, but CAN be carrying significant current, which if it comes from an inductive load can push very hard against being disconnected.

Mark
Click to expand...
While that is what the code requires, I have never seen the grounded conductor of a corner grounded system actually identified that way in the field.
 
ptonsparky said:
Stop at my projects in NE.
Might be hard to find. Only had two at 480 and one of them has been torn out. IDR of any at 240 now.
Click to expand...
There are not that many around me...I have worked on a dozen or so, and none had the grounded conductor identified as required by the code. Never installed a new corner grounded system, these were all service calls at facilities around me.
 
Last edited:
don_resqcapt19 said:
There are not that many around me...I have worked on a dozen or so, and not had the grounded conductor identified as required by the code. Never installed a new corner grounded system, these were all service calls at facilities around me.
Click to expand...
Not saying it’s right, but I have never seen a corner grounded system marked that way either, but it’s been around the northeast a lot longer than the south.
 
You must log in or register to reply here.
Top