480 VAC, 3∅ Distribution Question

Wire-Smith

Senior Member
Location
United States
the main transformer bank is likely 3 single phase transformers(being that old), so it would be easy to see wye. Bretzel, are all of the secondaries(one side of each transformer) at the main transformer bank connected at one point? and the three legs you have are connected to the other bushing's of the transformer's and are all alone(only one wire on that bushing and that wire is not connected to the other wires anywhere else). omit the neutral (N) that runs down the pole in the photo




or again just omit the neutral wire running away from the bank
 
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__dan

Senior Member
Two things to keep in mind:

With a solidly grounded system, the line to ground fault passes enough current to trip the breaker. The contiguous bonding / grounding system limits voltage differences between any (attached) exposed metal to mitigate any shock hazard. Then the breaker trips, removing the fault from the supply, the sense of urgency to fix it becomes immediate because production is down, and the cost of production downtime is the driving imperative. So the safety hazard is removed by automatic operation of the safety device, the location of the fault is indicated by the circuit out, and fixing it is indicated at the highest priority.

A floating or ungrounded system supply has one set of issues. The electrical distribution supply without the contiguous bonding / grounding system, using local peg grounding with ground rods but no EGC conductor or effective fault clearing path back to the source, has a different set of issues. You are describing the coincidence of both, floating and no effective fault clearing path back to the source.

Floating or ungrounded systems are still legal with marking or signage and ground fault detection (not ground fault trip). Typically the words I see are 'with qualified onsite supervision'.

A floating or ungrounded supply, first line to ground fault simply references the system to ground and it can go undetected for a long time. But with no EGC, there can be and probably is, a local elevated voltage potential above the earth level on the exposed metal frame.

This would be especially hazardous outside or around a body of water. The man standing on the ground and grabbing a handhold on the structure / ladder / frame could have the full system voltage across him, from the earth level through the fault to the supply transformer. This would be 1000x if he were standing in water and then grabbed the metal that was faulted to the line.

Since the fault is not localized, it can be any piece of metal frame or structure anywhere on the site.

Since the breaker does not trip, the hazard is neither indicated or removed. The original design intent of this system is that the onsite qualified personnel will find and fix the fault, with the same or better urgency as if the breaker tripped and production went down.

So while possibly a legal and grandfathered system, you are missing several components. No awareness of the very specific hazards, no automatically operating safety devices that are normally expected to be present, no indicating or alarming for voltage hazards on normally grounded and bonded exposed metal.

There is a lot else going on, possibly passing enough current to show up on your electric bill but not enough to trip a big breaker.

What to keep in mind is that a shock hazard likely exists, and when you are normally expecting your breakers to trip for this type of problem, you have a system that by its original design intent, the breakers do not trip. The original design intent is the power stays on for line faults to ground.
 

kwired

Electron manager
Location
NE Nebraska
If it was a wye, there would be a neutral, likely grounded, leading to a fault.
Wye has to have a neutral point to function. If neutral was never utilized for any purpose other then to tie the three coils together and never grounded and all that was connected to the source was the three phase conductors, it wouldn't really act much differently then an ungrounded delta system, as long as the neutral remains intact.

With same setup but intentionally grounding a phase conductor - it would act a lot like a corner grounded delta.
 

Wire-Smith

Senior Member
Location
United States
Two things to keep in mind:



A floating or ungrounded supply, first line to ground fault simply references the system to ground and it can go undetected for a long time. But with no EGC, there can be and probably is, a local elevated voltage potential above the earth level on the exposed metal frame.

This would be especially hazardous outside or around a body of water. The man standing on the ground and grabbing a handhold on the structure / ladder / frame could have the full system voltage across him, from the earth level through the fault to the supply transformer. This would be 1000x if he were standing in water and then grabbed the metal that was faulted to the line.

Since the fault is not localized, it can be any piece of metal frame or structure anywhere on the site.
.
so do you think corner grounded deltas are installed initially with that hazard?

everything has to be bonded like a grounded system, the secondary winding's are just not tied to ground
 
The physical plant I manage buys power from the POCO at primary. We have our own 3Mw substation. The beast was built c. 1935. It is, unfortunately, an operating electical museum. The secondary is 480v 3∅ Wye, 3 Wire, local grounds. We have a number of stepdown transformers running our buildings. We have a number of rotating loads up to 100 hp. Recently, we have observed some irregular voltage readings. We're seeing 460 to 495 between the three legs. Two legs fairly consistently measure 460 and 480 to ground. The third leg is 1.75 volts to ground. All rotating loads appear to be operating normally. Some transformers, depending on the legs they are connected across, are 120v on one side and 95 - 102 on the other.
Its a bit hard to understand the exact setup. From what I gather, your building is served by customer owned primary. This is stepped down to 480, which is then stepped down to 120. So you have multiple seperately derived systems. Each SDS has its own system grounding configuration that will not effect the others' so you need to start by figuring out which building/system/area has the issue.
 

kwired

Electron manager
Location
NE Nebraska
so do you think corner grounded deltas are installed initially with that hazard?

everything has to be bonded like a grounded system, the secondary winding's are just not tied to ground
If seconadary windings of a separately derived system are not tied to ground - that secondary has no ground reference and is an ungrounded system. With ungrounded systems you still run EGC's and aGEC and bond them to all non current carrying conductive parts - maybe that is what you were trying to say here?

Its a bit hard to understand the exact setup. From what I gather, your building is served by customer owned primary. This is stepped down to 480, which is then stepped down to 120. So you have multiple seperately derived systems. Each SDS has its own system grounding configuration that will not effect the others' so you need to start by figuring out which building/system/area has the issue.
Correct, every time voltage is transformed you start all over with grounding of each individual system. But stray voltage on a grounded conductor that originates on an upstream system can find it's way across the entire grounding network.
 
If seconadary windings of a separately derived system are not tied to ground - that secondary has no ground reference and is an ungrounded system. With ungrounded systems you still run EGC's and aGEC and bond them to all non current carrying conductive parts - maybe that is what you were trying to say here?
My take is he was responding to some of the posts that were implying this is a dangerous situation - it may be, but there is nothing dangerous about ungrounded systems and even first faults - it just becomes a grounded system. Sure you can come up with scenarios where there is a dangerous situation but you can do that with grounded systems too. I found a first fault in an old 600 volt system once. It had prolly been faulted for decades.
 

kwired

Electron manager
Location
NE Nebraska
My take is he was responding to some of the posts that were implying this is a dangerous situation - it may be, but there is nothing dangerous about ungrounded systems and even first faults - it just becomes a grounded system. Sure you can come up with scenarios where there is a dangerous situation but you can do that with grounded systems too. I found a first fault in an old 600 volt system once. It had prolly been faulted for decades.
I can agree with that, what might be dangerous though is not having any EGC system - which from OP's descriptions so far doesn't appear to be any.

If this were originally a grounded phase system, then as old as it is, should have been legal to run just three conductors to separate buildings and then ground at each building like you would have grounded a service. Today you can't do that anymore. But don't sound like anything is grounded period, and no ground fault indication either.
 
I can agree with that, what might be dangerous though is not having any EGC system - which from OP's descriptions so far doesn't appear to be any.

If this were originally a grounded phase system, then as old as it is, should have been legal to run just three conductors to separate buildings and then ground at each building like you would have grounded a service. Today you can't do that anymore. But don't sound like anything is grounded period, and no ground fault indication either.
Maybe, it's just not really clear....he said "local grounds" only......I don't know if he is talking system grounding or equipment grounding or bonding. If it's old, it's probably all in rgs.
 

__dan

Senior Member
If he has all RGS conduit feeders, all steel boxes, and proper attention was paid to the system so that all the site metal with electrical power to it is bonded to the contiguous EGC system, the hazard would be greatly reduced. The floating ungrounded supply would still be legal with signage and GF detection.

We would say he has 3 phase 3 wire with 4 conductors.

With multiple buildings spread out over a site dating to 1935, you have to assume, of all the guys who worked on the power there, some of them should not be touching wiring except with sufficient supervision. Not a bet I would take.

Even if he has a lot of visible RGS, you really won't know if all the loads and structures are connected to it without a survey. Knowing this is in the future.

3 phase 3 wire 3 conductors using local "Peg Grounding" has been installed enough times that it shows up in MSHA fatal accident investigation reports. These were not installed by mistake. It is a system that was repeated. Then lots of guys will come along and just copy something because ... that's the way it has always been done. Or that's the way we do it here ...

According to accident reports, operators have tried to use earth as their safety grounding system. When this has been done, fatalities have normally been the end result. The following accident report details the hazards of using earth as a safety grounding system.

https://arlweb.msha.gov/S&HINFO/TECHRPT/GROUND/MNMELECT.HTM

In the modern times, all the guys will just guess or assume the breaker will trip if there is a problem. They will wait for the breaker to trip. And if the breaker resets, they will assume the problem is gone. These guys are not ghostbusters.

One I found is exactly what the OP describes. I don't know if he has local only grounds or RGS. Site I saw had two large (old old old) industrial building complexes and then three sites in the mine with 480, mostly 3 phase 3 wire 3 conductors. Main transformer was floating Y with three conductors leaving.

One of the mine power site was a floating 20 hp dewatering pump. There was a mix of 4 conductor SO, EMT, but the underground feeder was 3 conductors. Any fault and all the metal would go to 277.

This site had passed many MSHA required annual grounding inspections. How I don't know, but it took me all of five minutes to fail it.

In a large site feeding a load 1000 ft away underground, installed cheap way way back, do you think they ran RGS. Point is, you will not know the answer until you survey it. And you have to know what you are looking for.

You have to verify your distribution system has an "effective fault clearing path back to the source".
 
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mbrooke

Batteries Not Included
Location
United States
Occupation
*
To late to the debate, but I too wonder if the there is a grounded or grounding conductor missing to the source.

Technically it is possible in such a case for everything in the building to behave like an ungrounded delta for a phase fault, but have 277 volts to remote earth. Such a system is dangerous because the ground current between the building and substation can result in fire or electrocution.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
Are you certain this is a wye system? What (if any of the source) conductors are grounded?

The primary is 13,200 Wye, the secondary is 480 Wye. (Switchable to 600, 575, 535, 480, 460, 230 with the turn of a bakelite knob.) The primary is grounded at the transformers but the secondary is not and appears never to have been grounded. The entire distribution system is three wire. The older (1920s patent dates) oil-filled transformers are a mix of single and three phase installtions with local grounds to earth. This construction appears to be original. Newer dry transformers are installed in a similar manor. There is a mix of Wye and Delta transformers on the system.

We have a number of ancient line surge arresters which appear to be of the carbon-filled ceramic canister type. I have to assume these require at least periodic maintenance or replacement. If one of them had a lightning strike at some point it could easily be the source of our ground fault.

Apparently, no one knows exactly when the voltage issues arose but with the addition of some newer equipment with PLC and VFD controls it has become much more obvious.

My plan at this point: While I find a consulting electrical engineer to bring in, I'll have my electricians start checking the grounding of the newer transformers and downstream equipment working backward in installation time looking for the lower voltage leg to be grounded at some location. We'll also check the very few underground runs we have for ground faults.
Can you post pictures or the substation and transformers?
 
Really interesting conversation (photos, too, please).

It does sound like a shutdown for testing would he useful; it's also an excellent opportunity to make some new "as-built" drawings of the transformers/switchboards/etc (complete with more photos :D).
 

Bretzel

Member
Location
Golden, CO
Unfortunately, due to the nature of our work, cameras of any kind are not allowed on the property. It's a check your cell phone at the gate kind of place. Otherwise is a really cool operating museum!
 
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Bretzel

Member
Location
Golden, CO
Thank you to all who have added to this thread, it has been most informative. A special tip of my hat to Wire-Smith for the link to the GE transformer theory paper. I learned a lot and cured my insomnia at least 7 times this weekend! Really, it was very good information.

I have visually verified this is a 3 wire, no ground/neutral wire downstream of the primaries on the substation transformers.

At one time, we had our own seasonal hydro generating station and a small coal fired boiler which ran the generator during the dry season. It appears to have produced 600 DC which ran a motor-alternator to turn out the 480 at 60Hz. The motor-alternator would have been easier to regulate the frequency than would the hydro or stream.

The POCO supplies primary at 13,200. Each of three, 999,999 Watt transformer has two terminals on the top. One ties to a hot leg of the primary and one ties to a common with which is both bonded to the POCO ground and to a ground peg just outside of the building housing the transformers.

The 480 secondary side of the transformers has only two terminals. There are only three wires leaving the transformer building. T1 connects across leg A and B of the 3 plant distribution wires; T2 connects across leg B and C; T3 connects across legs C and A. There is a stud welded to the bottom of the side of each transformer can. This stud connects to a conductor which bonds each can to the next and then to the same ground peg as the primary. There is no distribution ground or neutral wire from here downstream.
View attachment 20968

The distribution is mainly on poles and cross arms, open wire. There are three locations with piped, underground wiring. Some more recent main installations have used 4 wire runs which are grounded to earth at each end but do not run all the way back to the transformer building. In these and all other areas of the plant there is a ground peg at each transformer, transformer bank, or machine. I understand these local ground pegs and earth cannot return a fault back to the source if we have an unintentional ground fault of one of the distribution legs. It appears the location of this fault is what we are now searching to find. There are no indicator lights on the system for faults. Through planned outages, we have found an area when we believe the ground fault has most likely occurred. The electricians are working the area de-energized. They are using a megger where appropriate and an ohm meter where necessary such as the arresters.

I have initiated discussions with business management to replace the transformers with a package substation with remote primary and secondary switching and proper distribution. I'm still looking for that EE who is knowledgeable, thorough, and old enough to have seen something like it before and who is reasonable enough to help plan a measured, staged conversion to a modern system.
 

kwired

Electron manager
Location
NE Nebraska
Maybe, it's just not really clear....he said "local grounds" only......I don't know if he is talking system grounding or equipment grounding or bonding. If it's old, it's probably all in rgs.
I was going to reply to this, but looks like OP clarified some in last post. Appears to be an ungrounded system (at least the 480 volt system is) and is grounding done "locally" at each building? But likely only to non current carrying components leaving the system still ungrounded.

Can make for a real mess with stray currents when you have a ground fault on phase A in one building, a fault on phase B in another building and a fault on phase C in a third building.
 
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