Ungrounded delta system

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Dsg319

Dsg319

Senior Member
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West Virginia
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Wv Master “lectrician”
I’ve recently realized the station we are working at (additional construction). Operates from 480v ungrounded delta system. As many others I’m sure that I just never knew about than but have Just learned about recently. And I understand that even if a phase fails nothing with happen with an ocpd until a second fault occurs, than with that happening you have a 480v fault on your hands. But I have two questions I haven’t been able to find exactly to my likings to know for sure. 1. If you touch any 1 phase to ground will you still receive a shock?
And 2. I know that you still have to pull an egc with all the 480v ungrounded circuits, and I know you must have detectors to show something event of losing a phase. But wondering how they work and where are they usually located?
 
1. Yes
It is possible you could get a shock if you are in the line-ground path. It has to do with stuff called coupling capacitance and charging currents.

2. Poorly and hidden
Actually, there are several different method of detection/indication with the most familiar being 3 lights connected into a wye configuration which is grounded. The voltage for the bulbs is typically chosen to be the L-L voltage. When everything is good the wye configuration causes each bulb to see effectively L-N voltage therefore the bulb is usually just barely glowing. When a phase goes to ground the voltage across it goes to zero so it goes dark, the other two lights now get L-L voltage across them so they glow at full brightness. Sometimes a pushbutton is included in the ground connection, so when it is pushed the reference to ground is lifted and all three lights go back to being dim. These detectors are usually located at the service entrance location, which means they may not be seen a a regular or timely basis. also familiarity can breed laziness, after seeing the lights at normal intensity for year upon year, many people stop noticing them altogether.

Actually this is a good topic for discussion.
 
jim dungar said:
1. Yes
It is possible you could get a shock if you are in the line-ground path. It has to do with stuff called coupling capacitance and charging currents.

2. Poorly and hidden
Actually, there are several different method of detection/indication with the most familiar being 3 lights connected into a wye configuration which is grounded. The voltage for the bulbs is typically chosen to be the L-L voltage. When everything is good the wye configuration causes each bulb to see effectively L-N voltage therefore the bulb is usually just barely glowing. When a phase goes to ground the voltage across it goes to zero so it goes dark, the other two lights now get L-L voltage across them so they glow at full brightness. Sometimes a pushbutton is included in the ground connection, so when it is pushed the reference to ground is lifted and all three lights go back to being dim. These detectors are usually located at the service entrance location, which means they may not be seen a a regular or timely basis. also familiarity can breed laziness, after seeing the lights at normal intensity for year upon year, many people stop noticing them altogether.

Actually this is a good topic for discussion.
Click to expand...
Thanks for the info!
 
Generally is only a good design decision in a process situation where immediately shutting down one part of the process because an OCPD trips because of a ground fault can introduce more hazard than allowing the fault to remain and then shut the process down in an orderly fashion.

That concept becomes somewhat useless if you do ignore the first fault though, because if there is a second fault on another phase - OCPD's are going to trip anyway.

Ignoring possible capacitive effects, you don't get shocked in normal operation because there is no reference to ground until one conductor faults to ground.

Too many think a "phase conductor" is always "hot". Well it is in nearly all systems with a neutral simply because NEC usually will require the neutral to be the conductor that is grounded. Otherwise in reality you could ground any line in the system and that line is the one that is at/near earth potential and the actual neutral in such arrangement would have voltage to ground.

Many get confused with corner ground systems for similar reasons. Only difference here is one corner is intentionally grounded, but still carries current just like a neutral carries line to neutral current on a system with a neutral. For that reason you still run separate EGC and "grounded conductor" beyond the service equipment or bonding point of a separately derived system, just like we do for systems with a neutral conductor.
 
Dsg319 said:
I’ve recently realized the station we are working at (additional construction). Operates from 480v ungrounded delta system. As many others I’m sure that I just never knew about than but have Just learned about recently. And I understand that even if a phase fails nothing with happen with an ocpd until a second fault occurs, than with that happening you have a 480v fault on your hands. But I have two questions I haven’t been able to find exactly to my likings to know for sure. 1. If you touch any 1 phase to ground will you still receive a shock?
And 2. I know that you still have to pull an egc with all the 480v ungrounded circuits, and I know you must have detectors to show something event of losing a phase. But wondering how they work and where are they usually located?
Click to expand...

I agree with Jim Dungar's responses but would add two items. The shock potential is small if indeed the system is a fully insulated 3 wire Delta system and there is a small capacitance coupling to ground for every phase but also inductive mutual coupling to adjacent phases that will also contribute. The second item is this, are you sure you have a 3 wire Delta system? Did you actually confirm the transformer nameplate or the wiring on the transformer bank that you can see is wired as a Delta 3 wire service? The reason for the second question is that I have seen many 480 Volt three wire systems in industrial plants that are actual 4 wire 480Y/277 volt services that do not extend the neutral beyond the main switchgear.
 
NewtonLaw said:
I agree with Jim Dungar's responses but would add two items. The shock potential is small if indeed the system is a fully insulated 3 wire Delta system and there is a small capacitance coupling to ground for every phase but also inductive mutual coupling to adjacent phases that will also contribute. The second item is this, are you sure you have a 3 wire Delta system? Did you actually confirm the transformer nameplate or the wiring on the transformer bank that you can see is wired as a Delta 3 wire service? The reason for the second question is that I have seen many 480 Volt three wire systems in industrial plants that are actual 4 wire 480Y/277 volt services that do not extend the neutral beyond the main switchgear.
Click to expand...
Voltage measurements can help confirm this, but can mislead you if there is any wye connected load that for some reason has the wye point connected to ground even though that connection should have been left floating.

Good nominal 277 volts to ground with a low impedance meter probably means it is a wye with grounded wye point.
 
NewtonLaw said:
I agree with Jim Dungar's responses but would add two items. The shock potential is small if indeed the system is a fully insulated 3 wire Delta system and there is a small capacitance coupling to ground for every phase but also inductive mutual coupling to adjacent phases that will also contribute. The second item is this, are you sure you have a 3 wire Delta system? Did you actually confirm the transformer nameplate or the wiring on the transformer bank that you can see is wired as a Delta 3 wire service? The reason for the second question is that I have seen many 480 Volt three wire systems in industrial plants that are actual 4 wire 480Y/277 volt services that do not extend the neutral beyond the main switchgear.
Click to expand...
Yep. I’m almost certain it is 3wire delta. I looked at service drop and only brought in the three phases and haven’t got to look good into the transfer switch but I’m almost certain. Also both transformers Fed from the first and only 480panel I have seen are primary 480v delta fed. With one having a secondary delta high leg and the other a secondary wye.
 
Dsg319 said:
Also both transformers Fed from the first and only 480 panel I have seen are primary 480v delta fed.
Click to expand...

That would be a red herring. Customer owned general purpose transformers, with 480V primaries, are probably 99.99% delta wound.

You should check with your utility to see how their transformer is connected.
 
There’s a pad mount utility transformer in the station but I haven’t been able to find any info on it but I’ll check again. But I know going into it with the service drop it looks to me like they just brought the phase conductors Into it and brought the neutral (utility line) over to bond all the metal components that support the phase wiring going into the conduit on the pole.
 
There’s a pad mount utility transformer in the station but I haven’t been able to find any info on it but I’ll check again. But I know going into it with the service drop it looks to me like they just brought the phase conductors Into it and brought the neutral (utility line) over to bond all the metal components that support the phase wiring going into the conduit on the pole.
 
A couple of corrections:
Dsg319 said:
...And I understand that even if a phase fails nothing with happen with an ocpd until a second fault occurs, than with that happening you have a 480v fault on your hands.
Click to expand...
By "fail", it would have to mean a GROUND FAULT. The frst ground fault makes the ungrounded Delta into a Corner Grounded Delta, so the plant continues to run. If a phase opens, that is still a phase loss.

And 2. I...and I know you must have detectors to show something event of losing a phase. But wondering how they work and where are they usually located?
Click to expand...
Per the NEC, you need either GROUND FAULT MONITORING, OR you need to use a Corner Grounded Delta system. Phase loss monitoring is no different. Desirable, but not mandatory.

The "grounding lights" method is (was) cheap and easy and fulfilled the requirement, but just try to find a 480V rated incandescent bulb now. There are simple replacements like the Littelfuse PGR-3100 that emmulate the 3 light bulb system, or much more sophisticated ground monitoring systems available now that can connect to SCADA systems to alert operators of a problem or tie to an SMS message server to alert the maintenance crew.
 
Dsg319 said:
There’s a pad mount utility transformer in the station but I haven’t been able to find any info on it but I’ll check again. But I know going into it with the service drop it looks to me like they just brought the phase conductors Into it and brought the neutral (utility line) over to bond all the metal components that support the phase wiring going into the conduit on the pole.
Click to expand...
If there is a neutral from the POCO it will be bonded/grounded and is a Y system and not ungrounded. Remember, the definition of an ungrounded system starts at the source (the transformer). I've seen too many installs with out a neutral brought to the service and had the installer tell me they are running an ungrounded system. There is no such thing as an ungrounded system fed from a grounded POCO neutral. It is also very dangerous if the system is grounded at the source and then treated as an ungrounded system.
 
texie said:
If there is a neutral from the POCO it will be bonded/grounded and is a Y system and not ungrounded. Remember, the definition of an ungrounded system starts at the source (the transformer). I've seen too many installs with out a neutral brought to the service and had the installer tell me they are running an ungrounded system. There is no such thing as an ungrounded system fed from a grounded POCO neutral. It is also very dangerous if the system is grounded at the source and then treated as an ungrounded system.
Click to expand...
Thanks for the info! And from what I can tell there is no neutral from the service pole to the pad mount utility transformer. BUT on the service pole from the main power line they did bring a neutral and all’s they did with it from what I can tell by looking is bond it to metal parts on the pole that’s supporting the phase conductors as they enter into the conduit going down the pole to the transformer pad.
 
Jraef said:
A couple of corrections:
By "fail", it would have to mean a GROUND FAULT. The frst ground fault makes the ungrounded Delta into a Corner Grounded Delta, so the plant continues to run. If a phase opens, that is still a phase loss.


Per the NEC, you need either GROUND FAULT MONITORING, OR you need to use a Corner Grounded Delta system. Phase loss monitoring is no different. Desirable, but not mandatory.

The "grounding lights" method is (was) cheap and easy and fulfilled the requirement, but just try to find a 480V rated incandescent bulb now. There are simple replacements like the Littelfuse PGR-3100 that emmulate the 3 light bulb system, or much more sophisticated ground monitoring systems available now that can connect to SCADA systems to alert operators of a problem or tie to an SMS message server to alert the maintenance crew.
Click to expand...

Wait, they made 480 volt incandescent bulbs? Proof or they never existed.
 
I worked at a steel mill that was an ungrounded Delta system built around WWII where we had ground lights in the Electric Shop. They were big mogul base incandescents and I always assumed they were 480V, because I was told they had to be rated for the line voltage. There were only 3, so no series bulbs, but it never occurred to me that there may have been PTs behind the plywood panel they were mounted on. Now that I think about it more, there probably was.
 
Jraef said:
I worked at a steel mill that was an ungrounded Delta system built around WWII where we had ground lights in the Electric Shop. They were big mogul base incandescents and I always assumed they were 480V, because I was told they had to be rated for the line voltage. There were only 3, so no series bulbs, but it never occurred to me that there may have been PTs behind the plywood panel they were mounted on. Now that I think about it more, there probably was.
Click to expand...


Probably the case, but remember it could actually be either way, in a capitalist society if there is demand for it they will make it.
 
Drifting further off topic-
It was quite common in subway systems to light the tunnels with five 120v lamps in series and hook that from the 600v wire to the return. The Green line in Boston had them at least into the 1980s and might still.
 
zbang said:
Drifting further off topic-
It was quite common in subway systems to light the tunnels with five 120v lamps in series and hook that from the 600v wire to the return. The Green line in Boston had them at least into the 1980s and might still.
Click to expand...

I remember NYC had random incandescent lamps in their train terminals and tunnels.
 
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