ungrounded delta

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bob52

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Location
pittsfield ma
My company is under contract to replace an existing 4000 amp ungrounded delta service. we have included ground fault indication per NEC. My customer would like to add ground fault indication for each individual sub feed. any suggestions? Thanks for your help
 

GoldDigger

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Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
If there is a ground fault anywhere in the system all ground fault indicators would trip, so I do not see the point.
Except maybe to make it more likely that someone will notice.
If you put an indicator upstream of each feeder disconnect it would let you see easily whether the fault goes away when you open the disconnect.
A circuit that would specifically detect a downstream fault on an energized feeder seems overly complex, if even possible.
 

Tony S

Senior Member
I’ve worked on ungrounded systems for about twenty years, like GoldDigger I can’t think of a reliable method for fault detection on outgoing feeders.

With ours once a fault was detected that was it, the MV beakers opened.
 

iwire

Moderator
Staff member
Location
Massachusetts
I’ve worked on ungrounded systems for about twenty years, like GoldDigger I can’t think of a reliable method for fault detection on outgoing feeders.

With ours once a fault was detected that was it, the MV beakers opened.

What is benefit of an ungrounded system if the fault detectors kill the power?

Here ungrounded systems are used to maintain power during a fault, the fault indicators just let you know you have a fault to fix
 

jumper

Senior Member
With ours once a fault was detected that was it, the MV beakers opened.

Okay, I am asking as no nothing grunt on ungrounded and MV systems.

Is that not incorrect?

I thought the point of an ungrounded system is that it would be a second fault that shut the system down.

The first fault would be indicated by alarms/indicators/whatever and a factory/crane/system would have time to shut down safely before a second could occur.

What am I missing?
 

Tony S

Senior Member
Okay, I am asking as no nothing grunt on ungrounded and MV systems.

Is that not incorrect?

I thought the point of an ungrounded system is that it would be a second fault that shut the system down.

The first fault would be indicated by alarms/indicators/whatever and a factory/crane/system would have time to shut down safely before a second could occur.

What am I missing?

Ever worked on induction furnaces? If you had you would know why the main breaker opens.
 
Okay, I am asking as no nothing grunt on ungrounded and MV systems.

Is that not incorrect?

I thought the point of an ungrounded system is that it would be a second fault that shut the system down.

The first fault would be indicated by alarms/indicators/whatever and a factory/crane/system would have time to shut down safely before a second could occur.

What am I missing?

Tony being from across the pond may have something to do with it. I think ungrounded systems are used differently and are more prevalent over there no? But irregardless (not a word) I would like an explanation of how they handle keeping critical processes up and running.
 

big john

Senior Member
Location
Portland, ME
The only way I know to chase a fault through a ungrounded system is with a pulser. These devices can be turned on to automatically create a periodic high-resistance ground. This allows the fault crew to chase the pulsing ground-fault current through the system with ampmeters until they find the source.

If the customer is concerned on-line fault finding, then I would try to sell them on that over the cost of multiple banks of indicator lamps which, as others have said, won't provide much additional service.

The pulsers often have the added benefit of being sold in combination with the ground-fault detection and alarms that would give your customer indication.
 

Tony S

Senior Member
Tony being from across the pond may have something to do with it. I think ungrounded systems are used differently and are more prevalent over there no? But irregardless (not a word) I would like an explanation of how they handle keeping critical processes up and running.

I blame you lot, our furnaces were designed by a well known American company. The reason for the earth free system is the conductors and coils are water cooled. So yes it is a specialist application.

I redesigned the largest furnace so I suppose some of the blame lies with me.

If the leakage current becomes excessive it can indicate a refractory failure. Automatic shutdown before the furnace springs a leak. There could be other causes but the detection system just shows it as a fault, worse case scenario comes in to play.
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Simple ground fault indicators work by measuring voltage from the circuit conductors to ground. As lots of people have already mentioned, if a phase goes to ground then the voltage to ground will be low everywhere on that phase, and all of the indicators on that phase will activate, everywhere in the connected system. (Sure, there will be small differences in voltage, getting closer to zero the closer you get to the actual faulted system...)

To actually find the ground fault you need to look for current flow. big john mentions one technique, intentionally 'pulsing' a supply connection to ground and looking for the pulsing current in your feeders and circuit conductors. I believe that there are other possible techniques, such as injecting higher frequency common mode AC (AC voltage between the system and ground at some frequency different from the mains so that you can detect it.) But in all cases you would need a tool to look for current flow.

In North America, ungrounded and high resistance grounded systems are used for 'continuity of service', to permit a system to operate during a ground fault. It is entirely possible to design a system solely for the purpose of reducing ground fault current, not maintaining operation. This requires the use of ground fault detecting breakers, since ground fault current is low and would not trip OCPD. I don't believe such 'fault current reduction' fit well with NEC requirements, but there doesn't seem to be much in the way of market.

-Jon
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
What is benefit of an ungrounded system if the fault detectors kill the power?

Here ungrounded systems are used to maintain power during a fault, the fault indicators just let you know you have a fault to fix

I agree. The only good reason I can see for an ungrounded system is to keep the power on if there is a ground fault.

Generally, the fault detectors I see are just dim lights where one glows brighter on a ground fault. And I can't say I have seen all that many of them.
 

electricman2

Senior Member
Location
North Carolina
Occupation
Retired Electrical Contractor
The wastewater treatment plant I worked at several years was supplied with a 480 volt ungrounded delta system. It was built in the mid 60's before ground fault indication was required. I assembled a simple indicator consisting of 2 240volt lamps in series from each phase to ground.

The lamps burned somewhat dimly under normal conditions as to ground voltage was usually around 300volts. A grounded phase would light two pairs at full brilliance and the other would be very dim or complexly off.

After a while we learned the most likely place to start looking, shutting feeders off till we isolated the faulted circuit. Worked pretty well.
 
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