250.21(B), Ground detectors

Jpflex

Jpflex

Electrician big leagues
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Victorville
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Electrician commercial and residential
For ungrounded systems requiring ground fault detectors for ungrounded systems operating at not less than 120 volts and at 1000 volts or less, does this factor in whether "operation" is factoring utilization secondary or supply primary?

In other words as long as primary or secondary exceeds 1000 volts we no longer are required to install ground detectors?

An old ungrounded delta transformer i worked on was 4500 vac primary to 480 VAC secondary. The system did not have ground fault detectors. I assume this set up was correct because the primary side of the system exceeded 1000 vac. Is this correct?

The code doesnt destinguish the primary from the secondary portion of the system.
 
I think the answer is in your last sentence:
"The code doesnt destinguish the primary from the secondary portion of the system."

If you have a 480v ungrounded system, regardless of source, it should have ground detection.
 
I remember a few ungrounded systems from back in the 90's some developed a unique overvoltage problem
when intermittent ground faults happened, surges over 1kv would happen, some epic equipment failures.
I did not fully grasp it back then as I was just starting out, but I demoed a lot of burnt plastic because of the over voltage problem.
I think it was related to capacitance I still don't know. I do remember the old EE's debating which was better, convert to wye or corner grounded.
All the 480V ones went wye due to the new drive systems, some 240V ones were corner grounded, the last corner grounded 240 I knew of was converted to 240Y139 just recently.
 
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augie47 said:
I think the answer is in your last sentence:
"The code doesnt destinguish the primary from the secondary portion of the system."

If you have a 480v ungrounded system, regardless of source, it should have ground detection.
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If the system has a feeder primary operating over 1,000 VAC, despite the secondary at 480 VAC then it should not require ground fault detection based on the nec code wording
 
Jpflex said:
If the system has a feeder primary operating over 1,000 VAC, despite the secondary at 480 VAC then it should not require ground fault detection based on the nec code wording
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If the 480V is not grounded, it requires a ground detector regardless of the primary side feeder to it. Both sides of isolated systems stand alone.
 
Jpflex said:
If the system has a feeder primary operating over 1,000 VAC, despite the secondary at 480 VAC then it should not require ground fault detection based on the nec code wording
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If your system is 13kV to 480V then you will need ground detection on the low side.

IF your system is 480V to 13kV then you will already have ground detection on the primary since your ungrounded feeder would be from a ungrounded source like the utility service or another transformer.

The wording says just that. Your system, separately derived or service, would need ground detection if ungrounded under 1000V. The code even lets you choose the location. In my experience it is the first overcurrent device.
 
Jpflex said:
For ungrounded systems requiring ground fault detectors for ungrounded systems operating at not less than 120 volts and at 1000 volts or less, does this factor in whether "operation" is factoring utilization secondary or supply primary?

In other words as long as primary or secondary exceeds 1000 volts we no longer are required to install ground detectors?

An old ungrounded delta transformer i worked on was 4500 vac primary to 480 VAC secondary. The system did not have ground fault detectors. I assume this set up was correct because the primary side of the system exceeded 1000 vac. Is this correct?

The code doesnt destinguish the primary from the secondary portion of the system.
Click to expand...
The answer to your question is in your question. "an old ungrounded delta". While ground detectors were available, they were not required up into the 70's. They may not have them simply because they didn't require them. The same goes for "corner grounding" it wasn't required at the time. In my jurisdiction we had a 4K + system stepped down to 480v and it was an "open delta" without ground detection and when it finally failed, it failed dramatically. The switch gear was about 35' long and had multiple meters, 4 of the meters were blown across the room and the meltdown set 2 power poles on fire as far as 500' away.
 
jim dungar said:
If the 480V is not grounded, it requires a ground detector regardless of the primary side feeder to it. Both sides of isolated systems stand alone.
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Back when i worked at a mine as an industrial electrician, i told the company owner and employee associates that the system needed ground detectors.

Everyone including another former electrician said I was wrong and it was never done. Mines are not under NEC but MSHA so who knows if they are required to follow typical nec compliance.

So recently i revisited the code and thought the peimary being over 1000 Vac exempted this requirement.
 
Jpflex said:
If the system has a feeder primary operating over 1,000 VAC, despite the secondary at 480 VAC then it should not require ground fault detection based on the nec code wording
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The secondary side is a separate system (separately derived system) and the language applies to the system. If the secondary has a voltage of 1000 volts or less, it requires ground detection.
 
Are you sure it was a ungrounded system not a 'resistance or reactance grounded' system?
Were there any VFD driven 480V motors this ungrounded system you worked on ?
I would imagine some equipment like a VFD would have come with built-in surge suppression I wonder how that would work in a truly ungrounded ungrounded system if the wye point floated over the clamping voltage?
 
tortuga said:
Are you sure it was a ungrounded system not a 'resistance or reactance grounded' system?
Were there any VFD driven 480V motors this ungrounded system you worked on ?
I would imagine some equipment like a VFD would have come with built-in surge suppression I wonder how that would work in a truly ungrounded ungrounded system if the wye point floated over the clamping voltage?
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It was not an impedance grounded system. It was a delta delta 4500 primary to 480 secondary with no derived neutral.

This fed another transformer using 2/3 legs. Sencond transformer delivered 240/120 Vac. This was to power a lab and plasma particle analyzer for breaking down minerals

Also this unit used an isolated ground receptacle
 
tortuga said:
Were they running other 480 equipment with VFD drives ?
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i dont believe any of the motors were run from VFD drivers. We had 3 phase motors from 1 hp all tbe way tp 100 hp being ran from it qhile protected from older style Nema starters and heaters
 
don_resqcapt19 said:
The secondary side is a separate system (separately derived system) and the language applies to the system. If the secondary has a voltage of 1000 volts or less, it requires ground detection.
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I wish the nec would clearly specify which part of the "operating system" being over 1,000 VAC. Otherwise its speculation and arguable
 
Jpflex said:
I wish the nec would clearly specify which part of the "operating system" being over 1,000 VAC. Otherwise its speculation and arguable
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No it is not.
You apparently have two "operating systems", one >1000V and another at 480V. I am pretty sure you have some more like 208V and maybe even some DC.

Each system stands on its own. See the NEC definition of separately derived systems.
 
See the definition of a separately derived system.
Though now that I ponder that
one could in theory create a power supply that does not meet the definition of a 'separately derived system'
with one side over 1000V and one side 1000V or under both electrically connected, PV systems and wye-wye transfromers come to mind but nothing you'd find at a old gravel mine.
 
jim dungar said:
Just because they are wye-wye construction does not mean they are not separately derived systems. Auto-transformers are an example of a non-separately derived system.
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The only non-utility wye-wye's I know of are off a 4160 buss system; 4160/2400 : 480/277 or 4160/2400 : 600/347 transformers. The HO and XO are grounded, would you consider that a SDS?.
They are all 70's era, I know the definition has changed slightly I am genuinely curious what your take is?
 
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