what is the rational behind the code requirement for ground fault starting at 1000A

[mshields]

On a 480V service rated say 600A, the NEC would not require ground fault. But would ground fault provide added protection of the system? What is the rational behind setting the requirement at 1000A?

 

[ActionDave]

I was told that 480V Burns and flashes better than any other voltage above it or below it so ground fault protection makes it safe for mankind. 1000A may be rooted in a study somewhere or it may be somewhat arbitrary.

 

[__dan]

The rationale I've read is that GF trip was added because of the large number of large 480 V services that would (or did) burn down.

A small arc carbonizes over on a nearby insulating material and then the conductive path is (carrying fault current, but current limited by the path through the arc, so the breaker is too slow to respond before catastrophic damage.).

In the days before the GF trip requirement, I had my own guess or assumption that over 600 Amp, the breaker could see the arc like a normal load and just stay on. Fuses I would trust to open, but not the old breakers. Seen and heard of enough stories of stuff that just kept burning.

 

[Jraef]

...In the days before the GF trip requirement, I had my own guess or assumption that over 600 Amp, the breaker could see the arc like a normal load and just stay on. Fuses I would trust to open, but not the old breakers. Seen and heard of enough stories of stuff that just kept burning.

But the requirement for GFP on services 1000A and over applies to fuses too.

A clue as to the rational can be read into the wording of Informational Note 1 in 230.95.

...It serves only to limit damage to conductors and equipment
on the load side in the event of an arcing ground fault on
the load side of the protective element.

 

[mbrooke]

Unlike 120 volts, it is indeed possible to sustain an arc at 277 volts in typical building wiring. An arc has an impedance, so OCPDs may take their time clearing if they do.


Here is an arcing event at 230 volts L-G nominal:



https://www.youtube.com/watch?v=-qCuXF9eDBM

 

[ron]

I wouldn't be surprised if it wasn't the result of a negotiation.

1st guy: All services @any size.
2nd guy: 480V @4000A+
..... 2 weeks later
ok ... more than 150 to ground up to 1000 Phase to Phase and 1000A+

 

[jim dungar]

I want to say it involved data from fire reports, but I would have to look up the specifics.

I wonder if the GF requirement for 1200A and services and protective devices is related to the maximum GF setting of 1200A?

 

[romex jockey]

I wouldn't be surprised if it wasn't the result of a negotiation.

1st guy: All services @any size.
2nd guy: 480V @4000A+
..... 2 weeks later
ok ... more than 150 to ground up to 1000 Phase to Phase and 1000A+

Now i wonder if some ancient substaintiation exists....?


~RJ~

 

[kwired]

rated 1000 amperes or more and installed on solidly grounded wye electrical systems of more than 150 volts to ground, but not exceeding 600 volts phase-to-phase,

It is not a general requirement for 480 volts, it applies to solid grounded wye sytems operating over 150 volts to ground and no more than 600 phase to phase.

480 volt delta system doesn't fit that description, whether it is a high leg or a corner ground system. Ungrounded delta and impedance grounded wye doesn't fit the description either.

Most common systems it would cover are 480/277 and 600/347 volts. Those have a higher tendency to melt down equipment without tripping overcurrent protection is the reasoning I remember being told why GFP is required, just high enough voltage it can sustain arcing but at low enough current it won't trip standard OCPD devices, apparently those over ~1000 amps.

I will add I have still seen several cases where it melted down items even with 60 amp or less overcurrent protection and never tripped any OCPD.

 

[kingpb]

Simply for informational purposes; Exhibit 230.25 and 230.26 indicate the type of protection listed as ANSI 50G. Whereby an ANSI 50N would also provide earth fault protection but it would be by sensing current in each current carrying conductor and mathematically calculate to earth fault.

 

[pv_n00b]

Section 230.95 was first in the 1971 revision of the NEC. Unfortunately, the NFPA does not have the ROP and ROC files for that revision available on their website so we can't look at the proposal and substantiation for it. I have heard that it is designed to protect large 480V 3ph motor loads that were experiencing significant equipment damage due to ground faults.

What I find interesting is that there is little guidance on the setting of the ground fault pickup current. The NEC only says the maximum is 1,200A with a maximum 1sec time delay on a 3,000A fault. What trip setting to use less than 1,200A is up to the user. Is there any guidance on this setting?