3000 amp ground fault question

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powerpete69

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Location
Northeast, Ohio
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Professional Electrical Engineer
In our factory, we had a Roof top unit with a ground fault, the hot wire was laying on the chassis and immediately tripped the 70 amp breaker feeding it. There was a 600 amp breaker above it that did NOT trip. However, the 3000 amp main breaker above that 600A did trip. There is an attached picture of it.
The maximum short circuit for three phase at the RTU bus is 7,000 amps, and its maximum short circuit for line to ground is 5,500 amps per "EasyPower"
The instantaneous breaker curve of this breaker says it wont trip to 25,000 amps, so I believe that is out of play.

We believe the ground fault of the 3000 amp breaker is what tripped. It is currently set at 720 amps if you look at the picture.
Even if I set it up to its max at 1200 amps, isn't it still going to trip when a small unit like a rooftop short circuits at 5,500 amps?
Doesn't make sense to me. What am I missing and what is the main purpose of this ground fault if the setting is so low?
Knocking out a whole factory is not good when a small unit short circuits line to ground.
 

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  • 3000A MB.jpg
    3000A MB.jpg
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I found the breaker curve for the ground fault. If I crank up the I2 OUT delay from .2 to .5, this changes the start of the trip time from .08 seconds to .5 seconds.
This could help considerable.
 
Here is the .2 delay curve for the ground fault compared to the other breakers.
 

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  • .2 delay ground fault curve.pdf
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Here is the ,3 delay curve compared to the other breakers. Looks like just this small adjustment would help signifigantly.
I don't see any reason to change the long time setting from 750 amps.
Nothing like the power of one self!!
I would still appreciate any input if possible! Thanks.
 

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  • .3 ground fault delay curve.pdf
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This is the problem when your system only has one level of ground fault protection (NEC minimum).
In my experience it is very difficult to get a 'main' ground fault device to coordinate with any downstream thermal magnetic device larger than 30 to 70A. Although for practical reasons, I usually provide settings to coordinate with 30A breakers, because those seem to be the most likely circuits to be worked on hot.
 
As Jim mentions, this is the big downside to having GFPE on only the main. Nobody wants to spend the money to protect all the feeders and coordinate them unless it is an installation where it is required by code. Seen many a large building blacked out after small branch circuit faults.
 
Probably need an engineered study for the settings, that removes your liability if you turn it up on your own. We have a customer that says to turn the settings all the way up on their stores. I refuse due to liability.
 
Thanks.
I'm hoping cranking up the delay from .14 seconds to .30 seconds seconds allows the downstream breakers to clear.
I am the engineer. The buck stops with me.
 
Thanks.
I'm hoping cranking up the delay from .14 seconds to .30 seconds seconds allows the downstream breakers to clear.

I have found that time is more critical than the pickup point, yet it is amazing how many systems are dialed up to 1200A with minimal, if any, delay.
 
I have found that time is more critical than the pickup point, yet it is amazing how many systems are dialed up to 1200A with minimal, if any, delay.
I agree. The long time setting is not in play in short circuits. I was able to solve my own problem once I found the curve for the ground fault portion of the graph. Was just a simple click in Easypower under short circuit settings.
Thanks for your input.
 
Usually it’s equipment far from the gear that trips it, the impedance tends to limit the short circuit amps, along with any resistance in the return ground path, so the fault current doesn’t rise fast enough to trip the local breaker, but is high enough to cross the Ground fault relay’s threshold. The time delay should help.
 
I have found that time is more critical than the pickup point, yet it is amazing how many systems are dialed up to 1200A with minimal, if any, delay.
Usually I find the fault setting at its lowest setting, (factory set) and if tripping is a frequent issue, I will turn the delay and amp setting up one notch. Seems to work most of the time. If not, I advise the customer to get an engineered study.
 
Usually I find the fault setting at its lowest setting, (factory set) and if tripping is a frequent issue, I will turn the delay and amp setting up one notch. Seems to work most of the time. If not, I advise the customer to get an engineered study.
I am the engineer! haha....the breaker curves above are from my engineering study. I couldn't see what was going on until the ground fault curve showed up....then it was clear we needed to up the delay.
Engineers are people too! We don't always know everything, but we hopefully eventually get there!
 
I couldn't see what was going on until the ground fault curve showed up....then it was clear we needed to up the delay.

This is why I insist on having a typical 30A branch circuit protective device on all of my GF curves.
I see no use behind a GF curve showing a single device.
 
I am the engineer! haha....the breaker curves above are from my engineering study. I couldn't see what was going on until the ground fault curve showed up....then it was clear we needed to up the delay.
Engineers are people too! We don't always know everything, but we hopefully eventually get there!
Yeah, I know, I was just giving my experience on dealing with ground fault gears. LOL! I work with a lot of engineers giving them the electricians perspective on what works on paper doesn’t really work in the real world sometimes! LOL! We are all in it to make it work. Magnetic trip adjustments, transformers and generators are another combination that can give fits. LOL!
 
That was going to be my question.
Does the PPE and signage change now, or is it still within a specific PPE range?

No.

AF incident energy is based on 3-phase fault currents. The GF current is usually a single-phase line to ground event, and is not considered in an Arc Flash analysis.
 
No.

AF incident energy is based on 3-phase fault currents. The GF current is usually a single-phase line to ground event, and is not considered in an Arc Flash analysis.
Let's stop fencing here.
When I plug in the original settings of that ground fault at 750 A with .2 delay, I get a .2 calorie at the roof top unit in question. It has 6.9 KA available fault current at three phase fault and a 5.13KA fault current from line to neutral.

When I max out the settings of the ground fault to 1200A and .5 delay, I get a .2 calorie at the roof top unit in question. It has 6.9 KA available fault current at three phase fault and a 5.13KA fault current from line to neutral. I checked this for several other buses as well. This means that there is no change in calories, short circuit amps, PPE...or the like....and it means that Jim Dungar was correct.

EasyPower is a wonderful thing!
And....and...I was able to accomplish this without having to contact the engineering department!!! Haha
 
If anyone has any questions about how any of those breaker curves work that I posted above...ask away. We are all here to learn. I'm sure I will learn some things as well as the greatest minds in the world collaborate!! I have certainly learned today!

Incidentally, in the original .2 delay curve I have posted, the ground fault should not have tripped according to the graph. Per the graph, the 70 amp breaker should have cleared well before tripping the ground fault. However, breaker curves, times and amps or only approximate. I think moving the delay to .3 seconds should give enough time for that 70 amp breaker to clear. Maybe the 70 amp breaker took much longer to trip than expected due to it being old or something like that. Maybe the ground fault trips much sooner than expected. The next ground fault at this factory will tell the story.
 
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