3000A Ground Fault Protection breaker tripping

[powersoff]

I had a 3000 amp 480 volt breaker trip on a Ground Fault. The Hmi for the ground fault showed 79Amps. I did find a 36kw heater with all 3phases grounded. It was fed with (3) 50a LPJ fuses. 1 of the fuses was blown.
In the future I would like fuses to open before tripping the 3000 amp breaker. Does anyone have any suggestions as to what sort of protection should be implemented to act faster than a ground fault protection ?

 

[PHovnanian]

Tech support is stating that if ground fault relay set above 200amp pickup all sorts of bad things could/will happen and that I need to implement even more ground fault protection to be safe.

And they know this how? That's the sort of thing a protection study will reveal. So if they've already got one, have them cough it up.

They might be correct. There may be something downstream that needs that level of sensitivity for GF protection. Perhaps a GF function needs to be added to a branch circuit (or several) downstream.

Dialing in a time delay might fix your problem. But, aside from that 36 KW load, what else lies downstream of the 3000A breaker and what sort of TD value will achieve coordination with the rest of your system?

 

[waross]

I did find a 36kw heater with all 3phases grounded. It was fed with (3) 50a LPJ fuses. 1 of the fuses was blown.

Sorry for not being ,clear the connections at the heater were bad/grounded/in water.
I understand that the ground fault breaker did what it was supposed to.

You had a ground fault and the ground fault protection acted.
You may increase the trip setting on the breaker so that ground faults are cleared by the fuses, but, you may end up boiling a lot of water in a similar instance if a ground fault is not drawing enough current to blow the fuses. It takes awhile for 79 amps to blow 50 amp fuses. It is possible that the next time that a junction box is flooded the fault current may not be enough to blow the fuses. I don't see a higher setting on the only ground fault protection as a good idea.
The reason that the codes call for ground fault protection on large installations is to prevent large scale damage in the event that a serious arc to ground is not cleared by the over current protection.
OR
You may install ground fault breakers coordinated with the main breaker so as to clear ground faults without taking the whole plant down. The added breakers may be the main feeds for panels or installed on the individual circuits, or both.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[powersoff]

Thanks for all the replies.
Most of the end use is motors,roughly (40) 20 hp + 40 1/2-10hp motors.
125kw of heaters .
The biggest I line breaker in the switchboard is 800 and the smallest is a 60.

What concerns me about adding more GF protection is the possibility of more nuisance tripping. Granted it would save the 3000 amp main. But if say instead of a 15amp fuse blowing now I am taking out 200 amp GF breakers. I will not be getting a raise.

 

[HienN]

Could you post the diagram up here. We should see the diagram.

 

[rbulsara]

powersoft:

What concerns me about adding more GF protection is the possibility of more nuisance tripping. Granted it would save the 3000 amp main. But if say instead of a 15amp fuse blowing now I am taking out 200 amp GF breakers. I will not be getting a raise.

Your concerns are valid and your thinking as well. Plus adding GF at lower levels is expensive and is not normally done in systems like yours. The key is to set your GF pickup (and delay) on the main less sensitive than all or most of your feeder/branch breakers.

The NEC permitted maximum limit on GF setting, I indicated earlier, is there for a reason. In fact it is there exactly to address the situation you are in without having to go through expansive studies and engineering. Make "full" use of it.

The alternative is to have a coordination study done by a professional.





Rafiq Bulsara

http://www.srengineersct.com

 

[PHovnanian]

The alternative is to have a coordination study done by a professional.

Considering the impact of just tweaking the GF time delay and calling it good will have on things like arc flash protection and maintenance procedures, this would be a really good idea.

Since powersoff's technicians expressed concerns with changing the original settings, it would be a really good idea not to catch them by surprise the next time they get to wrenching on a live system.

 

[rbulsara]

Let us not confuse a proper GF setting and coordination with arc flash analysis. Basis of device settings should always be coordination.

No one should be wrenching on a live system to begin with and if they do, surprises should only be expected. Arc flash analysis and PPE although helpful, provide a false sense of security.

Working live should only be a last resort and requires a permint per NFPA 70E. If any settings are messed around for arc flash, they should only be temporary or at least not at the expense of permanent coordination.

PPE only protects against burns not against shrapnel or blast pressure. Even the burn injuries could be a second degree burn while wearing a PPE, which is most painful burn injury but curable. So anyone thinks that just by using PPE as we know today will save them from surprises or all injuries should think twice.



Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

Plus ground fault setting has no effect on arc flash analysis as it only considers 3 phase faults.

Rafiq Bulsara

http://www.srengineersct.com

 

[PHovnanian]

Plus ground fault setting has no effect on arc flash analysis as it only considers 3 phase faults.

Protection time delay does affect arc fault energies. And I do consider ground faults when doing these analysis. The worst case cumulative energies can occur for lower power faults (either ground or phase) if the protection for that case is slow.

 

[davidbeach]

But none of the calculations of arc energy are for single phase faults. You don't have any standard or code based reason to look at anything other than three-phase faults when doing an arc-flash analysis.

 

[PHovnanian]

But none of the calculations of arc energy are for single phase faults. You don't have any standard or code based reason to look at anything other than three-phase faults when doing an arc-flash analysis.

That depends on whose standards I'm using. Not everyone buys into NFPA or IEEE formulas blindly.

For systems above 600V, the code requirement is to determine the flash protection boundary at 1.2 cal/cm^2. If I can show where, for a combination of lower fault currents and longer clearing times* the energy exceeds that figure, I'll use my own judgment.

*For 600V or lower systems, although the code specifies the fault power formula, the exposure time is _not_ tied to this current level. If an actual fault is a single phase to ground and the applicable clearing time is longer, then that's the value of 't' you've got to plug into the formula.

 

[WhiteyWhitey]

Is it possible to replace the heater fuses with something more sensitive?

A 20A fuse or something?

I spose if your main breaker is popping out at 0s then it may not make too much difference but any changes could be minimised in the main breaker if you can size the heater fuse more accurately.

Thanks,
Andrew

 

[PHovnanian]

A 20A fuse or something?

A 20A fuse will open with 120A fault current in a couple of seconds. The GF setting of 120A, zero seconds delay is much faster than that.

So there is some range of fault currents for which the suggested fuse will not coordinate with the stated GF setting.