GFP on distribution panels - no big deal, or try to avoid?

[malachi constant]

I am designing distribution for a large school (about 500,000 SF). The mechanical engineer is leaning towards a distributed chiller system (a number of 400 to 800 MCA chillers located around perimeter and on roof). I was penciling in a couple 1200A MLO distribution switchboards to feed multiple chillers, then realized since these are 480V and over 1000A they would need GFP, which then triggers the need for a main - correct? Or are you required to put the GFP ahead of the feeder, which would put it at the 1200 BPS in the main switchboard, and would eliminate the need for a main at the DP? A main is not a terrible option, but it does increase cost and footprint. Thinking it through, I'm guessing putting GFP at the upstream OCPD is more common then adding a main the downstream equipment - right?

Another option would be feeding more chillers direct from the main service switchboard so that I have no downstream equipment over 1200A. This would eliminate the need for any GFP beyond the main service disconnect - and in my experience downstream GFP trip with enough frequency it seems good to minimize them.

For whatever reason I've managed to avoid thinking about downstream GFP for most of my career and am curious how the design community approaches larger scale 480V systems like this. Minimize GFP (keep equipment at 800A) for nuisance tripping purposes? Ignore concerns of nuisance tripping and just design to minimize feeder/distrubution costs? Does anyone see downstream GFP as something that should be embraced so as to increase safety, as opposed to avoided due to nuisance concerns?

I'm leaning towards the "keep distribution below 800A" - the largest chiller is closest to the service. I think all the rest are far enough apart from each other and closer to 400A that this is justifiable. But your thoughts/experience is appreciated. Thanks in advance!

 

[augie47]

Likely from a cost standpoint, when given a choice most the jobs I see steer away from GFP when possible making MLO panels with feeders less than 1000 amp common but I think you would have to factor in the potential cost savings of series rating the feeder OCP with a main vs fully rated feeder OCP..

 

[skeshesh]

Likely from a cost standpoint, when given a choice most the jobs I see steer away from GFP when possible making MLO panels with feeders less than 1000 amp common but I think you would have to factor in the potential cost savings of series rating the feeder OCP with a main vs fully rated feeder OCP..

Agreed with this approach but it really depends on the application and where you need to isolate the ground fault from an operational perspective.

In case of critical facilities, i.e. hospital, correctional facilities or military, there may be a need to isolate the fault downstream even if it's going to lead to extra cost.

 

[augie47]

:thumbsup:

 

[winnie]

Stepping well outside of my experience rating :

If you have ground fault protection at your distribution panels, and it is properly coordinated with your main, then you can avoid the issue of a small fault on a distribution panel taking out service for your entire campus.

My understanding is that there are systems which signal between the distribution panels and the main to enforce coordination.

Just another design issue to consider...

-Jon

 

[Ingenieur]

isn't it 1000 and over?
with a max trip of 1200
if service >=3000 a 1 sec td allowed

nuisance tripping should not be an issue at 1200/1 sec

http://www.cooperindustries.com/con..._Ground_Fault_Protection_Requirements_GFP.pdf

how many chillers?
total load/service transformer kva?
eg a swbd with 6x800 does not require it

without knowing bldg layout , equip locations, service location, etc, hard to say what is the best approach

this is likely a >$700 mil project, I would not let gf cb cost be a factor
100 mil elec?

 

[topgone]

I am designing distribution for a large school (about 500,000 SF). The mechanical engineer is leaning towards a distributed chiller system (a number of 400 to 800 MCA chillers located around perimeter and on roof). I was penciling in a couple 1200A MLO distribution switchboards to feed multiple chillers, then realized since these are 480V and over 1000A they would need GFP, which then triggers the need for a main - correct? Or are you required to put the GFP ahead of the feeder, which would put it at the 1200 BPS in the main switchboard, and would eliminate the need for a main at the DP? A main is not a terrible option, but it does increase cost and footprint. Thinking it through, I'm guessing putting GFP at the upstream OCPD is more common then adding a main the downstream equipment - right?

Another option would be feeding more chillers direct from the main service switchboard so that I have no downstream equipment over 1200A. This would eliminate the need for any GFP beyond the main service disconnect - and in my experience downstream GFP trip with enough frequency it seems good to minimize them.

For whatever reason I've managed to avoid thinking about downstream GFP for most of my career and am curious how the design community approaches larger scale 480V systems like this. Minimize GFP (keep equipment at 800A) for nuisance tripping purposes? Ignore concerns of nuisance tripping and just design to minimize feeder/distrubution costs? Does anyone see downstream GFP as something that should be embraced so as to increase safety, as opposed to avoided due to nuisance concerns?

I'm leaning towards the "keep distribution below 800A" - the largest chiller is closest to the service. I think all the rest are far enough apart from each other and closer to 400A that this is justifiable. But your thoughts/experience is appreciated. Thanks in advance!

Been there, done that and got burned using GFPs (1000A above). The last paragraph seems the best option for me. It is very hard to keep breakers coordinated when it comes to ground fault protection.

 

[Ingenieur]

the service requires it
it is lkely >=3000 so a 1 sec delay
he can use lsig cb's for the other >800 to get some adjustability/coordination

I would not complicate the system to avoid the gf protection
moot: we know service >= 1000 so the main MUST have it
so he better plan on using it on feeders to coordinate

215.10 ex 2
if he has it on the main with no xfmrs do the feeders require it?