Coordinating 4000A with 4000A

[lielec11]

So we have a situation with multiple gens paralleled on a common board. All the gen mains are protected by a combo 6000AF/4000ACB (the fuse is to ensure a 200kA rated board). There are also numerous loads set to 4000A. What is the protocol for trying to selectively coordinate these loads with the mains if they all have the same LTPU setting? At some point, they are going to overlap in the overload region regardless of the other settings.

I've looked at TCC curves in the past on a cogen board where we had a similar issue. We were able to adjust the gen curve based on the "relative" fault seen at the gens vs. the board (i.e. the fault current is much lower at the gen breakers compared to the paralleled board). So this shifted the curve to the right and showed proper coordination.
However, this was a bit ago and I do not remember all of the theory behind the thought process. Anyone else familiar with what I am describing? I have attached a snippet of the one-line for reference.

TIA

https://imgur.com/a/XymkZB6

 

[Elect117]

Please update your occupation.

Get with a licensed engineer in your state to conduct fault analysis and provide a study to coordinate protection on long time pick up and short time delay.

They will need info on the type of loads and the generator start / protection scheme. Ideally the generators would balance in power delivery and deliver more power than the loads can pull.

Long time pick up is usually referring to the overload protection which shouldn't be impacted. Granted, more detailed information is needed before anyone could make heads of tails of it.

 

[jim dungar]

Say you have only (2) 2000A generators and (3) 4000A feeders
Each generator only sees it's current, which is why the TCCs get shifted.
You do not coordinate between the individual feeder breakers.

 

[lielec11]

I am PE I just don't remember the exact theory, was looking for some insight to jog my memory. I understand there are more details needed, but what they are asking us to do for them is to simply prove that coordination between the mains and worst-case branch device is possible. We are not setting all of the breakers.

An inspector is going to look at the drawing, see 4000A LTPU for the mains, 4000A LTPU for the branches, and want to see the same thing on the curves provided. They also want them to coordinate, hence why I was asking about the shifting curve.

I understand the branch devices do not need to coordinate. I am going to mess with the software to see how to shift the curves again. Thanks!

 

[ron]

Because the generator main and feeder both have a LTPU of 4000A, the curves will overlap at least in the LTPU region. You can try to shift the feeder breaker LTD, STPU, STD and Inst down and to the left of the main to provide some level of coordination, but hopefully you don't need to show full selectivity, because you never will be able to.
Crummy design to have 4000A mains and 4000A feeders

 

[lielec11]

Because the generator main and feeder both have a LTPU of 4000A, the curves will overlap at least in the LTPU region. You can try to shift the feeder breaker LTD, STPU, STD and Inst down and to the left of the main to provide some level of coordination, but hopefully you don't need to show full selectivity, because you never will be able to.
Crummy design to have 4000A mains and 4000A feeders

I agree and one of the reasons we may be passing on this job. I don't like touching existing paralleling/EM gear where I don't have more insight on the existing load conditions. Thanks for your comments everyone.