Coordination Question 3

[wbd]

Hello,
I have been reviewing a coordination study done in-house at corporate level. I am in the plant. There appears to be some lack of coordination with the utility fuse, main breaker (2000A) and feeder breakers (600A). I am being told that the reason is breakers are large in relation to utility transformer 750kVA. Apparently, there is no way to improve coordination unless either feeder breakers are smaller or utility transformer is larger. Is this correct? Looking for independent viewpoints as I have to live with this at my plant. The TCC is attached (I hope)
Thank You

 

[rcw retired EE]

Wihtout spending a lot of time comparing curves, I would agree with the analysis. Unless the feeder breaker's pickups can be reduced to move the curves to the left, there is not much room for coordination.

 

[dpc]

In my view, it is more important to coordinate the main breaker with the feeder breakers downstream than coordinating the main breaker with the primary fuse So I would increase the main breaker Long-time delay and short-time pickup to better coordinate with the feeders. This will make coordination between the main breaker and the primary fuse worse, but both of these interrupt the same amount of the system - all of it. Although the transformer and primary fuses appear to belong to the utility, for what it's worth, I think the fuse could be larger. The main breaker will provide the thermal protection for the transformer, except for a fault between the transformer and the breaker. As long as the fuse is covering the mechanical damage portion of the damage curve, I could live with that for a 750 kVA transformer.

 

[jghrist]

You don't say what the loads are or the fault current at the ends of the feeders. If the fault at the end of the feeders is above the instantaneous of the feeder breakers, then bolted faults will coordinate. If there is a chance that the total load will damage the transformer fuse, then you want the main breaker to trip first. Getting the plant back on line after a short overload will be a lot easier if you can reduce load and reset the breaker rather than get the utility out to replace the fuse.

 

[healyx]

Can the downstream feeder breakers be replaced with Micrologic 5.0 or 6.0 variants? If so you could likely adjust their short time settings to coordinate with the main breaker. Will be dependent on the loads attached, large motors might cause nuisance tripping if you do this.
Other than that, I agree with dpc, you might need to sacrifice coordination between the fuse and the main breaker.

 

[burnt2x]

Honestly, you don't need to coordinate your utility fuse with your main breaker: they are serving the same load. If the main breaker trips ahead of your fuse, you lose your trafo in the same way with a utility fuse break.

I agree with others on adjusting the main breaker trip settings to the right; say Inst pu = 8000A, stpu = 5,500A.

 

[dpc]

Well, in an ideal world, it would be preferable for the 480 V main breaker to operate before the primary fuses melt - much easier to re-energize as jghrist mentions and reduced risk of single-phasing. But we have to live in the real world and compromises are often required when trying to coordinate with primary fuses, especially current-limiting fuses.

 

[rcw retired EE]

Another thing to consider is how ground fault protection affects coordination. This is a 480V, 2,000A service which would require GFP in North America. Most faults are ground faults or start that way. The main will trip long before the fuses see the ground fault so there is no problem with main-fuse coordination. But if there is no ground fault protection on the feeder breakers, it will be difficult to coordinate the main with the feeders and still provide resasonable GF protection. Production managers tend to see tripping the main for feeder ground faults as nuisance trips, instead of protection from further damage.

Sliding the main tripping characterisc over does reduce the chance of nuisance trips on temporary overloads.

Check the impact on the arcflash levels at the switchboard before changing the main breaker settings. Increasing the trip time might increase the exposure.