Breaker Coordination Study
Breaker Coordination Study
(OP)
I've hired a company to perform a coordination study for a facility and I'm reviewing their recommendations. The system is 600V, with an 800A main breaker with LSI trips and adjustability. The largest downstream overcurrent devices are 400A drive fuses.
The study recommends that the main breaker be set such that it coordinates with the utility fuses. The problem is that with these settings, there is some overlap between the main breaker and the drive fuses. The overlap only occurs between 2500A and 3000A fault levels. The calulated bolted fault is 15kA, calculated arcing fault is about 11kA.
Is this overlap a concern? Is it even possible to have a fault in the 2500-3000A range which would result in the main breaker tripping? What would the fault conditions have to be?
The study recommends that the main breaker be set such that it coordinates with the utility fuses. The problem is that with these settings, there is some overlap between the main breaker and the drive fuses. The overlap only occurs between 2500A and 3000A fault levels. The calulated bolted fault is 15kA, calculated arcing fault is about 11kA.
Is this overlap a concern? Is it even possible to have a fault in the 2500-3000A range which would result in the main breaker tripping? What would the fault conditions have to be?






RE: Breaker Coordination Study
A prolonged phase fault in a narrow current range on your main bus is fairly unlikely.
RE: Breaker Coordination Study
As you surmised, the odds of a fault in that range is small. The utility short circuit level would have to be reduced to 20% for that to happen. That is not likely unless your system is unusual because your service transformer's impedance represents the major portion of the utility system impedance and that is not going to change much.
It may be possible for an unusual arcing fault to limit current to that level, but I doubt it could maintain that level long enough to hit the trip unit trip curve. More likely the fault will escalate and go to higher currents.
RE: Breaker Coordination Study
How big is the drive fuse, and what type of fuse is it? And what size are the utility fuses?
RE: Breaker Coordination Study
RE: Breaker Coordination Study
The maintenance personnel are pushing for full coordination and I am looking to justify the acceptability of the current installation and the recommended breaker settings. The breaker and fuses are already installed and we don't want to make any costly changes.
RE: Breaker Coordination Study