Breaker Coordination Study

[X49]

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?

 

[dpc]

Anything is possible. I would not be too concerned about a small area of overlap, since even if the fuses melt before the breaker trips, the load interruption is essentially the same, since the fuses and main breaker are basically in series.

A prolonged phase fault in a narrow current range on your main bus is fairly unlikely.

 

[rcw retired EE]

It sounds like the main breaker can either coordinate with the upstream utility fuse or the downstream drive fuse but not both. It is easier to recover from a mis-coordination inside the plant than from a utility fuse blowing.

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.

 

[richanton]

The short time setting is usually adjustable in that range, and should be able to be adjusted slightly upward and to the right to clear the curve interference with any downstream fuse. Does the breaker have an I2t adjustment. That could help also.

How big is the drive fuse, and what type of fuse is it? And what size are the utility fuses?

 

[healyx]

What country are you in? Is it a code requirement to provide coordination at that level? Are there any safety services on the load side of the main breaker? If there is you should really ensure coordination. As richanton said try any use a breaker with I2t On/off setting, this setting is specifically provided to sneak above a fuse curve.

 

[X49]

Coordination in this case is not a code requirement. A trip of the main breaker will result in a maintenance call to the site, which is not regularly staffed. There is no safety equipment.

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.

 

[dpc]

It would be hard to justify spending any money to try to eliminate this small area of miscoordination unless there were some serious issues related to the process or systems involved. Some lack of coordination between a main secondary breaker and upstream utility fuses is common. You also need to evaluate the coordination for a low side ground fault, since these are much more common. It is likely that the main breaker has some type of ground fault protection, so will fully coordinate with the primary fuses for ground faults.