Coordination between switchboard and local starter

[shkim2000]

The local motor starter is powered by switchboard(fuse contactor) and it has branch circuits i.e. have 3 motor starters. But the problem is fault current rating of local starter is 10kA while the fault current rating of switchboard is 65kA. For this reason, fuse in the switchboard is limited to less than 10kA in order to protect local starter. In the case of one of three motors earth fault, I think that circuit breaker for motor should be opened before fuse is blown on switchboard. If fuse is blown, is there problem ?

 

[NYedison]

Shkim2000

You are right about the “ the Motor starter Breaker should open first if the motor developes fault”
The coordination expects that the load side motor breaker should sense the fault and act before the switchboard fuse blows.

Even if the single fuse in switchboard side (upstream) blows in the event of any motor fault one phase voltage to all motor starters is lost . If the starters have single phasing protection all motors will trip.

About fault clearing capacity of 65kA and 10 kA . Is that (65kA) the fault level at that location? You may confirm the fault level.
Do you have breaker at every motor starter and No breaker upstream on the switchboard. Need more details
Marvin

 

[RRaghunath]

Though the fault level at switchboard is 65kA. It reduces rapidly at the load end, depending on the cable length. This can be estimated once the cable size and length are known.

With the present arrangement considering the fault level at load end is greater than 10kA, it is important that the fuse operates faster than the circuit breaker (which is rated 10kA). Otherwise, there is a possibility of damage to the circuit breaker. For this reason, it may be necessary to disable the short circuit release in the circuit breaker (in order to allow fuse to blow for all short circuits).

It may be desirable to have a fuse in the local starter that is rated at >65kA, upstream of the circuit breaker.

 

[mc5w]

Almost every motor controller manufacturer requires class J or class CC current limiting fuses because of your situation. Each motor controller and each motor overload relay should be marked with the maximum class J fuse allowed. For the smaller motor controllers and overload relays there will be a marking for class CC fuses as well. This is for U.S. rules.

U.S. current limiting fuses are starting to penetrate the international market partly because people want to standardize and some countries do have strange fuses, particularly Germany and Japan. There are also some European current limiting fuse designs but when machines are imported into the U.S. we like to refit for Class J or Class CC.

 

[alehman]

The starter must be rated for the fault current available unless the starter assembly is series rated in combination with the upstream device. Also, the NEC doesn't allow use of series ratings if the motor current exceeds 1% of the AIC of the starter. You need to determine the actual fault current available at the starter, not the upstream board. Many fusbile starters with class R or J fuses are rated 100kAIC.

"Almost every motor controller manufacturer requires class J or class CC current limiting fuses..."

Not sure if this is completely accurate. I think many styles of combination starters are available for class R fuses.

http://ecatalog.squared.com/catalog/173/html/sections/14/17314030.html#1008458

 

[buzzp]

Real similar discussion going on here,
thread238-118419
I think you would find it useful. I posted some stuff there which I am being to lazy to cut and paste in here.