How to fix Corrdination between breakers

[tlona]

I have an 2500A, 480V distribution panel (call it DS1) with a 800A current limiting fuse. The fuse feeds an 800A, 480V MLO panel (Call it Panel P1). P1 has a 225A branch feeding another 480V MLO panel. The 800A Fuse and the 225A coordinate under overload but not fault currents. There are not adjustments that can be made. How do you correct the coordination between two overcurrent protection devices whether it be between a fuse and breaker or 2 breakers. Thank-you

 

[dpc]

1. Assuming these are molded case circuit breakers and you are in North America - the breakers must be equipped with instantaneous trip elements, so in general, complete selectivity between two molded case circuit breakers is often simply not possible. It will depend on the magnitude of fault current available. In some cases, an adjustable inst. trip can be set high so it will never actually trip.

2. Similar issue coordinating between upstream fuse and downstream MCCB. It will depend on the magnitude of fault current. Full coordination may not be possible.

3. If it absolutely must coordinate, current limiting fuses (low voltage) are the best option. If the upstream fuse is 2X the size of the downstream fuse, it should always coordinate. In some cases the ratio is 3:1, so check with manufacturer.

Or just live with the possible lack of coordination.

 

[Mbrooke]

What code or standard requires instantaneous pickup?

 

[tlona]

Yes these are MCCBs in North America. Thank-you for confirming my suspicion. And yes the lack of coordination we are seeing is between a ration of less than 2:1 between 2 OCPDs. Not sure what standard requires instantaneous pick-up, may be UL 489.

 

[dpc]

What code or standard requires instantaneous pickup?

UL 489

 

[LionelHutz]

Did you check to see what fuse would be required to co-ordinate, or if there was a different fuse that would work? It simply might not be possible to achieve co-ordination with fuses upstream of a breaker. You may need to replace the fuses with an adjustable breaker.

 

[tlona]

Thank-you, yes looked at replacing fuse with adjustable breaker but then I run into a coordination issue with the main. Is just not acheivable. Thank-you all for your input.

 

[waross]

Thinking out of the box, will it help to reduce the fault current a little?
In the early part of the last century there was a device in use called a "Wireless Reactor".
It would add a few percent to a transformers impedance to allow efficient parallel operation of transformers of different impedances.
It was a stack of laminations about 6" or 8" tall. One of the secondary conductors passed through the center of the stack.
You may be able to get the same effect with a couple of turns through an old window type CT.

Bill
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"Why not the best?"
Jimmy Carter