Circuit breaker delay coordination

[siena]

I'm accustomed to spec'ing circuit breakers so that the downstream breakers have a shorter delay than the upstream (main) breakers.

But what do I do when I have a load with a high inrush current? I pretty much have to use a breaker with the longest available delay for that load, so I won't be able to make the main breaker delay curve any longer. I risk tripping the main breaker before the branch breaker.

Can anyone shed any light on this?

 

[Manindemand]

What type relays do you have?

If you have micro-processor relays you can set your coordination pretty tight. You can set logic in the main relay to block tripping as long as the feeder is timing out. You would need a 2 conductor cable from the feeder to the main so the feeder can send the block trip signal to the main.

 

[rbulsara]

Coordination is often a trade off between competing goals and compromises are almost unavoidable so do not feel bad about it.

Since a ground fault is more likely to happen than a 3 phase bolted fault, one of the ways to minimize mis-coordination is to provide ground fault relays which are much easier to coordinate by setting low pickup and fast time for the downstream breaker. If required, you can employ zone interlocking as suggested by 'Manindeman'. ( I am not crazy about zone interlockings).

By the way what type of breaker, trip units, current and voltages are you talking about here?.

 

[dede61]

There are existing numerical protection relay who can take in account the inrush current of tranformers. Like suggested by "manindeman" interlocking with blocking signals is a common way in this case to prevent tripping of the main feeders.

regards,

Danny

 

[siena]

Thanks for the tips, all of you.

This is probably a much smaller, simpler application than you are accustomed to. The supply voltage is 120/208 and the main breaker is 100A. One of the loads is a 3-phase air conditioner that draws 46A steady state and 239A locked rotor.

The breakers are basic magnetic type with fixed delay curves (they are not programmable).

 

[dede61]

In that case selectivity is normaly made using B-C or D curves

regards,

Danny

 

[rbulsara]

The AC unit cutsheets usually have maximum OCPD rating on it (fuse or brekaer), indicated by MCB or MCF, just use that rating.

You can not coordinate any breaker or fuses with less than 2:1 ratio, more so with breakers.

You may forget about coordinating this one, as you will need minimum of 60A breaker. It will not coordinate with a 100A breaker.

 

[Borti]

You can specify a molded case circuit breaker (MCCB) equipped with a solid state trip unit having short time delay feature as the main breaker.
The branch breaker can be a MCCB with a thermal-magnetic trip unit, possible with an adjustable magnetic trip (usually between 5 and 10 times pug rating).
The selectivity between these two breakers is possible, as the short time pickup and delay can be adjusted on the main breaker. The only concern is the instantaneous override typical for MCCB's. The manufacturer should be consulted regarding the current value. You may have fault level below this value, in which case you would have acceptable selectivity.