Short Circuit Device

Please check out this recent thread....

http://cr4.globalspec.com/thread/71318

It will offer no better protection than the lowest-tripping-rated device.

What is the setting of the short circuit release in each MCCB?

Both having Icu=Ics=50kA .

I am not asking the Breaking Capacity. I am asking the setting made in the Instantaneous (Short Circuit Magnetic) Release made in these MCCBs.

The upstream MCCB setting is 100% & downstream have 80%.

1. EE65 has asked about the Instantaneous (Short Circuit Magnetic) Release setting..i think you are talking about the overload. Yes?

2. Like most people, you don't seem to have checked out the earlier thread on Cascading and Discrimination that i gave ... you would have learnt something i think.

WE have a PCC having 2000A incomer & 4 NOS. 650 A MCCB outgoing feeder & one of these outgoing we connect a MCC having 650A MCCB INCOMER.......

both MCCB having fixed type thermal magnetic short circuit setting = 6xIn.

Then, you will have co-ordination problems, for sure! If there is a short circuit fault beneath the MCC Incomer 650A MCCB, then there is every possibility that the PCC Outgoing Feeder 650A MCCB might trip first. And, you are not sure if the probelm was in the MCC or in the feeding cable, unless you make necessary testing & fault finding exercise.

In this case what is the correct co-ordination according to you?

With MCCBs, one cannot have time discrimination as MCCBs are Utilisation category 'A' breakers and hence would not have an Icw (Short Time Withstand Current) rating. You can try current discrimination, but for that the upstream breaker instantaneous setting has to be set above the fault level beneath the downstream breaker. The danger is that there is no back-up, in case the downstream breaker does not trip for a fault beneath it.

There are talks of energy discrimination among some switchgear manufacturers, but I have my own set of doubts on the effective functioning of the same.

In a nutshell, with MCCBs in series you cannot have an effective co-ordination. If possible, change the PCC feeder breaker to an ACB and set a time delay in its short time release (of course, after due calculations).

Worst case, you have increased the time it will take to troubleshoot a problem because you will not immediately know which one tripped. Not a big issue, but one that is better to be avoided if possible.

In this application it not clear that how you are using these MCCB . In series means MCCB in MAin Panel and second in next panel then you have to set the MCCB for Overload and Shortcircuit protections to achive cascading and discrimination , then MCCB near to fault will clear the fault and trip. If setting are not proper both may trip in case of Shortcircuit