LV MCCB at Incomer / Short circuit withstand time

[cherryg]

This question is specific to a MCC Incomer where ABB MCCB is being used rather than an ACB.
The MCCB short ckt interruption capcity is 70kA at 380V. But its short time withstand rating is only 7.3kA at 1 sec.
Now the prospective short circuit current at the MCC bus is 62kA.
If the Manin Incomer fails to trip on a MCC bus fault, the Upstream Switchgear breaker wil trip at 180 to 220 millisec.
But calculating the I^2 t, it appears that the MCCB is not adequately sized for the short time withstand.
Please correct me if I am wrong, but the short time withstand value here is important only if the breaker itself fails to trip.
Please discuss.

 

[che12345]

[highlight #F57900]"...This question is specific to a MCC Incomer where ABB MCCB is being used rather than an ACB. The MCCB short ckt interruption capcity is 70kA at 380V. But its short time withstand rating is only 7.3kA at 1 sec".[/highlight]
1. I have the following opinion for your consideration.
It is normal a MCCB short ckt interruption capacity is 70kA at 380V; short time withstand rating is only 7.3kA at 1s . An ACB, at a higher cost; would have a 1s rating.
2. This MCCB is fine as an incomer breaker. The main problem is impossible to coordinate with the tripping of the down-stream feeder breakers. The only reason is lower cost.
3. Check the tender specification whether coordination is specified.
Che Kuan Yau (Singapore)

 

[cherryg]

I have two questions for che12345:
a. If the Incomer breaker fails to trip and the upstream Swgr breaker trips, the let through energy still far exceeds the Incomer MCCB short time withstand rating. So how is this fine?
b. For Incomer MCCB to cooridnate with the feeder breakers, the feeder breakars need to be current limiting type, which should trip in less than half cycle. Only then can coordination be achieved.
Please offer your technical opinion on above.

 

[che12345]

"...a. If the Incomer breaker fails to trip and the upstream Swgr breaker trips, the let through energy still far exceeds the Incomer MCCB short time withstand rating. So how is this fine? b. For Incomer MCCB to cooridnate with the feeder breakers, the feeder breakars need to be current limiting type, which should trip in less than half cycle. Only then can coordination be achieved..."
I have the following opinion for your consideration.
a. The MCCB as the incomer breaker is only responsible for faults down-stream i.e. in the MCC. MCCB or ACB may failed to trip. This unfortunate event, if occurs is not within the duty of the incomer.
b. i) It is unlikely that current limiting feeder breakers can coordinate well with the incomer MCCB. Check OEM coordination table. It may be fail, partial or full. Attention: The incomer MCCB and the current limiting feeder breakers are of the same manufacturer.
ii) check: It is very unlikely that the MCC feeders are installed with current limiting breakers.
Che Kuan Yau (Singapore)

 

[LionelHutz]

There are a lot of MCCBs installed that have the problem you're describing. Won't the feeder MCCBs in this MCC have the same problem if you change the main to an ACB?

As mentioned, using a main ACB allows you to co-ordinate the main and feeder trips. By using the MCCB as the main, a fault on any of the MCC feeders will trip the main shutting down the whole MCC. It impresses me how many people wrongly thing the instantaneous trip level dial on a breaker is a current limit setting when it really just sets the trip level.

 

[che12345]

I have the following opinion for your consideration.
"...#1. There are a lot of MCCBs installed that have the problem you're describing. #2. Won't the feeder MCCBs in this MCC have the same problem if you change the main to an ACB?...#3. By using the MCCB as the main, a fault on any of the MCC feeders will trip the main shutting down the whole MCC..."
#1. Technically, the board is safe. The only reason is low cost, but sacrifice on the coordination. In practice, short circuit say > 14 x Im of the incomer MCCB which shuts down the whole MCC is not expected to happen frequently; and if the MCC can tolerate this nuisance tripping occasionally.
#2. I don't understand the statement. Perhaps, English is not our mother tongue?
Could it be " the MCC would not have the same problem if you change the MCCB to an ACB."
#3. Only for short-circuits say > 14 x Im of the incomer MCCB, the whole MCC is down. As for over-loads, coordination between the incomer MCCB and feeder MCCBs is possible.
Che Kuan Yau (Singapore)

 

[che12345]

I have the following additional opinion for your consideration.
1. High short-circuit current > incoming MCCB instant trip current level, either within the MCC or after the feeder breaker is not expected, but can happen. In case this unfortunate event happened, it is certainly advisable to shut down the whole MCC as a precaution from further faults. An unfortunate >> over current (short-circuit) trip of this nature called for a thorough investigation on the damage, the root cause and rectification.
2. Attention: When open (trip) on high short-circuit, the incoming MCCB and the feeder MCCBs may be damaged !
3. If the incoming is an ACB with 1 s rating and is correctly coordinated, the MCC would remain healthy (on) , even though the feeder MCCBs may be damaged by external faults.
Caution: The MCC has to be shut down anyway, in order to investigate the condition of the ACB for any damage.
4. Conclusion: Incomer with ACB is better than with MCCB, but at a higher cost.
Che Kuan Yau (Singapore)