Main Distribution Board

[cosfi]

Dear Friends,
We're designing a switchgear for a power plant. But we don't use subdistribution panels at this project. Customer want from us to supply all loads from Main D. B. directly. There are no local or another panels. So i want to ask a question , We supply small loads like motor operated valves (they are maximum 3 kW) from MDB directly. We use thermal magnetic Circuit breaker. Their cable size are 2,5 or 4 mm2 selected. And short circuit current is 65 kA at MDB bus bar. Can this cables withstand the S.C.C ? If they can't , what can be done. Using fuse and thermal magnetic C.B. together , can be a solution? or must we use another bus bar for all small loads and protect this bus bar with fuse (disconnector switch with fuse) ?

Thank you...

 

[davidbeach]

What is the fault current at the load end of the cable? Not much point worrying about the cable withstand for faults in the cable as it would already be damaged anyway. Doesn't take much length of small conductor to drastically reduce the prospective through fault current.

 

[cosfi]

Thank you davidbeach,
For 160 kW motors , i calculated 185 mm2 cable for 50 meters. And short circuit current at the end of cable (at the load) is 20 kA. But , for 3 kW MOV i selected 2,5 mm2 cable for 50 meters and short circuit current at the end of the cable is approximately 5 kA. I calculated this values but i'm not sure. When cable size decreases , impedance of cable is increases and for 2,5 mm2 cable , Could s.c.c rating(5kA) be correct ?

Which short circuit current must we consider while sizing a cable , Bus bar s.c.c or s.c.c at the load terminals?

 

[alehman]

Yes, 5kA is certainly possible. You should plot the time-current damage curve for the cable against the time-current curve for the overcurrent protection.


Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.