Current Limiting Capability in MV Circuit Breakers?
Current Limiting Capability in MV Circuit Breakers?
(OP)
I wonder if MV CBs have, just like LV CBs, current limiting capability? The current limiting capability makes us able to lower the size of downstream breakers, cables and other equipment.
Thanks in advance
Thanks in advance






RE: Current Limiting Capability in MV Circuit Breakers?
RE: Current Limiting Capability in MV Circuit Breakers?
RE: Current Limiting Capability in MV Circuit Breakers?
Yes, I too have only heard of fused-contactors to limit current in MV.
As for the LV, correct me if I'm wrong but I believe that is why LV cables don't need to be sized for short circuit condition.
Regards
H
RE: Current Limiting Capability in MV Circuit Breakers?
No, definitely not.
Also, when using the current limitation to lower the kA rating of downstream circuit breakers means that you lose discrimination. In the IEC world called cascading.
RE: Current Limiting Capability in MV Circuit Breakers?
RE: Current Limiting Capability in MV Circuit Breakers?
If someone has seen actual cases of LV cables being thermally damaged from short circuit current on a through-fault, I'd be interested to hear about it. I've never seen it.
RE: Current Limiting Capability in MV Circuit Breakers?
LV air circuit breakers do not, and I don't believe that SF6 is used for LV breakers as its not really needed.
As I understand it, its the magnetic forces during short circuit conditions that force the circuit breaker contacts open, thus limiting the through current and giving the inherent 'current limiting' condition. Of course, this means that the upstream source must be stiff enough to provide enough current to allow for this, thus the breakers can experience issues on soft systems (such as small generator powered sites).
RE: Current Limiting Capability in MV Circuit Breakers?
RE: Current Limiting Capability in MV Circuit Breakers?
Current limiting action is achieved by cooling the arc when the arc is drawn thereby increasing the arc resistance which will limit the current.
In a LV current limiting bkr (mcbs), the arc is drawn into a seperate chamber and the arc resistance is increased and thereby limiting the current.
In side a current limiting fuse the arc is drawn inside the fuse barrel and cooled by sand thereby limiting the current.
Therefore, beyond 1000V, to draw the arc and cool, it needs considerably large arc chutes. Therefore. over 1000V current limiting breakers are
not practicable.
RE: Current Limiting Capability in MV Circuit Breakers?
RE: Current Limiting Capability in MV Circuit Breakers?
RE: Current Limiting Capability in MV Circuit Breakers?
To dpc:
interesting fact dude.
To ScottyUK and Squeeky:
Thank you very much for your insight. But do you accept Mr. / Ms. dpc's comment that current limiting has nothing to do with cable short circuit sizing? He /She says that short circuits (according to NEC) will not create any problems to cables regardless of provision of current limiting capability for CBs or cable short circuit sizing.
RE: Current Limiting Capability in MV Circuit Breakers?