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Selection of CD Circuit breaker

Selection of CD Circuit breaker

Selection of CD Circuit breaker

(OP)
Hello,

Is there any recommendation to select a DC circuit breaker?
In a facility there have been some trips and nobody knows the reasson.
The only clue is that DC circuit breaker only has "instantaneous element" settable between 1600 and 3200 A and operates at almost 1 cycle.

Why DC circui breakers do not have thermal element?

Any answers or clues will be welcome!
 

RE: Selection of CD Circuit breaker

You haven't stated what application the DC breaker is being used in.

I've seen DC breakers without thermal settings used in UPS systems but I've also seen many other applications where a thermal trip unit is used.

RE: Selection of CD Circuit breaker

IF this is a back up battery application (such as in the UPSs), the battery source does not have enough capacity to damage the conductors due to overload (does not last more than a few minutes). Therefore no overload protection is necessary. It is cost effective to use a DC breaker just with INST.

Rafiq Bulsara
http://www.srengineersct.com

RE: Selection of CD Circuit breaker

That's a fairly conditional statement - I agree it possibly is true for high rate, short duration standby applications but I've seen enough big standby batteries designed for long duration, low rate applications which would certainly deliver enough current to get the conductors in trouble over a shorter period.

Another possible reason is that the battery protecting the critical installation is itself only protected against catastrophic faults such as a dead short, at which point the load is already lost.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Selection of CD Circuit breaker

The NEC requires overcurrent protection for battery conductors, but from there it refers to generally applicable overcurrent requirements. I don't think instantaneous only devices would comply. That requirement could probably be argued as not applicable for "listed systems", such as a packaged UPS.

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

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