Air v Moulded Case

[panelman]

We have to get an incoming device for a board.

Normally it’s easy, 1kA and down get MCCB, 2kA and up get ACB but this one is 1.6kA and ACB and MCCB will both deal with the load current and fault levels.

Anybody got a view on other things I should consider in making my choice?

 

[dpc]

MCCBs in series will not fully coordinate. If selectivity is important, you will need a power circuit breaker w/o instantaneous trip.

Power circuit breakers are maintainable, MCCBs are not.

Obviously the MCCB will be much less expensive.

 

[ScottyUK]

ACB will allow use of a definite-minimum-time protection scheme to ensure coordination with downstream breakers. On a high fault level system this is invaluable because an MCCB will trip instantaneously to clear a fault while the ACB will hold in. Thus an ACB is a good choice for an incomer to a board with multiple outgoing circuits protected by MCCBs where a fault on one load must not cause loss of the whole board.

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Sometimes I only open my mouth to swap feet...

 

[rbulsara]

please do not cross post

 

[ScottyUK]

rbulsara,

That's a bit harsh!

thinking about it this would be better over in the switchgear & boards forum so if a passing mod would care to delete this I'd appreciate it

The other thread has been flagged for deletion.

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Sometimes I only open my mouth to swap feet...

 

[rbulsara]

OK

 

[rbulsara]

except I had posted a reply there..

 

[rbulsara]

here it is: (as other post was not yet deleted):

its not as simple..

Items to consider and evaluate are:

Cost
application
degree of reliability required
maintainability required ( ACB are maintainable)
electrically operation required or not
Plug in, bolt-on, drawout, stationary- what is important to you
Short circuit ratings- switchboard or switchgear?
Short time ratings- for coordination purposes
ACB have more flexibilty to help coordination
Size of equipment (space )

 

[panelman]

I’d thought of the coordination aspect and passed my thoughts on to the consultant.

His suggestion is a 1600 mccb incomer with dummy trip unit (so basically a switch only) and relying on the upstream device at the transformer for protection. The board will have two 800A switchfuses as outgoing devices.

My initial suggestion was ACB incomer and MCCB outgoing but, as always, cost is an issue

At the end of the day, he’s got to be happy paying for it but I’ve got to be happy building it