MCBs and MCCBs in the sub main Distribution Board

AJJewsbury: 
 

Discrimination between MCBs (regardless of rating) is typically non-existent - so I wouldn't have MCBs upstream of other MCBs if you want to have selectivity. So I'd go for MCCBs (or HBC fuses) rather than MCBs for distribution circuits even if is the rating is low. Perhaps for final circuits too if the load incorporates MCBs or similar protective devices that you'd need to discriminate with.

The enclosures for MCBs and MCCBs can differ - which might influence your choice depending on what sort of maintainability might be required. E.g. MCB enclosures are normally a simple boxes with a single common lid - so to access one outgoing device you'd need to expose just about everything - so H&S usually means that the entire board would have to be isolated before connecting in a new circuit or replacing a protective device - thus losing power to all the other circuits. In critical situations, some of the higher ‘form factor’ enclosures available for MCCBs have internal partitions and separate lids that can allow some safe working on one outgoing circuit while leaving all the other circuits live.

Also consider actual availability - i.e. what can be reasonably obtained from suppliers as distinct from what's in a manufacturer's catalogue. MCBs with a breaking capacity over 10kA are rare beasts in the wholesaler, likewise MCCBs below 16A. Promises that things can be ordered as specials often have small print that you have to order some ridiculous quantity or cost a fortune. Not just for initial installation, but on-going repair and maintenance. If something business critical unexpectedly goes bang, local availability of spares can become a surprisingly important issue.

   - Andy.

I was of the opinion that, it does NOT matter whether it is a MCCB or a MCB as long as the MCB meets or exceeds the prospective short circuit current rating.

I think that in the overload region, selectivity can be achieved while this cannot be achieved for higher short circuit currents.

AJJewsbury: 
 

Discrimination between MCBs (regardless of rating) is typically non-existent - so I wouldn't have MCBs upstream of other MCBs if you want to have selectivity. So I'd go for MCCBs (or HBC fuses) rather than MCBs for distribution circuits even if is the rating is low. Perhaps for final circuits too if the load incorporates MCBs or similar protective devices that you'd need to discriminate with.

The enclosures for MCBs and MCCBs can differ - which might influence your choice depending on what sort of maintainability might be required. E.g. MCB enclosures are normally a simple boxes with a single common lid - so to access one outgoing device you'd need to expose just about everything - so H&S usually means that the entire board would have to be isolated before connecting in a new circuit or replacing a protective device - thus losing power to all the other circuits. In critical situations, some of the higher ‘form factor’ enclosures available for MCCBs have internal partitions and separate lids that can allow some safe working on one outgoing circuit while leaving all the other circuits live.

Also consider actual availability - i.e. what can be reasonably obtained from suppliers as distinct from what's in a manufacturer's catalogue. MCBs with a breaking capacity over 10kA are rare beasts in the wholesaler, likewise MCCBs below 16A. Promises that things can be ordered as specials often have small print that you have to order some ridiculous quantity or cost a fortune. Not just for initial installation, but on-going repair and maintenance. If something business critical unexpectedly goes bang, local availability of spares can become a surprisingly important issue.

   - Andy.

I was of the opinion that, it does NOT matter whether it is a MCCB or a MCB as long as the MCB meets or exceeds the prospective short circuit current rating.

I think that in the overload region, selectivity can be achieved while this cannot be achieved for higher short circuit currents.