MCCBs

If you want a simple assessment as a starting point, then basically allow at least two frame sizes between the MCCB's


Eg - if you have MCB's in a typical Type B dist.bd, then the board has a nominal rating of 125A   - so protect the sub main with a 160A MCCB, (say an F Frame) set down on the thermal curve to 125A.  - that should be good for MCB's in a 32A Range


Those 160A MCCB's sitting in a board feeding several Dist Bd's  will need protection, the smallest of which is likely to be a 400A MCCB protecting the panel board supply (say a K frame)  - those 400A MCCB's will probably need protection by a minimum of 800A (so say an M Frame) to each panel board - allowing for all the other incidental loads, that would give you a 1250A primary protection device in the facility main switchboard. You could put 1250A BS 88 fuses upstream of the 1250 MCCB, principally to take advantage of the cut off characteristics and give you a more economical board and you also gain a huge advantage for arc flash protection. Upstream from there you now have a classic 1.5MVA switchboard, with perhaps 3 or 4 outgoing 1250A ways - these feeders being ACB protected (to what ever level of protection you desire - eg, simple electronic trips through to a full blow CT based protection scheme, reaching upwards into the HV Network.


The above is not a guarantee, but would provide reasonable discrimination on a typical 25kA fault system - it's just to give you a flavour of "missing a frame size" akin to missing a fuse size for discrimination - it won't be perfect, but will protect you from most credible faults, whilst losing discrimination on the abnormal faults (JCB bucket thorough the plant room wall for example)


Regards


OMS

If you want a simple assessment as a starting point, then basically allow at least two frame sizes between the MCCB's


Eg - if you have MCB's in a typical Type B dist.bd, then the board has a nominal rating of 125A   - so protect the sub main with a 160A MCCB, (say an F Frame) set down on the thermal curve to 125A.  - that should be good for MCB's in a 32A Range


Those 160A MCCB's sitting in a board feeding several Dist Bd's  will need protection, the smallest of which is likely to be a 400A MCCB protecting the panel board supply (say a K frame)  - those 400A MCCB's will probably need protection by a minimum of 800A (so say an M Frame) to each panel board - allowing for all the other incidental loads, that would give you a 1250A primary protection device in the facility main switchboard. You could put 1250A BS 88 fuses upstream of the 1250 MCCB, principally to take advantage of the cut off characteristics and give you a more economical board and you also gain a huge advantage for arc flash protection. Upstream from there you now have a classic 1.5MVA switchboard, with perhaps 3 or 4 outgoing 1250A ways - these feeders being ACB protected (to what ever level of protection you desire - eg, simple electronic trips through to a full blow CT based protection scheme, reaching upwards into the HV Network.


The above is not a guarantee, but would provide reasonable discrimination on a typical 25kA fault system - it's just to give you a flavour of "missing a frame size" akin to missing a fuse size for discrimination - it won't be perfect, but will protect you from most credible faults, whilst losing discrimination on the abnormal faults (JCB bucket thorough the plant room wall for example)


Regards


OMS