12-02-2015, 08:03 AM
No, you can't use an RCBO like that, or at the very least it would be a rather non-standard arrangement. The CU itself doesn't need any further protection. Normally it's protected by the supply utility's fuse, in your case by the fuse at the sending end of the cable. The big cable is also protected by that fuse. Also if you use a 50A MCB or RCBO in the workshop CU there won't be adequate discrimination between that and the 60A fuse at the source of the cable. There also may not be adequate discrimination between the 50A RCBO and a 32A MCB for a ring final.
This sort of situation can arise in a domestic installation where a very powerful eelctric shower is fitted, needing a 50A MCB and the utility's fuse is only 60A. Ideally the utility's fuse shoul dbe uprated to 80A or 100A. In practice a blind eye is often turned.
Incidentally, you wrote "100A cable" in an earlier post. Cable has a size in sq mm. The current rating depends on various factors, mainly on method of installation. A long cable may need to be larger than required for current carrying to meet voltage drop and loop impedance criteria.
This sort of situation can arise in a domestic installation where a very powerful eelctric shower is fitted, needing a 50A MCB and the utility's fuse is only 60A. Ideally the utility's fuse shoul dbe uprated to 80A or 100A. In practice a blind eye is often turned.
Incidentally, you wrote "100A cable" in an earlier post. Cable has a size in sq mm. The current rating depends on various factors, mainly on method of installation. A long cable may need to be larger than required for current carrying to meet voltage drop and loop impedance criteria.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
