On industrial it is often the case (example) that say DNO have 200Amps and busbar main isolator for instance might be the same but if you reference the type that DNO use they tend to give allowance i.e. type 3, 4 or 5 against your bs88 type 2 this give your fuses priority, where as some commercial and domestic use standard type 2 DNO fuses and even with reduction of your fuse size in the case of a short circuit full discrimination wont be achieved anyway only partial in some cases..... its usually accepted that domestic cannot achieve full discrim' but to check if 2 fuses up and downstream of each other give full discrimination then check that the total let through energy (I^2t) of the downstream device is less than the pre-arcing energy (I^2t) of the upstream device.... if so then you have acheived full discrim'
Example if you have a bs88 63amp fuse Gg1 it will only achieve full discrimination to a 6amp (b) mcb, 10amp - 40amp will give partial discrim' decreasing respectively as your mcb rating increases but no discrim will be acheive between bs88 63amp and a 50amp and obviously a 63amp mcb type b.
Hope you can appreciate now how hard it is to get full discrimination on short circuit and as my example shows if your front end is a bs88 Gg1 63amp fuse then anything above 6amp (b) mcb wont acheive full discrim'.
Discrimination for overload current is an easier thing to understand and meeting full discrim' in this case isn't rocket science so wont bother explaining that one
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Ill just add to this (edit)... when discriminating against overload my comment just then is in ref' to similar devices but if you are comparing 2 differing divices a trick is to print the time current curve of one device on paper the the second device on tracing paper and overlay to see what needs to be done to achieve discrim' under overload condition as sourcing comparison charts cn be a pain.
re' short circuit discrim', Hagar's fuseboards catalogue has a tech' section that will compare in chart form different devices from bs88 to mccb's etc etc very helpful reference an im sure you'll be able to get it online.
