Discriminating of protective devices from manufacturers

[Ian1981]

has anyone ever been in contact with manufacturers regarding providing adequate discrimination?
Example I have a 80amp type c at a 3 phase db which supplies a 3 phase bemco unit which supplies a single phase db to a flat protected by a 63amp fuse.
It's a queer set up an mcb feeding a fuse.
Zs of the cooker circuit is 0.28ohms on a 32 amp type B mcb which is using the nominal voltage calc 821 amps of fault current.
The upstream mcb is a 80amp type c and if you use the tables in bs7671 it will trip at0.1 seconds to 5 seconds with 800amps of fault current so may take out the whole db power.
However I know manufacturers may give different values as bs7671 are generic values across the board.
Also the let through energy may need to be considered for short circuit conditions.
Anyone been down this road before?

 

[Richard Burns]

Normally an MCB will not discriminate on fault current from a fuse of similar value.
The manufacturers website normally have discrimination tables that show which values and up to what fault current different devices will provide full or partial discrimination.
However it does tend to be their own products (unsurprisingly) so if you mix manufacturers then you may have a problem.
They should also provide I²t values for their devices.