selectivity 60898 b6 to same

newfutile · Feb 12, 2024

I have a small Consumer Unit fed from a 6 amp type B mcb to a 6 amp type B,it only feeds 1 light in a bin store .
clearly we have no selectivity here ,would changing the feed mcb to a type C possibly 10 amp help achieve this?
its only on 1.5 swa so I can’t go too high ,my ZDB is 1.05 ohms

timhoward · Feb 12, 2024

You need to look at pages 416 and 417 of regs book, and also see if the MCB manufacturer publishes a selectivity chart (assuming they are both same brand).

newfutile · Feb 12, 2024

the time current curves don’t appear to say , also I guess they will only be selective for over currents , at a fault current of 228 amps there cant be any selectivity Eaton seem to suggest?

[ElectriciansForums.net] selectivity 60898 b6 to same

davesparks · Feb 12, 2024

Why not just remove the second CU and join the cables through? Having a 6A MCB in line with another 6A MCB is pointless

davesparks · Feb 13, 2024

newfutile said:
the time current curves don’t appear to say , also I guess they will only be selective for over currents , at a fault current of 228 amps there cant be any selectivity Eaton seem to suggest?

As a general rule of thumb for MCBs within the standard range of 6A - 63A they don't achieve selectivity on fault currents.

This is why MCBs are generally unsuitable for protecting distribution circuits.

BS88 fuses on the other hand have a selectivity ratio of 1.6 so if an upstream fuse is rated at least 1.6x the rating of the downstream device you will get selectivity.

pc1966 · Feb 13, 2024

As suggested above, you don't need the 2nd MCB here. A switch would do if some local means of isolation is needed.

However, is it a problem? True under fault conditions it is pretty much sure both will trip, but if that likely to present danger or unacceptable trouble for the owner?

If the supply was from another locked location I can see why it would matter, but if it is just two locations within one property, and nothing else is dependant on the upstream MCB remaining closed, then not worth worrying about.

newfutile · Feb 14, 2024

It's a remote bin store fed from the flats communal area,it is in a locked cupboard but it's only a fb key which all the tenants and contractors should have

newfutile · Feb 14, 2024

I have changed the supply to a 16 amp mcb , my ZS at the bin store light is 1.29 ohms ,fault current of 186 amps.
my read of the time current curves is that the local 6 amp will trip possibly the 16 amp as well ,so I may not have achieved much at all!