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Discuss selectivity 60898 b6 to same in the Electrical Forum area at ElectriciansForums.net

newfutile

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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
 
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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).
 
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?selectivity 60898 b6 to same IMG_0045 - EletriciansForums.net
 
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.
 
Sausages (just for tracking a min lol needed a word, I must be hungry)
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.
 
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
 
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!
selectivity 60898 b6 to same IMG_0047 - EletriciansForums.net
 

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