Type B RCDs VS Type C RCBO's


Postby markgoodl » Thu Oct 30, 2008 3:04 pm

Hi all, my first post so here goes...

I have recently changed my dis board as weve had a kitchen extension done. ive put in a new kitchen ring, new lights and cooker supply. I have fitted all the circuits with C type RCBO's at there correct rating.
When the work was inspected, i was told that i they should be type B RCBO's.
I have scanned the regs and the OSG (17th etd) but can find no regulations stating that i MUST fit B type.
Two reasons that type B RCBO's werent fitted are that they are impossible to get for a merlin gerin board and due to the oven having a fan (motor), the fridges having Motors and the lights being florescent then type C's are more appropiate any how.

Any advice would be appreciated...

Many thanks (in advance)
Mark
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Postby ericmark » Thu Oct 30, 2008 5:51 pm

So long as the neutral loop and earth loop impedances are within required limits and you have not used Ali-tube cable I can see no reason why type C should not be used.
The prospective short circuit current is the limiting factor and this should have been all noted on the installation certificate.
Very simple a type B will allow 5 times the rated current before the magnetic part of the RCBO works and a type C will need 10 times rated current before it works.
So for a 32 amp RCBO you need 320 amp for it to trip so the line / neutral impedance needs to be less than 0.7 ohms so assuming Ze of 0.35 then R1 + Rn needs to be 0.35 ohm so what are the reading taken?
All best Eric
ericmark

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