TN Distribution circuit to an outhouse

[Matty7814]

Evening gents,

I'm just looking for some guidance/clarification on a supply to a summer house I installed. I had to put in a small CU (the existing one had no spare ways left) with a 63A 30mA RCD main switch and a 45A B type feeding the distribution circuit. 6mm2 2 core SWA around a 25m run to the summer house that fed a garage CU with 40A 30mA RCD main switch.

The small CU I put in the house had a Ze of 0.2 ohms and the Distribution circuit had R1 +R2 of 0.33 ohms. The Zs of the circuit/Ze of the garage CU was 0.79 ohms.

Now looking at table B6 in the OSG shows that max measured Zs for 45A b type is 0.82 but I wasnt sure if that includes distribution circuits. Reg 411.3.2.3 states that for a distribution circuit a disconnection time not exceeding 5 seconds is permitted and 411.4.5 says: Zs x Ia < U0

Am I right in thinking that 0.79 x 225A = 177.8 so that it complies?

Apologies in advance for the wall of text and as you might of gathered I'm fairly new to this!

Thanks for your time

Matt

 

[Dave OCD]

The 45A mcb won't discriminate fully with the 20A or possibly even the 6A mcb
The RCDs won't discriminate with each other at all, which ever one has been calibrated to trip faster will do so.
The problem you do have is that you have a 40A RCD supplied by a 45A mcb so the RCD could be overloaded without the OCPD operating

Why ? The circuits supplied by the 40 amp/ 30ma RCD are protected by 20 and 6 amp MCBs respectively , so there's no way the RCD will ever suffer overcurrent.

 

[Leesparkykent]

I'm in Islington North London

There was someone on here looking for sparks and mates a few weeks ago in London called Cornhill electrical.

 

[davesparks]

Cheers Dave

If I changed the 45A to a 40A would that discriminate with the 40A RCD?

No it won't discriminate, it's not possible for an RCD to discriminate with an mcb because they are different devices entirely!