how to test a cct with more than 1 CU

krishcanag · Apr 8, 2008

Hello

I was wondering if anyone here might have come acorss this one and which way they went about it.

1. Mains --- main CU (MCB)-----SWA to garage------CU(SW + RCD)-----ring

2. Mains --- main CU (MCB)-----SWA to garage------CU(SW + RCD)-----SWA to shed--sockets

There is two ways I guess;
1. test from main to garage CU and secondly from garage CU treat it as normal ring test, etc

But how do you work out, because first part is radial and then ring?

2. Can you test straight through in one step? maybe continunity you can but want about other tests?

krishan

Bane · Apr 8, 2008

Shakey · Apr 8, 2008

krishcanag said:
Hello

I was wondering if anyone here might have come acorss this one and which way they went about it.

1. Mains --- main CU (MCB)-----SWA to garage------CU(SW + RCD)-----ring

2. Mains --- main CU (MCB)-----SWA to garage------CU(SW + RCD)-----SWA to shed--sockets

There is two ways I guess;
1. test from main to garage CU and secondly from garage CU treat it as normal ring test, etc

But how do you work out, because first part is radial and then ring?

2. Can you test straight through in one step? maybe continunity you can but want about other tests?

krishan

hi

just treat them as entirely seperate CU's

presumably by 'sw' in garage you mean MCB, or is the RCD actually an RCCB (ie an RCd with inegral overload protection)

either way, you would do your normal ring checks, IR etc, but your Zs value for the ring in the garage would only be applicable for the MCB in the garage protecting the ring, however this Zs would include original Ze, and the R1+R2 for the garage supply

so your Zs for garage ring would be

Zs = Ze (main CU) + (R1+R2)(SWA to garage) + (R1+R2) (garage ring)

thats if i am reading your descriptions right!

so effectively, the supply to the second CU treat as a sub-main, or a radial circuit in its own right

when i have done these, on the schedule of test results, i include the swa to garage as a seperate circuit (with its own R1+R2 value, IR value, Polarity, and Zs value)

you could also raise a seperate schedule of test results for the second CU if you wish

hope this helps

oh, and listen to Bane

he's the MAN