The testing required before cu change

[MrsS]

I’m about to change an CU with rewireable fuses to one with RCBOs, the reasoning behind this is, only a small CU will fit the space and the property only has 4 circuits
5amp Upstairs Lights
5amp Downstairs Lights
30amp Sockets
30amp Cooker
If I use MCB all circuits are protected by an RCD, therefore all circuits would be interrupted under fault conditions; an RCBO ensures that only the circuit with the fault is interrupted.
I plan to test the installation before removing the old CU and because I will be fitting RCBOs I want to avoid any problems with N-E.
Now I’ll get to the point (typically of a woman to go all around the houses)
Tests are:
Ze and Psc tests
Continuity of protective conductors test ( R1 & R2 test at the furthest of the 2xlight circuit and the cooker circuit, adding it to Ze to get Zs). But how does this test check neutral
Then the end to end testing on the sockets circuit
Followed by the cross over test, I know the reading at each socket should be (R1+Rn)/4 or (R1+R2)/4 (spurs higher)
Then Insulation resistance test on all circuit.
Then Zs test of all circuits
Is there anything else I should do or lookout for to avoid problems with the RCBOs
By the way, Which RCBOs are the best to use.
Feedback Welcome
Many Thanks

 

[Amp David]

Don't understand this method very well

 

[WayneL]

hi mrs s
to do the continuity of protective conductors
1 link line and cpc at db test between line and cpc at all relevent points if its a lighting circuit test at the ceiling rose between sw/line and cpc record highest as R1+R2 value for the circuit operate switches to confirm polarity whilst carrying out the test
2 if you want to you can also measure L- N by linking line and neutral at DB the tesing at all relevent points between line and neutral this result not recorded and value will be substantially the same if same size conductors are used ie 1mm T/E the line , neutral and cpc will give same results as they have same csa
if 1.5mm T/E used then the line to cpc will be higher than the line to neutral as cpc has a smaller csa
use this test for line to neutral (R1 + RN) when testing radial socket outlets to confirm polarity

Would you really do all this before a CU change??


MrsS, have a read of this : http://www.esc.org.uk/pdfs/business-and-community/electrical-industry/BPG6_09.pdf

 

[brucelee]

no mate not before changing cu she was asking how to check the neutral in post 3 and she put
neutral to cpc i was just explaining that its carried out between line and neutral R1+RN and its whats used to check radial socket outlets to confirm polarity and doesnt have to be carried out for lighting circuits
but not neded before a board change

 

[pushrod]

Don't understand this method very well

I am not surprised on reading my explanation again!

Interestingly it does make some sort of sense and would identify a circuit that for some reason had its own neutral and another neutral from another circuit which would cause rcd problems. This is not the usual problem though and it was not what i meant to say.
I managed to miss out was that you test between the "live of a circuit and the other" neutrals. As the more usual problem especially with 2-way lighting is that the current returns via another circuit.

If IR testing you need to do it at 250v and obviously light switches should be closed.

The easiest way of doing it is to just take the neutral, of say, upstairs lights out of the neutral bar and then test between the live of that circuit and the remaining neutrals on the bar - you only have to separately test all the neutrals if you get a zero reading. Then do the same for downstairs lights. As i'm sure you know it is most likely to be the lighting if there is a problem and it is a quick way of checking.

Hope that makes it a bit clearer and apologies for the confusing first post

 

[WayneL]

Much Better, Pushrod

 

[pushrod]

Much Better, Pushrod

Cheers Wayne, don't know what has been happening to my posts recently - mistakes all over the place , i couldn't even put two numbers in the calculator the other day

 

[JUD]

That job at the in-laws has knocked you for 6 I think

 

[pushrod]

That job at the in-laws has knocked you for 6 I think

yep still recovering - pass me a beer!