Another good example why R1+R2 isn't equal to Zs-Ze.

[1Justin]

Be warned it you are tempted by shortuts. I know in domestic work on old houses, we often have a race against the clock, and it can get stressful and sometimes very difficult, but some cut corners could bite.
I have just fixed errors left behind by NIC member who fitted new 17th ed'n consumer unit a couple of years ago. TN-S install.
Table of results taped neatly next to CU, all the labels in place, looks good.
It turns out that lighting R1+R2 were fabricated, presumably from Zs-Ze.
Half the house lighting had open circuit CPC, neatly hidden by a perfect N-E short on the far side of the break. "Zs" values throughout all the fittings were believable, (but not really Zs anyway since no CPC). Very obvious with proper R1+R2, or even with NE IR test done at at far end. (All IR of course was good at consumer unit)
The error included all the lighting throughout a large flat roof extension and bathroom. All class1 light fittings.

 

[colshaws]

If a 17th edition board with RCD all circuits then surely the N-E short would have caused the RCD to trip from day one?

 

[Dan23584]

If a 17th edition board with RCD all circuits then surely the N-E short would have caused the RCD to trip from day one?

Probably due to the lack of continuity of the CPC.

 

[1Justin]

If a 17th edition board with RCD all circuits then surely the N-E short would have caused the RCD to trip from day one?

Not if the CPC were broken in one concealed JB, and immediately off the same JB a builder had left an un-terminated spur with NE short! (No CPC so the RCD doesn't see this part of the house at all). "Zs", meanwhile gives a very believable live reading first via the unconnected part of the CPC, then via neutral.

 

[Deleted member 9648]

Not generally necessary to carry out an R1+R2 test when changing a CU, that test is really for initial verification of a new circuit prior to energising, in the case of CU change it's already been energised. The Zs tests will verify disconnection times are met and cpc continuity to the furthest point on the circuit.

 

[colshaws]

Not if the CPC were broken in one concealed JB, and immediately off the same JB a builder had left an un-terminated spur with NE short! (No CPC so the RCD doesn't see this part of the house at all). "Zs", meanwhile gives a very believable live reading first via the unconnected part of the CPC, then via neutral.

Yes realise this now, been a long day and engaging my typing finger before my brain.:6:

 

[1Justin]

.. The Zs tests will verify.. cpc continuity to the furthest point on the circuit

Not always, as this example shows.