How Many People Test L-N Loop With Rcbo's?

[Stephen Wattam]

I have carried out an EICR in an installation with multiple DBs and many RCBO devices for ring circuits. The copy of the original installation cert listed max Zs as 1666 Ohms (30ma trip). The RCBOs were mainly C type 32A. After testing ring continuity on several circuits it became apparent that a significant number of the circuits were far too long, 2.5/1.5 T/E L-L 1.65 Ohms, E-E 2.8 Ohms for example.

The earth loop reading 1.25 Ohms exceeds the max Zs for a C type thermal/mag trip (now 0.68 Ohms) but is well within RCD limit so should be OK, however if you look at L-N loop reading 0.93 Ohms which does NOT have RCD protection, the disconnection time for thermal/mag trip is not met.

My question is how many people on here would test L-N loop on an RCBO in these circumstances?

A quick straw poll of peers had interesting results!

 

[Sidewinder]

I do, how else can you verify that the circuit will disconnect in the event of a short circuit fault, and verify volt-drop unless you have undertaken and presented the cable design calculations.

Also, I do not believe in relying on the earth leakage protection, unless this is ABSOLUTELY essential where the EFLI cannot be reduced by any other robust & reliable means in an economically viable manner.

 

[steptoe]

yep,

me too,

I see RCD protection as secondary, you still need to meet continuity measurements IMHO,

RCDs are only there to allow you to meet disconnection times in a TT situation, or other non TN installs,

anyone who relys on an RCD to meet disconnection times on a TN system is NOT an Electrician,

under general circumstances,

unless a LOAD of other precautions have been taken. IMHO

 

[Doc Hudson]

Did the L-L & E-E readings match those on the original certificate. Do the figures also match up with the rough expected cables lengths, somewhere approx 200m of cable round the ring. It wasn't a type B 32A originally by any chance that has been changed later? What sort of loads are on the circuit.

Doc H.

 

[Essex1]

I honestly think this is a no brainier. The circuit should be designed to meet the requirements it is needed for then if RCD protection is required it is added. So the circuit should meet the requirements for maximum Zs first then if the circuit needs it RCD protection is added. Unless of course it is a TT system but the same process is involved. To put the max down as 1666 says to me either that RCD is being used as the be all and end all by the installer or (probably more likely) he has mucked up on his cable calcs and tried to cover his tracks by putting in the 1666 figure in the EIC.

 

[Steve3948]

Amtech and NICEIC certificate software input max ZS limitation once you enter the RCD details, so maybe that's why.

 

[binky]

even on TT systems I expect good circuits to meet the dead test figures for TN systems, to me anything else is a circuit with faults.

 

[Essex1]

even on TT systems I expect good circuits to meet the dead test figures for TN systems, to me anything else is a circuit with faults.

Spot on.