RcD x5 failure on TT system

[Sparkie Stow]

Hi Guys, recently installed a new radial socket circuit from CU.
When ramp testing 30ma RCD at unit, without loads, everything was fine. When I energised the circuit and tested again, the furthest two sockets failed the x5 test giving a 40ms reading. Other sockets on the same circuit came in fine. It's a TT system. So was wondering what the likely cause of the failure x5 would be. Could it be higher resistance due to cable length, or something else?
Thanks in advance for your thoughts.

 

[SparkyChick]

What sort of Zs readings are you getting at those two sockets?

 

[timhoward]

Does a quick Zs test at the last three sockets give sane values?
You could also try carefully testing with probes on connections behind socket to prove it still fails and is nothing to do with sockets.
Beyond that it's dead tests again really.
Maybe stick L and N in earth bar and check r1+r2 = rn+r2 at the last 3 sockets to prove connections are sound.

 

[Wilko]

Hi - some RCD need more than 5x to trip in 40ms, and that’s now permitted iirc.

 

[cliffed]

Hi - some RCD need more than 5x to trip in 40ms, and that’s now permitted iirc.

Only the A type I do believe?

 

[Sparkie Stow]

Thanks for the replies guys...Zs are the same across the last 3 sockets being 401.6 ohms, well within range for TT. The RCD is AC type.

 

[timhoward]

Have you double checked Neutral connections at the last 3? (as R1 and R2 seem fine or at least consistent)?
Are you able to share Ze and/or Zs @DB to give us a more complete sense of the install?

 

[Strima]

As the RCD passed when you tested it at the device with loads removed then it's a pass.

Although your Zs is within the 1663 permitted for a 30mA RCD it's still high, this could well have an impact upon the test current the meter puts out to trip the RCD.

 

[Rockingit]

As the RCD passed when you tested it at the device with loads removed then it's a pass.

Although your Zs is within the 1663 permitted for a 30mA RCD it's still high, this could well have an impact upon the test current the meter puts out to trip the RCD.

I'd agree and probably do a ramp test to prove this...... 400 Ohms is a significant resistance at 20-40mA

 

[Dartlec]

Hi - some RCD need more than 5x to trip in 40ms, and that’s now permitted iirc.

Some of the first Hager Type A RCBOs had to be tested at a larger current than 150mA to pass the 5 times test within 40ms, and the wording of the standard allowed it - but they've since come back into line with every one else I believe.

 

[pc1966]

Thanks for the replies guys...Zs are the same across the last 3 sockets being 401.6 ohms, well within range for TT. The RCD is AC type.

401.6 ohms is troubling as the regs suggest no more than 200 ohms to have confidence the rod is in good stable condition (even though for a 30mA RCD the theoretical limit for 50V max is 1.67k)

Back to the original question, can you ramp-test the device alone and with the sockets connected?