RCD's in series and how to test

[leep82]

Im currently working on a site where im testing the supplies to the site cabins. Each cabin has its own 6 way single phase DB with a double pole RCD as a main switch. The DB's are then fed via commando plug and socket arrangement from a 32a RCBO within a TP&N DB. When the cabins were originally 1st fixed the dead tests were performed and recorded ( r1-r1, r2-r2, rn-rn, R1 + R2 and IR ). Now i am in the process of energising the cabins and performing my live tests and as you can see have two RCD's connected in series.

In each cabin i have turned the main RCD switch off and in turn performed my RCD test on the supply side to prove the tripping times of the 32a RCBO back in the main DB. My question is will the readings ive obtained here be suitable to record as my RCD tripping times when i fill in the test results for my final circuits within the cabins? All the readings are within permitted values, only when i turn on the main RCD and try to perform the same test on the final circuits the RCBO in the main DB trips at x1 and the main RCD in the cabin DB trips at x5. Meaning that i havnt been able to prove that the main RCD in each cabin trips out at within the required time for x1.

Im not sure why there are two RCD's connected in series like this as i havnt done the design. I would have thought the RCBO in the main DB would have been more than adequate

 

[Rpa07]

Is the submain SWA? Does it need protecting? One way around it if it's ok as a special location requirement.

 

[westward10]

It is standard for cabins to have rcd protection and obviously additional rcd protection is provided for the supply sockets. I know what I would do but it probably isn't advice I would give on the forum.

 

[leep82]

Is the submain SWA? Does it need protecting? One way around it if it's ok as a special location requirement.

Yes the submain is a 4 core 35mm swa. TT system. A couple of the cabins were originally fed from a generator and so live readings were taken at the time. Now as more trades get onto site there is a need for the remaining cabins to be completed. I was going to use the readings taken for the the two cabins that were originally in use, would this be ok?

 

[westward10]

Are the cabins which were originally in use the same cabins you are now needing to test.

 

[leep82]

Yes only

Are the cabins which were originally in use the same cabins you are now needing to test.

yes only two of them though, 9 cabins in total. The two that were already tested were fed from a generator which is no longer in use, and in both cases the supplies to each have been reterminated as the original cabling was not long enough

 

[westward10]

How long since they were last connected then reconnected.

 

[leep82]

How long since they were last connected then reconnected.

Only a couple of months

 

[westward10]

Only a couple of months

Well you have obtained the x5 test which is the requirement for additional protection so personally I would state the x1 as N/V as fault protection is provided at source.

 

[leep82]

Well you have obtained the x5 test which is the requirement for additional protection so personally I would state the x1 as N/V as fault protection is provided at source.

Thanks for the advice

 

[westward10]

You never know if you try it enough times you may eventually obtain a x1 reading

 

[Leesparkykent]

No RCD protection for the tails entering the 3phase DB that's TT? Would of been cheaper and possibly a better design to have an upfront time delayed RCD in a plastic enclosure with the tails run through it which would provide fault protection. Preferably bs1361 fuse carriers in the 3phase DB supplying the distribution circuits then 30mA RCD's as the main switches in the cabins.

 

[Wilko]

Hi - if you are in control of the site that may perhaps allow you a temporary wiring option to perform your testing?
When I've struck this issue I've been fortunate enough that both have tripped during testing. Feeling optimistic I'd persist a while as Westward suggests

 

[westward10]

No RCD protection for the tails entering the 3phase DB that's TT? Would of been cheaper and possibly a better design to have an upfront time delayed RCD in a plastic enclosure with the tails run through it which would provide fault protection. Preferably bs1361 fuse carriers in the 3phase DB supplying the distribution circuits then 30mA RCD's as the main switches in the cabins.

This sounds a better set up however, it is now common practice for cabins to be connected by way of 32A couplers and assuming they are 32A in the OPs case they require additional rcd protection in compliance with Reg 415.1.1, that being 30ma.

 

[Lucien Nunes]

I've always overcome this problem by testing the second RCD using the test probes across its load-side line and supply-side neutral. The tester doesn't care whether the test current returns to earth or neutral, it should still perform a valid test on that RCD provided only the line goes through it. But because the test current returns via the the first RCD's neutral, that sees the tester as a legitimate load and shouldn't trip.