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

 

[leep82]

View attachment 37919

 

[leep82]

Just to be clear of the complete setup heres a little picture. The armoured cable that you can see is the 35mm 4 core that supplys the TP&N DB which in turn supplies the site cabins

 

[leep82]

View attachment 37920 Just to be clear of the complete setup heres a little picture. The armoured cable that you can see is the 35mm 4 core that supplys the TP&N DB which in turn supplies the site cabinsView attachment 37920

 

[leep82]

 

[leep82]

sorry for the duplicate posts

 

[leep82]

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.

Just tried this method and it still trips the RCBO in the main DB

 

[Lucien Nunes]

With no connection from tester to earth? Interesting, not sure why, anybody?

 

[Wilko]

Just tried this method and it still trips the RCBO in the main DB

If I understand correctly, it should by L and N to RCD load side as normal, but E probe to the incoming N for that RCD. So no test current in the supply earth.

 

[Karin]

If I understand correctly, it should by L and N to RCD load side as normal, but E probe to the incoming N for that RCD. So no test current in the supply earth.

that's how i do it on my kt64 with 3 probes

 

[davesparks]

It's hard to see from the picture but does that Earth leakage module not have adjustable settings?

 

[davesparks]

I've just googled the part number and it is indeed adjustable, so you can wind its settings up to stop it tripping for the duration of the test.

I'd suggest that, if it is compliant to do so, the trip should be set to something other than 30mA without time delay.