Why was the RCD test button not working?

[HappyHippyDad]

Evening all...

I have just got back from a CU change. All was going very well and I was finishing early which was lovely for a friday evening, and then.......

The RCD on the opposite side to the lights trips when the lights are switched on... arghh. I eventually found the problem, I had screwed through the shower cable when attaching the CU to the piece of wood I was using to hang it on.

Whilst the screw was still through the shower cable (touching N-E) why did this stop the test button from working on the RCD? There was 240V going through the RCD and by using something on the other side of the dual RCD board it would trip, but it would not trip with the test button?

Cheers.

 

[telectrix]

a hard N-E fault will sometimes stop the RCD from working, both on the button and when trying to get a reading with MFT. this also has the effect of tripping the other RCD due to fault current paths.

 

[Tes369]

Deleted

 

[Richard Burns]

Found this diagram I had drawn earlier whilst I was starting to search for a diagram to work from to draw just this diagram. (or whatever) test current can go through the fault and by pass the sensing coil.

 

[Ian1981]

You should go into teaching Richard

 

[Richard Burns]

I know some of the information and can sometimes manage to waffle enough to put it across, but looking at the blank incomprehension I might receive I would kill too many students if they were within reach, it is safer online!

 

[Rpa07]

I know some of the information and can sometimes manage to waffle enough to put it across, but looking at the blank incomprehension I might receive I would kill too many students if they were within reach, it is safer online!

Yours has always been appreciated input and having helped innumerable posters, increasing a depth of knowledge that this industry requests.

Sounds a bit creepy but honestly intended.
Now the chanting of, ‘for He’s a jolly good fellow’, would take it to the realm of creepy!
(Song is stuck in my head now!)

 

[Richard Burns]

Yours has always been appreciated input and having helped innumerable posters, increasing a depth of knowledge that this industry requests.

Sounds a bit creepy but honestly intended.
Now the chanting of, ‘for He’s a jolly good fellow’, would take it to the realm of creepy!
(Song is stuck in my head now!)

Thank you, glad it has been of help.
(Chanting not needed!)

 

[telectrix]

Thank you, glad it has been of help.
(Chanting not needed!)

this might be more appropriate:

or this:

 

[telectrix]

it's saturday, raining, nothing to do, so, for your entertainment ( turn the volume up) and enjoy.

 

[SparkyChick]

And I'd like to flip this on it's head and ask why a Memera RCD pod would trip with the test button, not when tested with my MFT from the terminals with nothing connected but then work 100% (bit slow at rated fault current ~ 50ms, but still acceptable) when tested from a newly installed double socket?

I usually test RCDs at the board with nothing connected, but this had me all confused this morning

 

[westward10]

The line conductor must pass through the pod where there is a sensing coil. Connecting direct to the terminals with any tester will not activate it unless your test lead passes through the pod tunnel. If you want test at the board make an extension socket with fly leads to connect at the device.

 

[plugsandsparks]

what he said, the sensing coil is downstream of the screw terminal.
Had me once , fortunately i tested a few more and smelled a rat when they all behaved the same. Googled the manufacturer and hey presto, there was the warning notice...

 

[westward10]

You have to remember the line conductor connects to the mcb and not the ad hoc rcd pod.

 

[SparkyChick]

Aha!!!! Thanks chaps. That makes complete sense. DOH!

Have to say I'm not keen on them because you can't see the line terminations. Don't like it!