Rcd testing, no trip at DB

[devon1]

On some RCDs when I test at the board I can't get any Reading and the device will not trip, but if I do the same test at the socket it works. Why is that? I usually test at the distribution board the full test.
Thanks in advance

 

[WDMDL]

Poor or incorrect connection of the test leads is all I can think of.
There's no other reason why the test would complete correctly at a socket outlet but not at the board.

Are you using the "13A plug" test lead for socket outlets and then the standard test leads at the board?
Test them all for continuity, wiggling them to see if there is an intermittent connection on any of them!
Remember it could be on that third lead you don't use so often...

Simon.

 

[devon1]

Thanks, no it's not the test leads. All connections are good.

It happens with MEM RCDs. A lot of them, so far about 30 of them.

I have recently tested about 1100 Rcds at large department stores and funny enough someone told me that if an RCD fails at the board it can pass if you plug in at a socket and test. I didn't really understand how that could be so, but only one week later i have come across this same problem with the MEM rcds.

Non of the MEM rcds will pass at the DB , but will pass when plugging in at the socket. When i'm testing at the DB the RCD tester shows ALL green good connections , i push the button and no trip and time out.

in a couple of days i will check the MA and settings of the MEM rcds,
and keep you posted of any solutions.
thanks

 

[apesolutions]

We have come across this a few times, and found that with some MEM rcd's and rcbo pods, the sensing circuit for the rcd is actually after the terminal where the wire is terminated ie. the coil is above the terminal screw. These devices will not trip if you test them by probing onto the terminal screw as the fault is introduced before the coil. As long as it passes the test at the socket, I would suggest there is no problem.

 

[BillyTheSparks]

I have read in the professional electricians mag, there was a part from megger that went along the lines of:

The circuit is part of the coil and so when tested at the RCBO/RCD it will not trip as your not testing the complete coil.
I didn't understand this myself, and for the life of me I can't find that mag anywhere....

If I do find it I'll post it

 

[devon1]

We have come across this a few times, and found that with some MEM rcd's and rcbo pods, the sensing circuit for the rcd is actually after the terminal where the wire is terminated ie. the coil is above the terminal screw. These devices will not trip if you test them by probing onto the terminal screw as the fault is introduced before the coil. As long as it passes the test at the socket, I would suggest there is no problem.

Thanks for the post, i think your post is the answer.
The MEM rcds are quite big devices and there is 'a part' (which i imagine to be the coil) after the live terminal. Therefore to test at the terminal does not trip the device.
Great post by the way on only your third post, cheers!

edit added; I have tested other circuits with the coil separate and after the connection terminal ( the coil being a separate device about 20cm away. These devices did still trip at the connection point. Perhaps it's the type of MEM coil that causes it.

 

[devon1]

I have read in the professional electricians mag, there was a part from megger that went along the lines of:

The circuit is part of the coil and so when tested at the RCBO/RCD it will not trip as your not testing the complete coil.
I didn't understand this myself, and for the life of me I can't find that mag anywhere....

If I do find it I'll post it

Thanks for info, Apesolutions answer would cover this.
Because the cable passes the coil after the termination at the RCD.

 

[WDMDL]

Every day's a school day!

Simon.

 

[J@mes]

Thanks for the info you have posted - we have been replacing loads of these as failures

 

[apesolutions]

Thanks for the info you have posted - we have been replacing loads of these as failures

You're not the only one - we did a few until we sussed this out

Liam

 

[devon1]

I have tested the MEM rcds at the DB.

Switch off the rcd. Disconnect the live cable, connect a test cable with a terminal block on the end ( secure safetly) . Test at the terminal block. reinstate. Continue testing all rcds at the DB.