help needed! my brains been frazzled!

[coxy84]

installed a new cu for some one yesterday, its a tt system incoming voltage 240v and incoming earth from spike 100ohms, all the circuits tested fine every thing works as it should!

now today i went to the same place to change the other small db that supply's 3 circuits the other end of the building. 1x ring and 2x lighting circuits, this small board is fed from the main db with a 40a type b mcb covered by the 63a 30ma rcd. and a 16mm2 3 core swa.

so i did the board, all circuits tested and are good,and with in coming ze of 78.6ohms.
where iv started to have problems is with the rcd test.

i cant get it to trip at x1 or x5 and it doesnt even trip the rcd on the main db side.

iv tried 3x new rcds thinking that it was a faulty rcd but still nothing and its not my tester as iv tried my other tester and have tested on a known working circuit on the main db.

iv had to leave the power turned off tonight as i wasnt happy about leaving it on knowing that it didnt trip when put under fault conditions.


any on got any ideas?

 

[mswesty]

i had a similar problem before christmas .
i was asked to connect a 10mm t&e to db for a new shower install.

i got there to find RCD board with a old brown bakalite wylex stand alone rcd also installed upfront (two rcds inline) checked bonds etc all good. ze appx (176ohms)ish
connected upto find niether rcd would operate ?
removed old style rcd suspecting it was throwing everything. still the same!
disconected the shower , explained problem and went next job.
ordered new main rcd . returned removed old style wylex thing and installed new . no joy!

i then stripped every thing in a rage and tested a bare board with just rcd . it worked!
connected circuits back one by one until i came accross a 6mm t&e which when connected stopped the rcd working . i asked about the unmarked cable and it turned out that this cable used to feed a old shower a while back and been "terminated" somwhere towards the back of the house by a builder.

nobody knew where the end of the cable was. i disconnected line/neutral and earth for this cable explained that further investigation was required and left becuase the customer didnt want to persue the issue any further.
i cant to this day work out the reason apart from a thought which just came to me

if the lost end of this cable had neutral and cpc connected together could it be that when the rcd fault test occurs it flows to neutral via this connection no earth leak is found and life goes on ?

30ma fault current flows via tester a high portion of which is able to flow to n via conn the high ze means a low portion of which will flow to earth?

i supose under normal loads etc the rcd would trip using my example though

ramble finished!!!

 

[coxy84]

thanks for the ramble mswesty! will bare that in mind!

 

[nickblake]

Some one may have mentioned this have you pressed the button on the RCD

 

[SKY]

Wonder what the outcome is?? maybe our friend is still there!

 

[telectrix]

hope he's not blown himself up

 

[coxy84]

i am afriad im still alive!

as it turns out it was the batterys on both my testers! when i used my fluke this morning the battery symbol popped up! i changed them and x1 =16ms x5 =8ms perfect! i didnt think it was my wiring!

i knew my metrel had a dodgy battery but the batterys on the fluke must have been on the way out plus it was - 4 when i was using it yesterday.

god knows why it worked fine on my house.

goes to show it is either tester or a faulty rcd!

cheers for all your coments!