Converting tt to tns

[jibjob]

Bit of an odd one this. Just to put you in the picture. Block of 4 flats on TT install. Upon completing some minor works in the basement I did all the usual tests etc, did a Ze, the reading was above 200 ohms (ugh )
The strange thing is there is a TNS connection (16mm) off the service cable prior to entering the service head which isn't terminated.
So, I decided to attach the 16mm to the basement with the intention of running all earths back to M.E.T. and disconnecting the earth rod from the rest once I was satisfied all was well. Did another Ze for the basement and got 0.14 ohms (nice ). However, all of the flats are protected by 100mA RCD's in meter cupboard and each time I attached the 16mm to the M.E.T. the top flat RCD trips. Top flat is empty but I can't get access until later in week. Anyone got any ideas that I can arm myself with prior to my inevitable fault finding mission, thanks in advance.

 

[Deleted member 26818]

Sounds like a fault in the top flat, and having a good earth is now allowing a larger earth fault current to flow.

 

[JD6400]

Hi jibjob afraid i have no idea on the rcd trip , but on a separate note is the tns connection sweated joint or clamped as this may be why it was disregarded in the first place

 

[IQ Electrical]

Hi jibjob afraid i have no idea on the rcd trip , but on a separate note is the tns connection sweated joint or clamped as this may be why it was disregarded in the first place

Be very careful, if at some stage the DNO advised that the TN-S connection was not stable or could not guarantee stability then TT would have been the option for the flat development.

 

[jibjob]

It's a sweated joint, flats are 19th century, in a very poor state of repair, loads of additions, an electricians nightmare to be fair. Ground is really soft and loose as I already tried putting in a second spike but to no avail. I will do some digging re TN-S being unsuitable, other flats in same terrace appear to have a mixture of the 3 main domestic earthing arrangements

 

[sjm]

How far above 200 ohms is the original TT system? Although the reccommended value of a TT system should not be over 200 ohms it is not stated that it must not be! The max value given in 7671 is given as 1667, when protected with a 30ma RCD. The main issue, IMO, would be the disconnection times, and reliability of disconnection, of the RCD. My advice would be to contact the DNO and make a PME enquiry, this should then confirm whether or not you can use the unused TN-S connection.

Good luck.

 

[JD6400]

Maybe spinlondon has it ? The reason i as asked about the type of joint is as far as i know if it is a clamp on a lead incomer you cant guarantee a good connection , a bit like not using a earthing clamp on a swa instead of a gland

 

[SimpleSimon]

Might be worth giving the supplier a call to see if there was any reason why it was disconnected.
As regards to RCD agree with Spin, obviously an extremely poor earth which is why it wasn't originally tripping.

 

[tony mc]

Can you not do a IR test on 250v between L E and N E on the load side of the top flats RCD this might give you a clue!!

 

[jibjob]

Can you not do a IR test on 250v between L E and N E on the load side of the top flats RCD this might give you a clue!!

Sounds like a plan and many thanks for all other ideas, comments and suggestions. As always this is the place to come when you need a little help