TT using next doors TNCS

[m4rk1]

Hi all,

My first post here after an exceedingly long time reading and studying this encyclopaedia of a fourm.

I had a job in on sunday night which was leaving the whole house in darkness the second the shower was turned on.

TT installation with front end RCD to cover every circuit.

Tested RCD - Fine
IR tested (shower) - Fine
IR test wiring - Fine

Ze = 85 - (decent for a TT)
Zs at socket next to board 0.7

The conclusion being the bonding to the water was stealing the benefits of next doors TNCS but the clash of two earthing systems was causing the trip. I quickly disconnected the bond at the pipe to test the theory and voila shower working perfectly no tripping.

Here's the question. Is there any requirement to bond back onto this water pipe when it caused this problem in the first place. If it had been plastic (or even gets changed to plastic later) it would not be necessary.

All other bonding is OK or mostly not actually required as the majority of the plumbing is in plastic polypipe and resistance to MET is hugely over 25Kohms at the nearest touchables, which in this case is a short length of copper pipe in between plastic pushfits housing the isolator valve.

The house is now fully back to TT , safe and not tripping but I thought I'd kick it around on here to get your views. Something always crops up worth knowing.

Lastly the house is up for tenancy but not yet occupied and hasn't had a full test yet so if it turns out removing the bond was for some reason a dozey idea, we have plenty of time to put it back again.

Thanks in advance.

 

[tony mc]

If all the circuits were RCBO protected my money is on only 1 cct tripping (the faulty one) even with the bonding reconnected !

 

[m4rk1]

Thanks Lucien and others for your advice much appreciated. I do follow what you say and also what you say about fault current bypassing the RCD.

When I go back I'll bypass the RCD altogether for test purposes and put clamp meters on every part of the isolated circuit to see what's going on including earths . I've done hundreds of RCD trips and this is the only one in nearly fifteen years I can't see an explanation for as there is no other effective connection to earth bar the CPC in the cable

All my test gear is properly calibrated too so I trust my readings. I'll do differentials and all other tests thoroughly later.

I'll end up lying awake tonight with obscure routes to earth going round my head.

 

[Hawkmoon]

Would be nice to find out what the fault is (when you eventually find it) so post it up

 

[ChrisElectrical88]

Haawwwwwkkkkk Mooooooooooon!

 

[Hawkmoon]

Haawwwwwkkkkk Mooooooooooon!

Spiiiiiiiiiiiiiiiinks - hmm, doesn't have the same ring to it - sounds like I'm abusing a group of foreign gentlemen across the road!

 

[ChrisElectrical88]

Haha don't say that, it sounds like I'm a Mexican eating nachos out of my sombrero!

 

[Lovell1991]

Hi there just working in friend of the family's place supplied by tt system will it past a test and inspection with the board being is isolated by a 100 ma rcd or does it need a main switch within the board

thanks

 

[telectrix]

what other purpose can a sombrero possibly serve?

 

[Silly Sausage]

Frisbee

 

[RogertheBodger]

Give your Donkey a bath.

 

[yellowvanman]

Shower is brand new and also passed the IR test after use. It was good and hot too as after initially getting it all working as flowrate was so very low and the thermal cut out came in.

I had my money on the shower too on the way to the job but absolutely fine.

Did you IR test the shower unit with it turned on, ie. IR tested the internal heating elements?

 

[m4rk1]

Obviously as a dead test but yes. Joined L+N and tested at 500v to earth point inside shower after use and so hot. Cut out had reset itself by then too. Reading >200Mohm so fine.

Or do you mean straight forward resistance test between L / N of element which I'd expect to be around the 7ohms region?

 

[Lucien Nunes]

I think he means with water on so that the flow switch is closed

 

[SPARTYKUS]

Great thread.

 

[itselectric]

could it be a high resistance on the neutral causing an imbalance.
possibly a loose connection either at cu or meter position beside cu or next door.