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.

 

[happysteve]

(Sorry to interrupt the flow, but...)

Best. Thread. Ever.

Appreciate reading folks' problem solving strategies. OP, hope you get it sorted. Thanks all who have contributed so far.

 

[Lucien Nunes]

I might even put an ammeter in series with the water bond and test the direction of current if something shows up

I am not sure what you mean by direction? In-phase with the supply would suggest the pipe is taking your leakage and out-of-phase would suggest the rod is taking someone else's via the pipe, unlikely given the relative impedances. You'll probably also find a tiny DC electrolytic current. But the RCD knows none of this, as it only sees L & N, until one of the fault conditions so far mentioned comes into play.

 

[Hawkmoon]

OP...

Two possibilities now:

1) You have made an error in your testing
2) This is a wind up

leaning toward 2 myself!

 

[Hawkmoon]

I can confirm there is no flickering or any loose neutrals my side.

Next line of inquiry will be change of RCD to a known reliable one plus a box full of clamp meters testing an isolated circuit testing at every point possible.

If so much as a milliamp escapes I'll be onto it.

Edit: I might even put an ammeter in series with the water bond and test the direction of current if something shows up.

lol - as I said - 2

 

[Silly Sausage]

[/U]


lol - as I said - 2

mooooooon!!!!!

 

[Hawkmoon]

View attachment 21616



mooooooon!!!!!

nice piccy - wallpapered!

 

[Deleted member 9648]

Yes...interesting thread.Have to say I agree with the view that this is a classic N-E scenario which often only trip the RCD when a big load is applied. Never ever known 'mixed' earthing/bonding to affect an RCD.
OP you have missed something....test again,or get a second opinion from another experienced electrician.

 

[itselectric]

Can you explain how it would do that?[/QUOTE

Yes you're right. that last whiskey and the sandman calling scrambled my brain.

 

[m4rk1]

I am not sure what you mean by direction? In-phase with the supply would suggest the pipe is taking your leakage and out-of-phase would suggest the rod is taking someone else's via the pipe, unlikely given the relative impedances. You'll probably also find a tiny DC electrolytic current. But the RCD knows none of this, as it only sees L & N, until one of the fault conditions so far mentioned comes into play.

Agreed but if there is anything incoming from next door then that's a problem. I'm not expecting it TBH for the reason in bold.

Anyway for those who are leaning towards this being a wind up, I've got better things to do than sit here doing that so I think we'll leave it there for now.

As I say,there are further tests required which weren't done at the time as I wasn't there to do a full test but will be done when I go back. I'll find the problem don't worry about that.

If it gets really strange, I know plenty of other sparks who'd be more than interested in pitting their wits over the good fault problem.

 

[Hawkmoon]

Agreed but if there is anything incoming from next door then that's a problem. I'm not expecting it TBH for the reason in bold.

Anyway for those who are leaning towards this being a wind up, I've got better things to do than sit here doing that so I think we'll leave it there for now.

As I say,there are further tests required which weren't done at the time as I wasn't there to do a full test but will be done when I go back. I'll find the problem don't worry about that.

If it gets really strange, I know plenty of other sparks who'd be more than interested in pitting their wits over the good fault problem.

Why take offense? - none intended, just banter

This forum thrives on it lol

 

[Lucien Nunes]

anything incoming from next door then that's a problem

I was actually trying to lay that ghost! Try drawing yourself a circuit diagram showing the path of the fault current from next door and how it has an effect on the RCD. If you are suggesting RF, you'll need to do the math too.

BTW pending getting these pics please can you tell us the make / model of RCD & confirm it has only four wires, incoming and outgoing L&N.

 

[DPG]

Agreed but if there is anything incoming from next door then that's a problem. I'm not expecting it TBH for the reason in bold.

Anyway for those who are leaning towards this being a wind up, I've got better things to do than sit here doing that so I think we'll leave it there for now.

As I say,there are further tests required which weren't done at the time as I wasn't there to do a full test but will be done when I go back. I'll find the problem don't worry about that.

If it gets really strange, I know plenty of other sparks who'd be more than interested in pitting their wits over the good fault problem.

Don't be put off by the odd comment. You've had some good advice on here by and large, and it has made an interesting thread. Daz

 

[polo1]

Just caught up with this thread - can't wait for the problem to be revealed. Just hope it's worthy of all the good input!

 

[m4rk1]

Apologies for the rather abrupt sounding post earlier making it sound like I'd thrown the towel in on here, no offence taken by the comments BTW, just a bit pushed for time as I was going out the door.

Reading back on my original post, I did rather make it sound like I'd jumped to the very early and unjustified conclusion that next doors earth was the cause before mentioning the tests I'd carried out beforehand. Looking back, as a first time poster I did sort of invite a few questions as to whether I knew what I was doing or not so there were quite a few obvious answers to address that I could have avoided by phrasing the question better.

I didn't have my full kit on me that day as I'd had a couple of pints earlier and wouldn't drive when I was asked to sort the tripping out so had a lift. Nothing to disorientate the senses though or need anyone to prop me up, just a strict no D&D policy.

I said I'd have the job sorted in about ten minutes at the most having done countless of these before, but we had a few anomalies that I didn't have the time to sort.

I was more interested in exploring the possibility of whether a neighboring installation could affect another rather than go through the standard test procedures for tripping RCDs which I know well, hence the reason for the thread. As I say, If you look back there are numerous examples where people claim this has happened so I thought the idea was worthy of discussion.

I'll keep you all posted when I get back fully armed with all my test kit but I'll be tied up all week and this job is the best part of 60 miles away so disappointingly it won't be this week.

As it stands, it's got a cat in hell's chance of getting an EICR off me in it's present form until I've gone through the lot thoroughly but I hope you all appreciate the desire to get back and sort it out before my 100% record of fault finding goes up the spout not to mention providing the answer to all you , like me, who are chomping at the bit to know yourselves.

Fingers crossed it's a nice juicy one and not some dodgy RCD that doesn't like loads over a few Kw.

PS - In response to the above query, the RCD is L/N in, L/N out as standard. It fronts a Hager board which seems to have been butchered into a high integrity as the RCD has been removed????

I'm pretty sure the RCD was a Chint but didn't take too much notice. They test well chints and I've no reason yet to think this one is faulty, but who knows.

I'll keep you posted.