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.

 

[Darkwood]

So the elements have been overheated and cutout due to thermal trip ... it could be the fact that the elements took some damage that has since cleared ... cheap shower if it dosn't have a basic flow/pressure switch.. but you get what you pay for.

 

[Lucien Nunes]

a high resistance on the neutral causing an imbalance.

Can you explain how it would do that?

 

[UKMeterman]

Have a read of the link, Try page 42 http://www.google.co.uk/url?sa=t&rct...,d.bGQ&cad=rja

 

[m4rk1]

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.

Any loose connections are not my end as I've tested for all that. A quick pointy ended screwdriver is one of the easiest tests.

If there's a neutral fault next door it could throw a high voltage through the earth loop assuming it isn't bonded very well to any another PME along the other houses to rescue it. We're the end house so in theory, it's only next door that could be at fault, assuming it's possible to affect it that is, which is under debate.

Let me say this. Looking at the standard of houses and tenants along the route, I doubt many decent sparks have been inside any of them. The ones that did venture inside were probably got rid of as anything north of fifty quid for a full test, remedial and cert was laughed out of the front door so there is no guarantee anything within the nearest four houses is safe.

 

[HandySparks]

Can you explain how it would do that?

I see that you're being diplomatic here. I've used the same sort of response myself from time to time. A bit more subtle than getting into a slanging match.

 

[Silly Sausage]

I see that you're being diplomatic here. I've used the same sort of response myself from time to time. A bit more subtle than getting into a slanging match.

No response is the best response!

 

[m4rk1]

So the elements have been overheated and cutout due to thermal trip ... it could be the fact that the elements took some damage that has since cleared ... cheap shower if it dosn't have a basic flow/pressure switch.. but you get what you pay for.

The shower is a Galaxy Aqua 1000. Not the best but not the worst either. Brand new and working perfectly as soon as the earth bond to next door is removed. This model has a pressure switch too.

But you make a point worth noting.

 

[Engineer54]

Not going to go into what's causing the problem at this stage, as you have some things to clear up and check out on your return.

What i can't make out, is if PME is obviously available on this street. why are there still TT systems in place?? 88 ohms is better than 200 ohms, but it sure isn't better than < 0.35 ohms. So no matter what the eventual cause of this problem is, I'd strongly advise this householder to have the supply PME'd....

 

[Silly Sausage]

You've never been to Stoke have you Eng?!?!?!

 

[m4rk1]

You've never been to Stoke have you Eng?!?!?!

^^ This.

You need to see the set up Eng. They still have what passes for normal streets wired by overhead telegraph pole with transformers nailed to them.

 

[Lucien Nunes]

Originally Posted by Lucien Nunes Can you explain how it would do that?



I see that you're being diplomatic here. I've used the same sort of response myself from time to time. A bit more subtle than getting into a

What I meant was, I can see a possible (albeit far-fetched) mechanism, where a loose N connection at the CU causes transients across the N-E capacitance of the installation when heavily loaded. I think it's unlikely as the flickering and probable overheating would have come to the OP's attention, and also he states that the trip occurs the moment the shower is switched on, every time. However a poor connection in next door's neutral seems like a red herring, and was curious as to the suggested explanation for that...

 

[m4rk1]

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.

 

[Lucien Nunes]

Excellent, keep us posted!

Also, some pics would be good, I don't expect they'll reveal anything but it's nice to see what you can see, rather than our own imaginary pictures that we concoct as the story unfolds.

 

[Engineer54]

^^ This. You need to see the set up Eng. They still have what passes for normal streets wired by overhead telegraph pole with transformers nailed to them.

Nothing remotely strange about that.... You'll find that most if not all overhead PME systems are ''True PME'' and not TNC-S!!

 

[Silly Sausage]

And lots of pictures of everything!!!



edit:

Lucien's already said that, but just to emphasise the point! They do make things a lot easier.