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.

 

[Lucien Nunes]

Unless I've missed something this sound like classic symptoms of an N/E fault on another circuit, i.e. RCD trips when heavy load applied (from a circuit that is itself OK) and when earthing is good.

 

[Silly Sausage]

No. I've got a clamp meter I use for earth leakage and it didn't even make a reading.

If there was any earth leakage from the shower still, the RCD would definitely trip. I'm fairly certain this isn't a faulty shower or cumulative leakage either.

I should have added, with the water bond in place...

(I think! )

 

[Hawkmoon]

No. I've got a clamp meter I use for earth leakage and it didn't even make a reading.

If there was any earth leakage from the shower still, the RCD would definitely trip. I'm fairly certain this isn't a faulty shower or cumulative leakage either.

A normal clamp meter (assuming that's what you used) does not have resolution low enough - earth clamp meters go down to about 0.1mA

If the leakage from shower was low it may not trip the RCD with your higher Ze value (TT alone)

 

[m4rk1]

It's got me stumped but the water bond was nearly 1Mohm isolated from the shower due to all the plastic piping. Even if there was huge leakage from the shower it wouldn't have had low enough resistance to cause it to trip.

 

[Hawkmoon]

Unless I've missed something this sound like classic symptoms of an N/E fault on another circuit, i.e. RCD trips when heavy load applied (from a circuit that is itself OK) and when earthing is good.

But then why only the shower causing the problem?

 

[m4rk1]

The clamp meter is a Dilog DL6507 It didn't even raise an eyebrow. IR test on 500v for shower when hot was >200Mohms.

 

[Hawkmoon]

The clamp meter is a Dilog DL6507 It didn't even raise an eyebrow. IR test on 500v for shower when hot was >200Mohms.

Sorry - assumed wrong


Then - shower has borrowed neutral from next door lol

 

[m4rk1]

The reason I posted this one was it's a massive mystery. Like I say though there are many examples on here over time where these nuisance trips are mentioned alongside 'mixed' earths.

[head scratching icon here]

Don't forget the original question too. Is there a requirement to replace the bond. If it does turn out different earthing systems can interfere with each other is there a reg anywhere I'm missing? Or one missing completely?

 

[m4rk1]

Sorry - assumed wrong


Then - shower has borrowed neutral from next door lol

It won't be borrowing any soap from next door, looking at them ,LOL

 

[Lucien Nunes]

But then why only the shower causing the problem?

Because it's probably the heaviest load in the property, and depending on the resistance of the N/E fault compared to the supply, it takes a certain minimum load to drive 30mA through the fault. You might get the same result with 9kW of heaters plugged in but the OP probably hasn't tried that. I did ask whether the IR on the other circuits has been tested but not sure about the answer yet.

 

[m4rk1]

Definitely no borrowed neutrals Hawkmoon. One of my first lines of enquiry that and ruled out by a straight visual as almost everything is on show via surface trunking.

 

[Flanders]

Unless I've missed something this sound like classic symptoms of an N/E fault on another circuit, i.e. RCD trips when heavy load applied (from a circuit that is itself OK) and when earthing is good.

Mark It Might be worth doing IR tests on other circuits to check this , if the place is empty then the only thing pulling any real load would be the shower but thinking about it, you would of thought you would get the same fault with the bonding disconnected as you still have the CPC connected, so It could be earth leakage from next door

 

[m4rk1]

Because it's probably the heaviest load in the property, and depending on the resistance of the N/E fault compared to the supply, it takes a certain minimum load to drive 30mA through the fault. You might get the same result with 9kW of heaters plugged in but the OP probably hasn't tried that. I did ask whether the IR on the other circuits has been tested but not sure about the answer yet.

I answered that just after you asked it Lucien (post #4). All IR tests were passed. It's not a complicated arrangement. One set of lights, ring, shower, water heater & cooker. No central heating pipes, no gas.

The shower is a 7.5kw as it's on a 6mm cable. As you say , i haven't tested a bigger load though and I'd agree the size of the load is relevant.

 

[m4rk1]

Mark It Might be worth doing IR tests on other circuits to check this , if the place is empty then the only thing pulling any real load would be the shower but thinking about it, you would of thought you would get the same fault with the bonding disconnected as you still have the CPC connected, so It could be earth leakage from next door

I'm thinking this Flanders.

I know its easy to think I've missed something obvious but i'm positive this is not the case and you'd all see my point if you could see the job yourselves.

I read not long back on here a guy, amongst many others who had something like 100ma running through his earthing but there was no tripping. Someone suggested it had re entered the system bypassing the rcd because one earth loop impedance was lower than a conflicting one in a different configuration.

The fact I've isolated next doors earthing and there is now no problem is telling to some degree. if the shower was at fault it would still be tripping now.

 

[m4rk1]

Next doors occupants don't look very savoury. I just hope the person who's wired up their cannabis farm has done a cert on it.

You never know.