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]

The condition and performance of the earthing system does not cause an RCD to trip unless there is another fault present. By modifying the earthing I think you might have hidden it. Did you IR test all circuits in the premises to identify any N/E shorts on other circuits?

 

[Hawkmoon]

The shower working again after bond disconnection may have been a coincidence (could you have 'disturbed' the fault during testing?)

Have you tried reconnecting the bond to see if the problem returns?

 

[m4rk1]

Yes I did and there were no irregularities.

There are quite a few threads on here where others have had tripping faults due to mixing earth systems together. On the face of it I can't see why but it does seem to happen.

There is also the possibility that something is amiss next door but I'm not likely to get access to find out.

 

[m4rk1]

The shower working again after bond disconnection may have been a coincidence (could you have 'disturbed' the fault during testing?)

Have you tried reconnecting the bond to see if the problem returns?

Yes. And the problem returned.

Also it was only the shower that caused this. It's brand new and flew the IR test.

 

[Hawkmoon]

You say the RCD tested ok - what tests did you carry out on it?

 

[Lucien Nunes]

mixing earth systems together

I am not sure what you mean by this. Fundamentally, all earthing systems are just connections to the neutral at the source of supply. If the installation and equipment are sound and the leakage is within spec, it won't matter to the RCD how that connection is made, because it's only monitoring L & N.

Old VOELCBs work differently and some RCBOs with FE connections could be funny about what goes on earth-wise, but an ordinary RCD should not be unless there is a fault.

 

[m4rk1]

You say the RCD tested ok - what tests did you carry out on it?

trip times on both sides of the wave at 1/2 x current / 1 x current and 5 x current
Ramp test

All passed perfectly.

 

[m4rk1]

I am not sure what you mean by this. Fundamentally, all earthing systems are just connections to the neutral at the source of supply. If the installation and equipment are sound and the leakage is within spec, it won't matter to the RCD how that connection is made, because it's only monitoring L & N.

Old VOELCBs work differently and some RCBOs with FE connections could be funny about what goes on earth-wise, but an ordinary RCD should not be unless there is a fault.

I agree in theory. But read back and you'll see plenty of this thing happening in other threads so I'm working on the assumption there must be something in it.

 

[Hawkmoon]

trip times on both sides of the wave at 1/2 x current / 1 x current and 5 x current
Ramp test

All passed perfectly.

Ok - I would say the shower (when running) has leakage to earth that only trips RCD when Ze is low (when you have bond connected)

Almost def shower's your problem

 

[itselectric]

If your Ze was depending on the connection to the water pipe. ie rod has a high resistance then you could have been testing the shower with no earth connection so it wouldn't trip. did you test the potential of the earth at the shower when you were doing your trial.

 

[m4rk1]

Ok - I would say the shower (when running) has leakage to earth that only trips RCD when Ze is low (when you have bond connected)

Almost def shower's your problem

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.

 

[Silly Sausage]

Have you tried clamping L & N together with an Earth Leakage clamp meter when the shower is running?

 

[m4rk1]

If your Ze was depending on the connection to the water pipe. ie rod has a high resistance then you could have been testing the shower with no earth connection so it wouldn't trip. did you test the potential of the earth at the shower when you were doing your trial.

No. I didn't do this.

The only earth on the shower came directly from the CU courtesy of the CPC. All the plumbing to it was in plastic and I'm fairly certain no other earth potentials derived from anywhere other than the CPC.

It's a simple radial ,visible nearly all the way from board to shower and no other wiring mixed in with it.

Ze was done with no parallel earths either.

 

[m4rk1]

Have you tried clamping L & N together with an Earth Leakage clamp meter when the shower is running?

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.