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]

Sorry, didn't connect that answer with that question. So the whole installation passes an IR test between N&E under the same conditions as the tripping occurs? If you unplugged appliances to do the test, and reconnected them afterwards, there is a small possibility that one of them contains an N/E fault.

Disconnecting the bond also points towards this explanation. With a normal case of earth leakage, whether Ze is 0.7 or 85 ohms won't make any difference, even that magic number 1667 ohms will only drop 50V of the p.d. driving the leakage. But with an N/E fault you have a low resistance path in parallel, the p.d. across the fault is tiny and increasing the Zs by a factor of 120 by disconnecting the bond would have reduced the leakage to a fraction of the current that first caused the trip. That is why I think this must be carefully ruled out, because the symptoms fit exactly

 

[Hawkmoon]

Does the shower live or neutral run very close to the RCD unit within the CU (have heard that older units can be imbalanced by closely adjacent loaded cables)

Though never had this issue myself


edit - scrub that as it would trip regardless of the bond

 

[m4rk1]

No Hawk. The RCD is in its own enclosure separate from the CU and nowhere near any wires.

 

[Flanders]

How do you think its getting the earth from next door is the mains lead piping ?

 

[Hawkmoon]

Going back to the having to bond question that has to be a yes - the incoming water supply (regardless of the pvc pipework around the house) is extraneous.

 

[tony mc]

What size is the up front RCD and what is your IR reading on the pipe you disconnected ?

The RCD is seeing a imbalance on the Line/Neutral upstream of the CU & IMO whatever leakage from outside the premises should not trip your RCD!

 

[Hawkmoon]

I would disconnect everything from the CU except for the shower to eliminate the possibility of other circuit faults contributing

If it still trips then it can only be either the RCD becoming faulty under load or the shower

I can't see how earth currents from next door's earthing system can cause an imbalance in the RCD (without L/N - E fault somewhere)

 

[m4rk1]

Sorry, didn't connect that answer with that question. So the whole installation passes an IR test between N&E under the same conditions as the tripping occurs? If you unplugged appliances to do the test, and reconnected them afterwards, there is a small possibility that one of them contains an N/E fault.

Disconnecting the bond also points towards this explanation. With a normal case of earth leakage, whether Ze is 0.7 or 85 ohms won't make any difference, even that magic number 1667 ohms will only drop 50V of the p.d. driving the leakage. But with an N/E fault you have a low resistance path in parallel, the p.d. across the fault is tiny and increasing the Zs by a factor of 120 by disconnecting the bond would have reduced the leakage to a fraction of the current that first caused the trip. That is why I think this must be carefully ruled out, because the symptoms fit exactly

I take your point on that Lucien but this is assuming there is earth leakage in the first place for the parallel path to lower the impedance to the point where it caused the trip. I'm reasonably confident there isn't for a number of reasons including direct measurement of leakage,IR testing, the lack of bonding to this circuit and the high resistance between MET and the nearest thing that can be even remotely considered an extraneous part.

 

[m4rk1]

How do you think its getting the earth from next door is the mains lead piping ?

Absolutely. I presume with no plastic fittings en route too.

 

[Lucien Nunes]

or the parallel path to lower the impedance to the point where it caused the trip

Sorry, I think we're at cross purposes, this is not what I meant. The parallel path I was describing is the resistance of the neutral conductor to the star point in the substation. When you have a N/E fault on a circuit, and another circuit is loaded, there are two ways for the neutral current from that load to return to the substation transformer once it arrives at the CU. One is directly along the supply neutral, causing it to pass through the RCD. The other is back from the CU neutral bar into the faulty circuit, back again to the MET through the faulty circuit CPC (and any other earth paths in contact with that circuit) and into whatever earthing system and hence to the susbstation. This path misses the RCD and causes an imbalance.

If you alter Zs, whether it is by adding or removing bonding, supplier's earth or even altering bonding in the neighbour's property, then you will alter the ratio by which the current divides between the neutral and earth routes, and hence the amount of imbalance seen by the RCD. The lower the Zs, the more likely the trip at a given load current, because a larger fraction of it will pass through the fault. Don't forget in this case it is only the voltage drop along the supply neutral, external to the premises, that is causing current to flow through the fault.

I may be wrong about the N/E issue but so far it is the only exact match for your symptoms

 

[midgetman]

disconnect other circuits - reconnect earth - try shower.

 

[Lucien Nunes]

And /or put a leakage clamp on the shower circuit and the meter tails

 

[Hawkmoon]

Isn't it a refreshing change to have an electrician's problem rather than the endless Electrical Trainee drivel!

 

[m4rk1]

What size is the up front RCD and what is your IR reading on the pipe you disconnected ?

The RCD is seeing a imbalance on the Line/Neutral upstream of the CU & IMO whatever leakage from outside the premises should not trip your RCD!

RCD is standard 30ma.

On the latter. I didn't take a direct reading of the pipe so I can't give you that figure now. I just did the ZE on the rod and them a quick Zs to establish we were running on two different earths.

I'll be honest I more or less homed in on the possibility of the neighbouring installation having a problem with it and disconnecting the link to test the 'theory',which worked. that coupled with all the other stuff before which I think rules out the actual shower circuit, borrowed neutrals etc

This will be ongoing and a full test is yet to be done to the point of certing it, and more to the point getting to the bottom of all this. I'll hook the shower up completely isolated next time and do more tests.

I know the whole street is a mix of TTs and TNCSs if it's anything of relevance (it's stoke on trent by the way and they're very antiquated up there and lucky to have electricity in the first place.:icon7:

I'd also be keen to look at the state of the installation next door just in case but can't see them even answering the door TBH.

I'll reserve judgment but if the regs insist on a bond, swapping the rising main over to plastic might help isolate the fault, assuming there is any mileage in next door having some kind of fault that somehow affects the electrics on our side

 

[Lucien Nunes]

Always beware of hiding the fault when you make these other changes! RCD behaviour can be infuriating but it's usually pretty logical and it doesn't work by voodoo; it is a differential transformer that measures the difference between line and neutral currents, period. If you measure this too, you will surely find the fault, regardless of the earthing setup. Every strange RCD problem I have ever seen, has been resolved completely by clearing an identifiable fault or changing the RCD. Back in the bad old days of VOELCBs, the earthing configuration was important to ensure proper operation of the device. This constraint absolutely does not apply to an RCD, so long as Zs is low enough for it to do its job, which in your case it clearly is. I would not make any moves with the earthing until I had leakage clamp readings and / or tested as per Hawkmoon & Midgetman's suggestions.