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dodger421

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Hopefully someone wiser than me can help explain some odd measurements I’ve taken at my own home. This is a long read, I’ve tried to give as much information as I can because I suspect I’m over thinking the problem and missing something obvious that’s causing insulated pipework to have earth continuity (IR at 250V 0.00 MOhm, 250 mA continuity >2000 ohm).

There’s no mains gas in the village and due to the property boundaries there’s no space for a bulk oil or LPG tank so gas for the central heating and water is supplied from LPG cylinders. 6 cylinders in 2 banks of 3 connect via a manifold and changeover valve to a single 28mm copper pipe that runs along the boundary wall for 1m before dropping through a plastic sleeve and into the ground. It comes back up, again in a sleeve, about 3m away at the side of the house and enters the subfloor void through another plastic sleeve. There’s an isolation valve, elbow and short length (50mm) of copper pipe visible outside before the supply enters the sleeve and subfloor void through the wall about 300-400mm above ground level. The sleeved section extends approx 300mm above ground.

There’s no visible main bonding conductor to the supply pipe, and I’ve no idea if it runs in a plastic sleeve or duct for its entire underground length. Given the types of connections going into the sleeve I think it’s likely the underground section is plastic, but I can’t confirm without digging up the monoblock or cutting the sleeve. Since I’ve moved in I’ve been wondering if it needs bonding, but testing it kept getting pushed down the priority list until now.

We have a split-con TN-S supply with a Ze of 0.25 ohm. It looks like it’s probably 16mm split-con cable underground but I’m not 100% sure on the size.

A 16mm runs from the MET to the CU Earth bar and a single 6mm runs from the CU earth bar, through the subfloor void to the incoming mains water. As expected it terminates in a BS951 clamp. For some reason the 6mm is connected to the hot water supply, with supplementary 6mm and BS951 clamps joining it to the mains in just above the stopcock and the cold out aftr a branch in the undersink cupboard. The pipework and conductor both read 0.14 ohm to the MET with everything connected.

Having spent more time than I’d like crawling around in the subfloor to replace a shower pump and most of the central heating pipework I’ve confirmed the following.

1) after entering the subfloor void the 28mm LPG supply runs directly to the boiler, suspended with metal pipe clamps from the joists. It splits to 22mm to supply the boiler and an old 8mm leg runs to a stop end in the living room where there either used to be an old back boiler or old gas fire. Both copper runs are again suspended from the joists with metal clamps.

2) the incoming mains water is a 35mm MDPE from at least from where it exits the ground in the subfloor to where it connects to the mains water. That’s about a 5m length of MDPE from where the supply exits the ground to the stopcock. I would imagine the MDPE runs to the mains supply on the street but again have not confirmed this.

3) A 2m length of 25mm MDPE exits the wall to supply an outside tap. There is a tee outside in a lagged wall box and more buried 25mm MDPE supplies another tap nearer to the LPG cylinders from that tee. No metalwork is contact with the ground.

4) the boiler is supplied by a 3A fused plug from a socket on its own radial in the airing cupboard. There was 6mm supplementary bonding joining the DHW, stored cold water, mains water and HWC cold supply pipework in the airing cupboard but I’ve removed that for testing purposes.

5) the immersion heater element has been disconnected because the supply cable to the flex outlet plate is damaged and the flex outlet plate needs moving from behind the CH pipework. The flex and both ends of the supply cable are terminated into separate wago’s for each conductor, just to keep them out the way. The MCB for the immersion is off, as is the DP switch that feeds the timer the supply cable normally terminates into.

6) the 6mm main bonding conductor is clipped direct to the joists, doesn’t appear damaged and doesn’t appear to come into contact with any pipework along it’s run.

Armed with that information I thought I’d go and have another look at the mains bonding situation. Given the incoming water supply is definitely plastic, although it is difficult to confirm from within the cupboard, my initial thought was that the main bond for the water was unnecessary so I got to measuring as follows;

All to MET with all supplementary bonding conductors removed. All readings from Fluke 1664 with nulled 50m wander lead. 250mA low ohm continuity tests and 250V IR testing.

To mains Water incoming - 6mm connected, boiler supply plugged in - continuity 0.14 ohm, IR 0.00 MOhm
Water - 6mm disconnected, boiler supply plugged in - 0.50 ohm, IR 0.00 MOhm
Water - 6mm disconnected, boiler supply unplugged - >2000 ohm, IR 0.00 MOhm

LPG supply at house - 6mm connected, boiler supply plugged in - 0.14 ohm, IR 0.00 MOhm
LPG supply at house - 6mm disconnected, boiler supply plugged in - 0.50 ohm, IR, 0.00 MOhm
LPG supply at house - 6mm disconnected, boiler supply unplugged, >2000 Ohm, IR 0.00 MOhm

LPG supply at cylinders, 6mm connected, boiler supply plugged in - >2000 ohm, IR 0.02 MOhm
LPG supply at cylinders, 6mm disconnected, boiler supply plugged in - >2000 ohm, IR 0.02 MOhm
LPG supply at cylinders, 6mm disconnected, boiler supply unplugged - >2000 ohm, IR 0.02 MOhm.

Clearly something in the house is providing an earth path to the copper pipework but from what I can see of the construction I’m at a loss to explain where it’s coming from. None of the subfloor pipework comes into contact with the ground at any point, and no services exit or enter the building in a way that would give contact to earth from what I can see.

Does anyone have any suggestions as to what could be causing the 0.00 MOhm to the MET from all pipework in the house, despite all contact points with earth appearing to be fully insulated?

Finally, from the measurements I think I’ll be running new 10mm bonding to the water and LPG supplies, unless I can find the source of the earth potential and remove it. The water is easy because it’s under the kitchen sink, just needs terminated to the correct pipework. The LPG though will need to be bonded outside to maintain access for inspection. I’ll use an external rated clamp, but is it necessary to box it in as well?
 
TL;DR
Incoming water and LPG services seem to have insulating sections >100mm where they enter the building and visually do not re-enter the earth. Continuity measurements to MET are off scale high (>2000 ohm) but IR at 250V is 0.00 at house end. What’s causing the earth continuity?

snowhead

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Unless I was asleep half way through reading it, the gas and water copper pipe only has continuity when the boiler supply is plugged in and connected at the C.U which is connected to the MET?
 

timhoward

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It looks as though the internal gas pipework is bonded after a fashion by via the boiler supply.
So in a nutshell...
1 - LPG supply at house - 6mm connected, boiler supply plugged in - 0.14 ohm, IR 0.00 MOhm
2 - LPG supply at house - 6mm disconnected, boiler supply plugged in - 0.50 ohm, IR, 0.00 MOhm
3 - LPG supply at house - 6mm disconnected, boiler supply unplugged, >2000 Ohm, IR 0.00 MOhm
1 - bonding via water pipe connection to boiler and cpc of supply to boiler, makes sense
2 - continuity entirely via cpc of supply to boiler, makes sense, looks like a typical R2 value.
3 - both routes not available, makes sense, though the IR in bold is the mystery.

Is this a combi boiler or is there a separate central heating pump somewhere?

It looks as though in any case the lpg needs bonding where it enters the house too in order to avoid relying on the boiler and other mystery source for a path to the MET.
 

dodger421

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It was a ridiculously long post, thanks to those who read through it all.

@snowhead yes, but the IR always shows a dead short regardless of the low ohm continuity readings.

@timhoward the boiler wiring is a bit of a mess to be honest, but here’s the general gist in another essay.

It’s a Y-plan system with the socket radial and wiring centre l, pump, three port valve and HWC are in the airing cupboard. The programmer (actually now a Hive receiver) is on the other side of the same wall in the kitchen. The boiler is in the utility room at the back of the house, about 5m horizontally from the wiring centre.

Here’s where it gets messy. The mains from the socket goes to the programmer in 2.5mm flex. Three more flexes, 2 three core and a two core then go from the programmer back to the wiring centre bringing all the switching inputs and power back to the wiring centre.

The three port valve and cylinder stat are wired straight into the wiring centre as usual. The switch live, neutral and earth then go out to the boiler on a 2.5mm flex which runs in the floor void. The pump the has another flex coming back from the boiler directly to it.

In the wiring centre there’s a disconnected T+E going to a now redundant room stat on the wall outside the airing cupboard. It’s disconnected and Wago’d at both ends inside the respective enclosures.

Writing that out and thinking about it actually makes me wonder if the pump CPC, which must be connected to the boiler casing as well, is causing the mystery IR source. Although surely that path would be removed when the boiler power supply is unplugged?
 

Dartlec

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the key seems to be to work out why the IR is giving a reading of 0 when the continuity isn't showing anything. Presumably that means continuity is somewhere between 2000 and the lowest the meter can read for IR? (0.01 I assume - which is 10,000 ohms)

With cabling wouldn't that usually point to a stray earth somewhere, with a connection that is maybe not good enough to show up on continuity testing?

Could one of the metal pipe clamps be touching, (perhaps with a very poor connection) something that is also earthed elsewhere? Or perhaps touching concrete in the foundations that is connected to earth somehow?
 

timhoward

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To be honest, if this was anyone else's house, you'd have said "I don't want to spend the rest of my life figuring this out" by now.
In pragmatic terms the bottom line is there is no evidence of gas bonding, just wierdness, so you add some bonding and move on.
I admit that in my house (will spare the essay of the former builder-occupier's own electrics) I would not be content with that either, but if you add the obviously missing bonding, there isn't a compelling reason to solve the riddle is there? It will likely become apparent at some point in the future when you are trying to solve a completely different problem!
 

dodger421

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Thanks everyone for the patience!

@timhoward is bang on, anywhere else I’d bond it and move on. It’s only because it’s my house and I’ve become more intimately familiar with the subfloor void than I’d like to that I’m perplexed and dwelling on it. Everything I can see with my eyes tells me there’s absolutely nothing making that pipework extraneous, and yet it tests as such!

@Dartlec yes, 0.01 MOhm is the lowest IR reading, so somewhere between 2001 and 10 kOhm sounds about right. There’s a SWA running through the subfloor wall to the garage and goes underground for a short length. It’s possible the outer sheath has been nicked and the armour is in contact with the earth and/or foundation brickwork.

As a final effort to aid understanding I’m going to try testing with the 16mm earth from the MET to the CU disconnected and see what comes back (once I can isolate the place obviously).
 

UNG

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Is this a fixing screw into an earthed metal back box that has grazed the insulation on a live cable type fault, wouldn't be the first time it has happened and left people scratching their head for hours
 

dodger421

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Given the sheer number of bodges and examples of shoddy workmanship from bathroom fitters, kitchen fitters, wet pants and not to mention the DIY electrical works by the previous owner you’re probably spot on.

I suspect when I get the chance to test the pipework just to the external earth the IR reading will improve, but I’ll let you all know when I get around to doing that test.
 

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