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?
