Disconnecting bonding with supplementary paths?

[Bronek536]

Hi guys. 

Just reflecting on the reasons for disconnecting of main protecting bonding conductor when carrying out continuity. As far as I understand it is to avoid parallel earth paths and effects on test readings. However in my dwelling, as I pick any conductor from MET and measure using Test method 2(wandering lead) I get continuity to pretty much every metal conductive part in my house(taps, kitchen sink, pipes gas and water, extraneous and exposed conductive parts). I guess it is a pretty good equipotential zone created. My question is in the above situation would you still disconnect bonding for continuity measurement(not even knowing if this is for water, gas or metal structure bonding because you get reading on all anyway) and if so then why? And finally, is there a way of determining which bonding conductor bonds what if values taken are all the same (I presume caused by parallel paths)?

Thanks

Bronislaw

 

[boltonsparky]

Disconnect bonding conductor at MET, long lead test to the pipe/structure, not the clamp, this way you are checking the effectiveness/continuity of the connection of the clamp to pipe/structure. If there is any doubt that the bonding conductor isn't continuous or is unidentifiable at the MET and the reading you'e getting is a parallel path disconnect at both ends and perform long lead test. Very low reading you have the correct cable, an open circuit you have the other cable. You disconnect them at the MET to test to rule out the parallel paths you mention. You need to be ensuring the reading/connection is via the 10mm cable not the 1.5mm CPC of the boiler circuit. However the reading you get should confirm this anyway, the 10mm will be a much lower reading than that of a 1.5mm cpc parallel path. Anything over 0.05 ohms for a bonding conductor should be investigated. 

Having a reading from the MET to every accessory and piece of fixed equipment, as you mention is correct, unless they're class 2. An earth reading will be being got at sinks etc through the taps from the copper pipe work via the main extraneous bonding conductors. Disconnecting bonding conductors is still likely to get you an earth reading although a slightly higher one through the CPC of the circuit to the connection of the boiler/heating/immersion through their connection to the pipework of the house. 

 

[Bronek536]

Love your answer. As always detailed and to the point. Sometimes the simplest solutions are overcomplicated. Next time I'll pay closer attention to minuscule variations in reading or simply disconnect at the clamp to determine the right cable. Many thanks

 

[binky]

drove myself  a bit potty one day tracking down parallel earth paths, it's amazing what you can get a reading from, especially with damp plaster on the walls. On that particular job, I was even getting earth continuity to the screws holding a door frame to a wall. Never understood how solar inverters can trace earth faults on the DC side through dry roofing timbers and the building materilas, but they can!  Just ignore these things and test as per guidelines, unless as per Boltons reply, you are trying to check extraneous parts using wander lead.

 

[phil d]

I had this once witha guy who was trying to trace a leakage current down to earth, he was adamant that putting his clamp around the main earth cable would reveal the answer, then I pointed out things like metal back boxes screwed to brick walls would allow some leakage to go to earth that way, the only way he'd get an accurate result measuring his way was if everything was on a plastic back box with no connections to any metal pipes and such like. It amazes me how many people don't know how to measure leakage current, I.E put the clamp around BOTH the live and neutral tails, if it reads zero fine, if it reads anything else then that is your leakage current. Then again I recently had a very amusing conversation with a young electrician, I asked him why he'd run a 10mm G/Y to both the gas and water pipes, 'because you have to bond them' he replied, so I asked him why and he replied that it was to prevent any dangerous voltage coming from outside into the building via the gas/water main.

I then asked how long plastic had been a conductor and he rather tersely informed me that it wasn't, it was actually an insulator! I had to laugh at his answer to my next question, having asked him how a voltage from outside could get inside via a plastic pipe, which, he assured me was non conductive I asked why then had he bonded the gas and water pipes. ' because we just do, it's an electrical thing, you wouldn't understand' he replied sounding rather annoyed and obviously not knowing I was in fact a spark myself. 'it's you who doesn't understand' I thought to myself as I walked away chuckling.

 

[boltonsparky]

In my opinion it would be deemed good practice if wiring a house to still run in a 10mm cable to the gas and water incoming pipes even if they are plastic (no need to connect them), just to future proof. 

Not so much of an issue if both supply and house piping is fully plastic but if there is a copper pipe system on the consumer side and any part of that piping is then run through a solid floor or solid walling either on the initial installation or at any point in the future, it would then become an extraneous conductive part and need bonding. 

Granted an unlikely scenario but for the cost of a bit of 10mm on a full rewire/build just stick it in then it's there.

 

[phil d]

In my opinion it would be deemed good practice if wiring a house to still run in a 10mm cable to the gas and water incoming pipes even if they are plastic (no need to connect them), just to future proof. 

Not so much of an issue if both supply and house piping is fully plastic but if there is a copper pipe system on the consumer side and any part of that piping is then run through a solid floor or solid walling either on the initial installation or at any point in the future, it would then become an extraneous conductive part and need bonding. 

Granted an unlikely scenario but for the cost of a bit of 10mm on a full rewire/build just stick it in then it's there.

The problem in this case was it was an electrician working for the local housing, now granted a short length of cable might not add much to the cost of one job, but say it's using 30 metres per house and multiply that by 500 houses, then add the cost of labour to do it and it soon adds up, which gets annoying when they are putting rents up due to the 'escalating costs of maintaining our properties'