Questions on bonding

[MickSmith5824]

Afternoon

A simply question but like confirmation from the more knowledgeable. Can someone confirm if there is a direct short between a line conductor to extraneous pipework I.e water or gas. In what scenarios will the MCB trip or not trip? My own thoughts are that if it has a negligible impedance to earth it will as this will cause the fault current to be high enough to trip the MCB. If it has a high impedance it may not as the fault current may not be enough for it to trip. So following from this situation if a main bonding conductor has been joined to this pipework this would lower the impedance and so make it more likely to trip out. Also if circuit that developed fault was RCD protected because this measures imbalance between line to neutral as soon as this fault became present on pipework it would trip out as we would now have more current travelling through the line and not returning via the neutral?

 

[telectrix]

basically correct. although, in the event of a person touching both an exposed conductive part and an extraneous part when there is a fault, bonding causes the extraneous part's potential to rise to approximate that of the exposed conductive part (under fault conditions) so that the body only has a small potential across it, ideally< 50V.

 

[MickSmith5824]

basically correct. although, in the event of a person touching both an exposed conductive part and an extraneous part when there is a fault, bonding causes the extraneous part's potential to rise to approximate that of the exposed conductive part (under fault conditions) so that the body only has a small potential across it, ideally< 50V.

Thank you Telectrix. So to clarify further then the 2.2K ohms measurement of true earth to any suspected/potential extraneous parts confirms whether these parts require bonding?

I still have limited experience and don't bite off more than I can chew but got asked to look at property for someone who had failed inspection for lack of water and gas bonding. On inspecting property I found meter located outside with 10mm directly taken to from MET to gas meter outside so he clearly didn't check this or decided not to. I also at this time noticed what must be main water pipes in plastic going into each property. I checked consumer unit and as well as 16mm main earth there was a 10mm earth conductor this did not go back to MET outside property so can only assume it was an old water bonding conductor and since all property's have been changed to plastic incomers. One thing that did throw me a bit tho was that on disconnecting main earth from consumer unit and using wander lead to pipework I was getting a reading of 55, 5 on Megger for low ohms reading. I was expecting a reading of Kohms and not sure what this comma is about? Surely if 55.5 ohms would be a dot not comma.....baffled : ) when set to insulation resistance I got > 299 so bit confused by this seems I've proved not extraneous but ohms reading has thrown me? Cheers

 

[telectrix]

did you isolate CU before removing earth? maybe the 55 was a warning voltage > 55V??? your IR test proves thar t water is not extraneous and does not require bonding

 

[MickSmith5824]

did you isolate CU before removing earth? maybe the 55 was a warning voltage > 55V??? your IR test proves thar t water is not extraneous and does not require bonding

I did telectrix it wasn't a voltage that was shown it was 55,5 ohms I think it must of been 55000 just that usually it would be 5.5K ohms on tester that is what has thrown me not sure if can get on with the Megger my old Kewtech seemed to be simpler and more reliable. On another point about bonding, said I had a few : ) I went to access a property for potential DB change no signs of any bonding at DB. Checked Gas had sticker saying plastic incomer. Disconnected main earth and tested to water stop cock getting dead short on ohms and IR test. Problem with the DB is it is set in brick work and recessed I think it would be an absolute nightmare and probably unsightly if ran trunking round kitchen to stop cock. I phoned NIC to check on options and they said that this 0.00 ohms reading is likely to be through the boiler cpc which I made the mistake of not disconnecting these before testing. He seemed to suggest that because of this low reading and installing fully rcd protected board I can get away with not running a new water bond and just note it on cert that I had tested it and readings are good. I'm not to sure about this though?

 

[telectrix]

I did telectrix it wasn't a voltage that was shown it was 55,5 ohms I think it must of been 55000 just that usually it would be 5.5K ohms on tester that is what has thrown me not sure if can get on with the Megger my old Kewtech seemed to be simpler and more reliable. On another point about bonding, said I had a few : ) I went to access a property for potential DB change no signs of any bonding at DB. Checked Gas had sticker saying plastic incomer. Disconnected main earth and tested to water stop cock getting dead short on ohms and IR test. Problem with the DB is it is set in brick work and recessed I think it would be an absolute nightmare and probably unsightly if ran trunking round kitchen to stop cock. I phoned NIC to check on options and they said that this 0.00 ohms reading is likely to be through the boiler cpc which I made the mistake of not disconnecting these before testing. He seemed to suggest that because of this low reading and installing fully rcd protected board I can get away with not running a new water bond and just note it on cert that I had tested it and readings are good. I'm not to sure about this though?

agree with NIC. bond is through the boiler manifold. gas to water .

 

[MickSmith5824]

agree with NIC. bond is through the boiler manifold. gas to water .

Ok thats good to hear. I thought we needed to bond in 10mm though if 16mm main earth? I realise the CSA of the boiler pipework is more than adequate for this but our original earth connection is made through the 1.5mm cpc of the circuit supplying boiler. Also I would think there would be a possibility if ever the boiler was changed or do away with it we potentially lost our bond to all water piping.

 

[pc1966]

There is a whole set of different tables on minimum bonding size depending on supply cable and type! But usually for domestic work (100A max, typically no more than 25mm supply phase size) then 10mm is OK for virtually every situation.