THE CAMPAIGN FOR REAL EARTHING

After looking at my particular earthling arrangement I thought I would take a risk based evaluation walk through of the application of BS7671:2018 to it.  The 11kV 400 volt substation is about 150 metres away. It is cabled in the street by repurposed 1930 era DC cabling. Thus we have two line phases and a smaller neutral in PILC.  We have concentric single phase cable jointed to it for 4 metres to the 60 Amp Henley service cut-out.  If we measure the Line to Neutral Z and the Line to Earth Z we get 0.19 ohms in both cases with prospective fault currents of 1324 amps.  I think we can assume we have TN-C-S.  No label to designate PME.


Looking at the equipotential bonding zone we have steel gas service pipe and plastic incoming water pipe both becoming copper internally.  But critically both the gas and water conductive pipes become external and exposed, as does the metallic flue of the boiler, thus exporting the PME to the outside. The external exposed extraneous parts are intrinsic to the design of the building.


One of the consequences of ESQCR has caused the DNO to state that the risk of a broken PEN conductor is significant and PME must not be exported.  This is then mandated in BS7671.


If we risk asses TN-C-S it has some pluses and some minus features.

 Firstly the prospective fault current Line to Neutral and Line to Earth being high means that the over-current BS88 service fuse will protect the tails and bus bar of the all metal consumer unit. Also all the sub circuit over-current protective devices will clear line to earth faults within the allowed time without the need for additional protection, (even though it is mandated for other reasons.


A risk that is not widely known about by the general public is that if type 1 appliances taken outside the house, it then exports the PME.  We have the risk that a broken PEN conductor then allows the Neutral to true earth potential to rise to dangerous levels with no means of detecting of clearing the fault.  The risk is suggested to be around one occurrence per day with little evidence of fatalities or prosecutions yet. Also some concerns about the Neutral to true earth potential being high enough to cause it to be noticed under adverse conditions.  For example bare feet on wet ground.


Should the public be concerned or is this a tolerably small risk?


Then contrast with the TT system that is mandated to be used if the TN-C-S is not acceptable. We have Ze anything up to 200 ohms.  No protection on tails or Bus bar of the consumer unit. None of the sub circuits can clear a line to earth fault using over-current protection. Thus we have to have RCD either in sections or per circuit in the form of RCBO to clear line to earth faults.

We must ask how reliable is an RCD? Some manufacturer evidence suggests that failure rates are around 3 – 10 % after 10 years.  The smaller figure being obtained if the device is regularly tested.  Therefore to maintain safe operation we have to have the system regularly inspected and tested to find and clear the inevitable fails in the RCDs. How often does a line to earth fault occur on a type 1 appliance?  And how often is this coincidental giving rise to the hazard?  Whilst some homes are regularly inspected and tested I would guess the majority are not. So should we risk asses using what the regulation says should happen or risk asses based on what is actually happening in practice. If you want to use the regulation based approach then maybe it should be compulsory to have all installations routinely inspected and it be enforced.  Good luck with that approach come the next election.


The obvious choice is TN-S.  It does not have the drawbacks of the other systems.  But that is not a choice for the customer. I do think that BS 7671 should more forcibly go in this direction and eliminate the risks of the other systems. In a way BS 7671 is being caught between conflicting requirements of interested parties. There is only one obviously safe system, the others suffering significant but currently tolerable risks. Adding ever more complicated protection relays or convoluted constraining regulations is the wrong direction to go.


So going back to my assessment I think on balance that using the exported TN-C-S is a marginally less risk than going to TT despite the regulations saying otherwise.

After looking at my particular earthling arrangement I thought I would take a risk based evaluation walk through of the application of BS7671:2018 to it.  The 11kV 400 volt substation is about 150 metres away. It is cabled in the street by repurposed 1930 era DC cabling. Thus we have two line phases and a smaller neutral in PILC.  We have concentric single phase cable jointed to it for 4 metres to the 60 Amp Henley service cut-out.  If we measure the Line to Neutral Z and the Line to Earth Z we get 0.19 ohms in both cases with prospective fault currents of 1324 amps.  I think we can assume we have TN-C-S.  No label to designate PME.


Looking at the equipotential bonding zone we have steel gas service pipe and plastic incoming water pipe both becoming copper internally.  But critically both the gas and water conductive pipes become external and exposed, as does the metallic flue of the boiler, thus exporting the PME to the outside. The external exposed extraneous parts are intrinsic to the design of the building.


One of the consequences of ESQCR has caused the DNO to state that the risk of a broken PEN conductor is significant and PME must not be exported.  This is then mandated in BS7671.


If we risk asses TN-C-S it has some pluses and some minus features.

 Firstly the prospective fault current Line to Neutral and Line to Earth being high means that the over-current BS88 service fuse will protect the tails and bus bar of the all metal consumer unit. Also all the sub circuit over-current protective devices will clear line to earth faults within the allowed time without the need for additional protection, (even though it is mandated for other reasons.


A risk that is not widely known about by the general public is that if type 1 appliances taken outside the house, it then exports the PME.  We have the risk that a broken PEN conductor then allows the Neutral to true earth potential to rise to dangerous levels with no means of detecting of clearing the fault.  The risk is suggested to be around one occurrence per day with little evidence of fatalities or prosecutions yet. Also some concerns about the Neutral to true earth potential being high enough to cause it to be noticed under adverse conditions.  For example bare feet on wet ground.


Should the public be concerned or is this a tolerably small risk?


Then contrast with the TT system that is mandated to be used if the TN-C-S is not acceptable. We have Ze anything up to 200 ohms.  No protection on tails or Bus bar of the consumer unit. None of the sub circuits can clear a line to earth fault using over-current protection. Thus we have to have RCD either in sections or per circuit in the form of RCBO to clear line to earth faults.

We must ask how reliable is an RCD? Some manufacturer evidence suggests that failure rates are around 3 – 10 % after 10 years.  The smaller figure being obtained if the device is regularly tested.  Therefore to maintain safe operation we have to have the system regularly inspected and tested to find and clear the inevitable fails in the RCDs. How often does a line to earth fault occur on a type 1 appliance?  And how often is this coincidental giving rise to the hazard?  Whilst some homes are regularly inspected and tested I would guess the majority are not. So should we risk asses using what the regulation says should happen or risk asses based on what is actually happening in practice. If you want to use the regulation based approach then maybe it should be compulsory to have all installations routinely inspected and it be enforced.  Good luck with that approach come the next election.


The obvious choice is TN-S.  It does not have the drawbacks of the other systems.  But that is not a choice for the customer. I do think that BS 7671 should more forcibly go in this direction and eliminate the risks of the other systems. In a way BS 7671 is being caught between conflicting requirements of interested parties. There is only one obviously safe system, the others suffering significant but currently tolerable risks. Adding ever more complicated protection relays or convoluted constraining regulations is the wrong direction to go.


So going back to my assessment I think on balance that using the exported TN-C-S is a marginally less risk than going to TT despite the regulations saying otherwise.