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THE CAMPAIGN FOR REAL EARTHING

John Peckham
I think that we were considering adopting PME earthing systems today on what we know now we would say no thanks?


I strongly believe that the use of PME earthing systems is inherently unsafe. I am keen to hear any technical arguments to defend the use of PME?


Most PME DNO new distribution cable use 3 core Wavecon cables for UG distribution with single phase concentric cables tapped off for single phase users. For overhead open wire supplies of newer installs ABC cable.


There is no reason not to use 4 core Wavecons and distribute a much safer TN-S earthing system other than the cables will be a 1/3rd more expensive.


 I believe that the DNOs having been tentatively asking government  for a £trillion pounds to upgrade their networks for when we stop burning fossil fuels and go all electric. No doubt the DNOs hope that the government, civil servants and politicians will have forgotten that these private companies purchased a public assets for a knock down price with the idea that the public would no longer have to subsidies a public body! 


A good start would be that no new supplies will be PME, no replacement cables will be PME and no repairs to cables will be PME conversions. For instance a new housing estate would have to be an all TN-S installation. I understand that WPD are already installing TN-S earthing systems for new housing estates. If this is the case then well done WPD. Can anyone confirm this?


I am also concerned about the degradation of the of the Global Earthing System with use of all plastic covered cables, no bonding to metallic service pipes and the failure on DNO contractors to install earth rods and joints to save time and money. Will we start to see 442 type over voltages?


Look at my EV charging thread and the measures we are having to deploy due to PME earthing, we are having to do this because the PME system is inherently unsafe!


Is there support for my proposed campaign?
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  • 0
    This is then mandated in BS7671.

    Which bit of BS 7671 do you have in mind?

     
    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.

    About 7% failure rate has been quoted elsewhere - so your numbers sound reasonable - although failure in this context can mean things other than just failing to trip - e.g. tripping slightly more slowly or at a slightly higher threshold than permitted - which might not be enough to make the difference between life an death in the end.


    A common approach is to have two or more tiers of RCDs - e.g. a 100mA S-type incomer and 30mA individual circuit protection - thus if the 30mA fails the 100mA unit would still trip for ADS. Similarly caravan setups duplicate the 30mA RCDs - having them both on the pitch supply and as a caravan incomer. That sort of approach rapidly reduces the risk - statistics might suggest 7% is then reduced to 0.49% - although in practice the two RCDs being subject to similar conditions simultaneously might make it not quite as good as that.

     
    The obvious choice is TN-S. It does not have the drawbacks of the other systems.

    TN-S isn't entirely ideal. It still has the problem that any L-PE fault will raise the potential on the earthing system well above safe touch levels until the fault is cleared - which can sometimes take considerably longer than the 0.4s generally regarded as a safe maximum. Faults on larger or distribution circuits within an installation can take up to 5s to clear and faults on the DNO network even longer - and a raised voltage is then impose on just about everything in the system - including Class I appliances outdoors.


    TN-S with an enlarged PE conductor so that the voltage about true earth is limited (say to 50V) might be an improvement - or indeed an arrangement with a neutral earthing resistor which can keep differences in Earth potentials even lower. If we were to go to the expense of scrapping PME, perhaps we could do better than TN-S?


       - Andy.
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  • 0
    This is then mandated in BS7671.

    Which bit of BS 7671 do you have in mind?

     
    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.

    About 7% failure rate has been quoted elsewhere - so your numbers sound reasonable - although failure in this context can mean things other than just failing to trip - e.g. tripping slightly more slowly or at a slightly higher threshold than permitted - which might not be enough to make the difference between life an death in the end.


    A common approach is to have two or more tiers of RCDs - e.g. a 100mA S-type incomer and 30mA individual circuit protection - thus if the 30mA fails the 100mA unit would still trip for ADS. Similarly caravan setups duplicate the 30mA RCDs - having them both on the pitch supply and as a caravan incomer. That sort of approach rapidly reduces the risk - statistics might suggest 7% is then reduced to 0.49% - although in practice the two RCDs being subject to similar conditions simultaneously might make it not quite as good as that.

     
    The obvious choice is TN-S. It does not have the drawbacks of the other systems.

    TN-S isn't entirely ideal. It still has the problem that any L-PE fault will raise the potential on the earthing system well above safe touch levels until the fault is cleared - which can sometimes take considerably longer than the 0.4s generally regarded as a safe maximum. Faults on larger or distribution circuits within an installation can take up to 5s to clear and faults on the DNO network even longer - and a raised voltage is then impose on just about everything in the system - including Class I appliances outdoors.


    TN-S with an enlarged PE conductor so that the voltage about true earth is limited (say to 50V) might be an improvement - or indeed an arrangement with a neutral earthing resistor which can keep differences in Earth potentials even lower. If we were to go to the expense of scrapping PME, perhaps we could do better than TN-S?


       - Andy.
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