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Found this on another forum while researching MicraMary's issue:
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- Thread starter securespark
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I'm not sure that I can make too much sense of all that, can you?
Kind Regards, John
He wants to turn PME into PM+1E.
Sure, but PME and P(M+1)E are the same thing - 'M' can have any value.
However, it's the 'logic' behind what was being suggested that I found difficult to follow - you?
Kind Regards, John
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I'm glad that it's not just me!
as long as it is >1...
It must be getting near the weekend!!Kind Regards, John
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Beer here we come!
Enjoy! I'm just about to open the first bottle (of wine)Beer here we come!
Cheers, John
Was it written by bernard green?
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Sorry to disappoint, guys, but it's neither of those two!!
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I can see a little of what he is getting at. Where TN-C-S is PME there is not too much of a problem. Although lost of neutral can cause the voltage to rise and fall the amount it can rise and fall is limited by the path through the ground and also the same with the voltage gradient as long as there are enough earth rods the voltage gradient is limited.
It is where the TN-C-S is lacking connection with the earth where voltages can vary along the whole 0 to 400 volt range. Also where the connection is not good enough to earth the currents involved can cause the connection to fuse where this connection is for example a gas pipe with some really dramatic results.
Where a group of houses all have gas and water bonded then loss of the neutral is shared between houses but where this is not the case and just one house has extraneous-conductive-parts (not called an earth rod when a TN-C-S system) then all the current can try to return to the sub-station through this one connection.
I saw the results of this where a radio amateur had a supply put to his shed and the electrician had bonded the radio earth (4 rods linked with tape around his garden) to the TN-C-S earth using a 4mm sq earth cable which ended up as copper balls on the ground when a workman's digger hit the cable.
Had the supplier used PME there would not have been a problem but after the fault site there were no earth connections to the system.
However I am assured this is unusual and very unlikely to be repeated.
As to the milliamp current required to trip the RCD I just can't follow his reasoning. With a TT system neutral and earth are likely at a different potential so earth - neutral faults are more likely to trip a RCD but where the 60ma comes from I am at a loss.
He fails to mention the problem with erosion I have seen a whole estate where the bottom of each down spout has been damaged in this way.
Also with a TT system there can be a different potential on each property so where houses are very close touching two down spouts could give one a shock under fault conditions. The same where TT and TN-C-S are next to each other and I question having electric cars and caravans parked next to a house with a different earthing system.
We are only required to have 200 ohms (Note 2) where we have RCD protection and we totally rely on the RCD tripping. If for any reason it does not trip the voltages could be quite large. To ensure a 100A supply would not lift the voltage above 50 volt would need 0.5 ohm which one is unlikely to get even just 13A needs 3.8 ohm and when I had to install a series of earth rods I needed between 3 and 8 rods at 1.2 meters to get 8 ohms so for domestic to get 50 ohms we are doing well.
So the only practical way to remove the problem with premises which are close to each other not having a lethal voltage between them is to bond the premises together which is what happens with the TN system.
Reach is shown as 1.25 meters (fig 417) so at 400 volt max between houses and 50 volt accepted as extra low voltage then premises should be 10 meters apart if not having same earth to ensure no more than 50 volt and with 240 volt i.e. garden shed looking at 6 meters. OK unlikely to have full 400 or 240 volt so maybe could reduce that a little but I am sure you can see my point.
To conclude both TN-C-S and TT have problems so unless we went to TN-S then it is down to risk assessment and to my mind the risk with TN-C-S is less than we TT so where we can TN-C-S would seem the answer.
However I am looking at a standard house. Mobile homes and like where there is a large amount of bonded metal work exposed to touch outside would come out completely different with a risk assessment.
I note with caravans and boats TN-C-S is not allowed and will also have to double up on RCD's so two need to fail however how one can test the caravan RCD I don't know as to test one would have to stop the site RCD tripping. So a non RCD protected socket would be required to test caravans and boats. May be the built in test button is enough when two RCD's are in series?
It is where the TN-C-S is lacking connection with the earth where voltages can vary along the whole 0 to 400 volt range. Also where the connection is not good enough to earth the currents involved can cause the connection to fuse where this connection is for example a gas pipe with some really dramatic results.
Where a group of houses all have gas and water bonded then loss of the neutral is shared between houses but where this is not the case and just one house has extraneous-conductive-parts (not called an earth rod when a TN-C-S system) then all the current can try to return to the sub-station through this one connection.
I saw the results of this where a radio amateur had a supply put to his shed and the electrician had bonded the radio earth (4 rods linked with tape around his garden) to the TN-C-S earth using a 4mm sq earth cable which ended up as copper balls on the ground when a workman's digger hit the cable.
Had the supplier used PME there would not have been a problem but after the fault site there were no earth connections to the system.
However I am assured this is unusual and very unlikely to be repeated.
As to the milliamp current required to trip the RCD I just can't follow his reasoning. With a TT system neutral and earth are likely at a different potential so earth - neutral faults are more likely to trip a RCD but where the 60ma comes from I am at a loss.
He fails to mention the problem with erosion I have seen a whole estate where the bottom of each down spout has been damaged in this way.
Also with a TT system there can be a different potential on each property so where houses are very close touching two down spouts could give one a shock under fault conditions. The same where TT and TN-C-S are next to each other and I question having electric cars and caravans parked next to a house with a different earthing system.
We are only required to have 200 ohms (Note 2) where we have RCD protection and we totally rely on the RCD tripping. If for any reason it does not trip the voltages could be quite large. To ensure a 100A supply would not lift the voltage above 50 volt would need 0.5 ohm which one is unlikely to get even just 13A needs 3.8 ohm and when I had to install a series of earth rods I needed between 3 and 8 rods at 1.2 meters to get 8 ohms so for domestic to get 50 ohms we are doing well.
So the only practical way to remove the problem with premises which are close to each other not having a lethal voltage between them is to bond the premises together which is what happens with the TN system.
Reach is shown as 1.25 meters (fig 417) so at 400 volt max between houses and 50 volt accepted as extra low voltage then premises should be 10 meters apart if not having same earth to ensure no more than 50 volt and with 240 volt i.e. garden shed looking at 6 meters. OK unlikely to have full 400 or 240 volt so maybe could reduce that a little but I am sure you can see my point.
To conclude both TN-C-S and TT have problems so unless we went to TN-S then it is down to risk assessment and to my mind the risk with TN-C-S is less than we TT so where we can TN-C-S would seem the answer.
However I am looking at a standard house. Mobile homes and like where there is a large amount of bonded metal work exposed to touch outside would come out completely different with a risk assessment.
I note with caravans and boats TN-C-S is not allowed and will also have to double up on RCD's so two need to fail however how one can test the caravan RCD I don't know as to test one would have to stop the site RCD tripping. So a non RCD protected socket would be required to test caravans and boats. May be the built in test button is enough when two RCD's are in series?
I think those who have concerns are contemplating the very rare scenario of the location of the neutral fault being such that, as seen from the viewpoint of the consumers affected, the TN-C-S supply is no longer (appreciably or at all) "PMEd".
Kind Regards, John
Fair enough, if you want to have it that way, he wants to take his PME supply and increment the "value" of M by 1.
Yes, that's another way of putting it - but to what end (the exercise, not the wording!)? Sure, it means that one will still have 'an earth' if a supply fault is such that one is cut off from all the supply's M earths (left with just one's additional one) - but in the 'bad case' scenario/location of a supply fault that some people seem to fear, that is going to make precious little difference to anything.
Kind Regards, John
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