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PME vs TT for charger

garywwess
Can someone please explain in common scence English (this has to be explained to a customer) ..  why it is permissible for a street light , which in itself is basically a big metal earth rod buried 5 feet in the ground.. is allowed to be PME`d  (customer used to install street lights) yet his charger has to be TT`d  and isolated from the PME at all costs..  his parking space has a street lamp smack in the middle of it ???  I neither agree or disagree with the instructions to do so… just need something in writing to show the customer to prove im not just generating extra income needlessly..

Gary
 
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    One of the arguments is that PME'd street lighting has an additional electrode that, should the CNE to the light become open circuit, keeps the neutral/earth at a safe voltage (usually below 50V).


    We can't practically do the same with EVSE because of the much larger load (typically not only of the EVSE itself but the entire installation that shares the same CNE). Whereas a quarter amp (say) street lamp could be held down to 50V with an around 200 Ohm electrode, you'd need something closer to a 0.5 Ohm electrode for a 100A supply. (Slight approximation as I've ignored the impedance of the loads).


    It's not a 100% water tight argument for allowing PME'd lampposts - if the break in the CNE for instance was upstream of several large installations as well as the the lamppost, the lamppost's electrode would be far less effective - but on the other hand lampposts tend to be either concrete (not as good a conductor as metal) or painted steel - so there's a degree of protection to anyone touching it, also people don't tend to go around touching lampposts very often - unlike a car - so again the chances of anyone coming to harm are reduced for the lamppost.


       - Andy.
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    One of the arguments is that PME'd street lighting has an additional electrode that, should the CNE to the light become open circuit, keeps the neutral/earth at a safe voltage (usually below 50V).


    We can't practically do the same with EVSE because of the much larger load (typically not only of the EVSE itself but the entire installation that shares the same CNE). Whereas a quarter amp (say) street lamp could be held down to 50V with an around 200 Ohm electrode, you'd need something closer to a 0.5 Ohm electrode for a 100A supply. (Slight approximation as I've ignored the impedance of the loads).


    It's not a 100% water tight argument for allowing PME'd lampposts - if the break in the CNE for instance was upstream of several large installations as well as the the lamppost, the lamppost's electrode would be far less effective - but on the other hand lampposts tend to be either concrete (not as good a conductor as metal) or painted steel - so there's a degree of protection to anyone touching it, also people don't tend to go around touching lampposts very often - unlike a car - so again the chances of anyone coming to harm are reduced for the lamppost.


       - Andy.
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