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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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    that'll do for me.. cheers Andy
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    I fear  that having just about got a lid on the can of worms, it all gets complex again if you do this.

    chargers to retrofit to lamp posts   I bet they are not going round converting all the lamp posts to be TT earthed.


    I also fear that while you may do one or two per street, the vision of one on every lamp post overlooks a problem of loading.
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    PME'd street lighting has an additional electrode



    Which begs a question - while this seems to be entirely acceptable good practice, required by many if not all DNOs (e.g. section 5.17 in http://library.ukpowernetworks.co.uk/library/asset/f6acd931-9c79-411f-a98f-f801bce6e63L/EDS+06-0017+Customer+LV+Installation+Earthing+Design.pdf) and presumably in line with BS 7430's recommendations, why doesn't the requirement for an additional electrode for street lighting appear in BS 7671 itself (presumably in section 714) - especially as the 18th DPC seemed very keen on the approach for PME installations in general. Anybody got any theories?


       - Andy.
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    Concerning separation of different earthing systems: is it permissible to place an earth rod close to the DNO's service cable? I cannot see how they would interfere with each other except in the event of damage to the latter.
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    Chris Pearson:

    Concerning separation of different earthing systems: is it permissible to place an earth rod close to the DNO's service cable? I cannot see how they would interfere with each other except in the event of damage to the latter.




    If the service cable is sheathed, the only problem you might think you have is striking the buried service ... not always possible to accurately locate them.


    However, surge protection (and likewise EMC) might be an issue. If you have premises where surge protection is required (or specified), the TT electrode should be separated physically to avoid importing impulses from capacitive coupling in the same manner as not separating circuits. In this case, you're importing through the earth electrode, rather than the run of the cable.


    So, the answer is, it depends on the particular installation.

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    On the subject of street lighting, I was told by the head of my local authority's Street lighting dept that they only earthed the head (light fitting) and not the column.

    Things may have changed since 2008 though!
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    BS7671 gives a specification for caravan and motor homes extension leads for connecting them to the site supply sockets,  it doesn't do that for EV charging leads does it?


    Surely there will have to be legislation and guidance in BS7671 to try and ensure that excessively long leads are used and also that there are restrictions on cables being laid across doorways creating trip hazard and across roads where vehicles will drive over them as well as being a hazard for cyclists. 


    Andy Betteridge.
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    Agreed Andy. The paucity of infrastructure is a significant impediment to the roll out of electric vehicles. The government is obviously keen on the benefits but perhaps slow to legislate. We have a multi- story car park underway at the moment with space for 400 cars. There are 4 charging points! Infrastructure requirements should have been, by now, solidly embedded in building regulations and planning.
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    On the subject of street lighting, I was told by the head of my local authority's Street lighting dept that they only earthed the head (light fitting) and not the column.

    Things may have changed since 2008 though!



    There's a school of thought that that's correct - the class I head needs t be earthed, the column (if metallic) needs to be bonded. (Which ties in with PME bonding sizes reducing to 6mm² for street lighting under some circumstances.)


      - Andy.

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