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Electrical Vehicle Charging Points - True earth on solid floor

Apostolos
Good afternoon.

The problem with EVC Points in a few words is that it introduces the PME (the MET in reality) voltage on the metallic car body. Then, if someone touches the car (under PME voltage) and stands on the ground touching the TRUE EARTH potential he will be introduced to a high voltage in case of an open PEN incidence. 

There are some gross guidlines ( https://www.electricvehiclechargepoint.com/ev-resources/earthing )that say

-If the EVC is outside then TT is needed (the person touches the true earth)

-If the EVC is inside and no charging happens on the outside then PME can be used (the person does not touch the true earth)

Then my question is:
WHAT IS THE POTENTIAL OF A CONRETE FLOOR IF THE EVC POINT IS INSIDE THE BUILDING????

Why do we assume that the concrete floor inside the building is on PME potential and so the EVC can be on PME?

If it is a metal building then I can see somehow that the concrete floor is in touch with the metal skeleton which is bonded to the MET so the concrete floor could be assumed to be on PME potential (correct me please if I am wrong)

If the building is concrete, how on earth this solid concrete floor is considered to be at PME potential????

 


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  • 0

    I think I can see a rational for retaining PME inside, say, a garage. It is likely that there will be lights, sockets, and maybe a garage door mechanism fed from the main distribution board in the house. Therefore their associated metalwork and that of anything connected to them will be at the same potential (save for under a fault condition) as the potential at the incomer's earth. It would be essential that the body of a vehicle on charge is at the same potential, to avoid the risk of shock potential being developed between the car body, and say a metal clad light switch or socket. However, as already pointed out, in the event of a neutral/Earth disconnect fault in the network, there is a risk of shock potentials arising between anything in the garage and "True Earth" which maybe the floor. It strikes me that if the risk of electrocution from touching a connected EV under such conditions is significant, then so to is that from touching any part of the garage's electrical installation. Clearly this cannot be the case, or all garages would have had to be made TT long ago. I think the answer to why PME is allowed indoors but not out is simply that outside regularly gets wet. Inside far less so and there is PME connected metal work around. Consequently the impedance to True Earth is likely to be very much lower outside than in. So outside, a  very rare event with very high impact; high risk of electrocution. Inside, same very rare event, but vastly lower risk of the consequences being catastrophic.

  • 0 in reply to Chrispy

    What about all of the P.M.E'd lamp columns and street furniture in the street? As Mike says, we do not see piles of dead bodies at bus shelters or piles of dead dogs at the base of street lamps.

    Z.

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  • 0 in reply to Chrispy

    What about all of the P.M.E'd lamp columns and street furniture in the street? As Mike says, we do not see piles of dead bodies at bus shelters or piles of dead dogs at the base of street lamps.

    Z.

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  • 0 in reply to Zoomup

    G12/4 Issue 1 had the answer right there for the reading ... it's related to the connected load downstream of a broken PEN conductor, larger the load, the greater the risk, as the less capable an earth electrode of given resistance is of limiting the rise in touch voltage.

    EVs are a large load.

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