Electrical Vehicle Charging Points - True earth on solid floor

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????

 


  • Rather depends on if there is a damp proof membrane or a tar layer in or under  the cement - true for a house, probably not a  garage, unless the floor is painted.

    Some but not all buildings will have steel re-bar in the cement, and maybe that is earthed or maybe not.

    It also depends if the cement is wet as to how well it conducts - in that sense a garage or even a car port is a better place than a driveway.

  • mapj1 thank you very much because you have given me in the past a really good answer on another EVC question I had posted in the previous IET forum.

    I just remembered it now ?

    To the subject now, lets assume that this is a car park (to make it more realistic) with several EVC points perimetrically.

    Theoritically according to what I have read online the EVCs should be on PME.

    The building is a metal building which is bonded on the PME as regular practise.

    Can I assume that the concrete floor being in contact with the droping metal columns is at the same potential (i.e. PME)?

    Even if we assume that there is some rebar in the cement how sure can we be that this rebar is in direct contact with the metal structure?

    I think you can see why I am a bit reluctant to use PME on the EVC points although it is inside the building.

    ...........

    I guess the problem becomes even bigger in case of a  concrete building because I do not see how the rebar in the columns and in the floor will be bonded to the MET...

    ..........

  • Thirty years ago the old Midlands Electricity board application for a new supply forms listed examples of when a PME earth terminal would not be install by them, leaving you to install a earth electrode.


    Quarry tile floors were listed as in older houses the tiles are simply laid on sand over soil, dairy's were on the list having permanently wet floors and so on.


    As Mike said the floor needs to be dry concrete or the like over a damp proof membrane.


    Andy Betteridge.

  • Apostolos Kasinalis:


    I think you can see why I am a bit reluctant to use PME on the EVC points although it is inside the building.


     




    OK - if you decide to go for a TT supply to the EVC's how do you segregate the TT electrode from the earthy frame of the building which has PME for all the other services.


    Consider if it would be safer making arrangements to connect the rebar under the vehicle areas to the MET and retaining PME to supply the EVC  - what potentials would exist between the vehicle body and the floor slab that a person might actually bridge between hand and foot in either case


    Regards


    OMS


     

  • OMS, silly questions....

    The shoes that are not conductors cannot prevent the current going through the human body into the earth?

    Or they are too flimsy to prevent the current running through them

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



    Inside buildings it's no just an EVSE issue - there's exactly the same issue with any class I appliance, luminaire, or wiring system. On the bright side a few decades of experience with such things (including washing machines and freezers in garages) suggest the actual risk isn't that great.

     

    The shoes that are not conductors cannot prevent the current going through the human body into the earth?

    Or they are too flimsy to prevent the current running through them



    Footwear is indeed usually a good enough insulator to prevent a shock - at least hand-to-foot or similar and when the footwear is dry. Absence of footwear is one of the factors (along with wet skin) that often demands more precautions than normal - e.g. in bathrooms or swimming pools. On the other hand we can't really assume that footwear will always be worn outdoors - especially in domestic situations (think kids on a hot day playing with a paddling pool and hosepipe on the lawn next to the car on the drive) - or that footwear will always be effective - e.g. if kneeling down to unload a washing machine or examine something under a car.


      - Andy.
  • It’s not unusual get an electric shock from a car with an internal combustion engine when you touch it due to static, indeed it can apparently damage hearing aids particularly those of children, according to some people healthy adults should feel the pain and take the shock to protect children and their hearing aids.


    The car charges up then discharges when touched by someone through their body and their shoes.


    So unless everyone is going to wear good quality wellies or the like I should not rely on footwear for protection from electric shocks from vehicles.


     Andy Betteridge

  • It’s not unusual get an electric shock from a car with an internal combustion engine when you touch it due to static



    That certainly used to be the case - I remember many a static shock from the car door handle as a child.  I'm not sure it's as prevalent as it used to be though - I beleive that modern vehicle tyres are designed to be slightly conductive specifically to dissipate static.


    I'm not sure insulating footwear would help much with static shocks - a small current can flow from a charged body to an uncharged one even if it's not to earth.


      - Andy.

  • Apostolos Kasinalis:

    OMS, silly questions....

    The shoes that are not conductors cannot prevent the current going through the human body into the earth?

    Or they are too flimsy to prevent the current running through them




     

    Shoes are quite effective insulators - we make a demand on them when we ascertain step and touch voltages in such places as HV substations - often to several kV


    You couldn't rely on them in an uncontrolled situation, though


    My comments were based on you thinking about the implications of having two methods of earthing with quite probably simultaneously accessible conductive parts - BS 7671 tends to not like that situation at all, for perhaps not so obvious reasons - you will have wildly different fault levels on the PME system compared to the TT system - so which do you design for ?. Could you actually have voltages impressed that exceed Low Voltage in some situations. Unless you can absolutely provide something along the lines of 3m of separation then relying on TT earthing inside a PME bonded installation could be very nasty for some individual.


    PME is generally less problematic than it's currently made out to be, and in general, trying to separate it out within a single enclosed installation is neither easy nor likely to solve the perceived problem


    Regards


    OMS


  • OMS:




    Apostolos Kasinalis:


    I think you can see why I am a bit reluctant to use PME on the EVC points although it is inside the building.


     




    OK - if you decide to go for a TT supply to the EVC's how do you segregate the TT electrode from the earthy frame of the building which has PME for all the other services.


    Consider if it would be safer making arrangements to connect the rebar under the vehicle areas to the MET and retaining PME to supply the EVC  - what potentials would exist between the vehicle body and the floor slab that a person might actually bridge between hand and foot in either case


    Regards


    OMS


     


     




    If a driver drops their car keys onto the concrete floor and reaches to retrieve them, they may be in contact with the metal keys on the floor with one hand, and the car as they steady themselves with the other bare hand.


    Z.