Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • +1 person also asked this people also asked this
  • Locked Locked
  • Replies 31 replies
  • Subscribers 33 subscribers
  • Views 3115 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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????

 


Parents
  • 0

    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.



    But concrete (being non-metallic) has a relatively high resistance - hence you need contact with a reasonably large surface area to get a low enough resistance to allow enough current to flow to get a hazardous shock - a bare foot with perhaps several tens cm² of contact with the concrete would be far more dangerous than a bunch of keys with perhaps only a couple of mm² contact.


    In some ways I agree with OMS's assessment that the PME situation probably isn't as dire as trying to apply normal BS 7671 rules on simultaneous contact might suggest - we've had earthy metalwork (e.g. barrier fencing) within reach of PME'd steel lampposts, all outdoors, with no bonding, for decades (as per reg 714.411.3.1.2) with little apparent problem.


      - Andy.
Reply
  • 0

    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.



    But concrete (being non-metallic) has a relatively high resistance - hence you need contact with a reasonably large surface area to get a low enough resistance to allow enough current to flow to get a hazardous shock - a bare foot with perhaps several tens cm² of contact with the concrete would be far more dangerous than a bunch of keys with perhaps only a couple of mm² contact.


    In some ways I agree with OMS's assessment that the PME situation probably isn't as dire as trying to apply normal BS 7671 rules on simultaneous contact might suggest - we've had earthy metalwork (e.g. barrier fencing) within reach of PME'd steel lampposts, all outdoors, with no bonding, for decades (as per reg 714.411.3.1.2) with little apparent problem.


      - Andy.
Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines