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
  • Locked Locked
  • Replies 6 replies
  • Subscribers 32 subscribers
  • Views 2518 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

Earth rod resistance

AlunW
Hi

Hoping someone can point me in the right direction!

On site we are trying to review our existing distribution earthing arrangements.  We have 11kV or 6.6kV distributed to local substations where we drop to 415v locally.  We generally have PNB systems with common HV equipment and LV neutral earth arrangements.  Typical one shown below
ebe497219bf639b0954936dc05b950cf-huge-substation-earthing-arrangements.png

I have reviewed several 'local authority' documents which suggest a maximum of 20ohms for earth rod resistance, but looking through BS EN 7430 it suggests on a similar arrangement in A.2 a maximum of 1 ohms?  Could anyone kindly advise what it should be? or point me in the right direction?

Many thanks

Al
Parents
  • Former Community Member
    0 Former Community Member
    HV feeders won't usually have high earth fault currents as these are limited by the earthing arrangements at the sending substation via the neutral earthing arrangements (earthing transformer, Liquid resistor, air cooled resistor etc or by using a resonant circuit (ie peterson coils)


    As an example, with a 6.4 Ohm liquid or air cooled resistor earthing the undistributed 11kV system neutral, the earth fault will not exceed 1000A


    If the HV is fully cabled, with earthed screens or armours, then it's almost certain the site is cold, as you are solidly connected to a "global" earthing system


    All of that said, you still need to calculate the RoEP and contrast that with the speed of disconnection  - and then calculate the step potentials based on the assumed male body weight, boots and a layer of something vaguely insulating like gravel - something in the order of 2kV might be expected, but usually we are under 430V or 630V RoEP dominated by the HV protection speed. 


    For the OP - noted the balanced EF rather than REF - it just looked odd on a first pass


    Regards


    OMS
Reply
  • Former Community Member
    0 Former Community Member
    HV feeders won't usually have high earth fault currents as these are limited by the earthing arrangements at the sending substation via the neutral earthing arrangements (earthing transformer, Liquid resistor, air cooled resistor etc or by using a resonant circuit (ie peterson coils)


    As an example, with a 6.4 Ohm liquid or air cooled resistor earthing the undistributed 11kV system neutral, the earth fault will not exceed 1000A


    If the HV is fully cabled, with earthed screens or armours, then it's almost certain the site is cold, as you are solidly connected to a "global" earthing system


    All of that said, you still need to calculate the RoEP and contrast that with the speed of disconnection  - and then calculate the step potentials based on the assumed male body weight, boots and a layer of something vaguely insulating like gravel - something in the order of 2kV might be expected, but usually we are under 430V or 630V RoEP dominated by the HV protection speed. 


    For the OP - noted the balanced EF rather than REF - it just looked odd on a first pass


    Regards


    OMS
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