Site cabin EICR

What is lowest acceptable resistance we are looking for when conducting an earth electrode test.

Presuming the generator is the source of a local TN-S system, then (from memory) I think BS 7430 suggests around 20Ω.


There doesn't appear to be much maths behind that figure though - and it's the same as DNOs use for MVA systems. So perhaps it's not the end of the world if it's somewhat higher for a ditty generator.


For conventional ADS (fault L-c.p.c) the electrode won't make any difference since the earth fault loop is all metallic on a TN-S system and doesn't involve the electrode. Ditto for additional protection within the unit (presuming extraneous-conductive-parts are bonded to the MET as normal). When it comes to additional protection from direct contact where the victim is in contact with true earth (e.g. where a circuit extends outside of the unit) a high resistance electrode cuts two ways - it'll reduce the fault/shock current which might prevent a protective device from operating, but it'll also limit the current through the victim and voltage across the victim - in the extreme case where the electrode had an almost infinite resistance the system almost becomes a separated system and the shock current becomes negligible. As usual it's the middle cases that are the most troublesome.


Section 717's diagrams make some interesting reading for the various different source arrangements - often 30mA RCD protection is only required for sockets supplying things outside of the unit (not ignoring the general requirements for sockets <=32A to have additional protection).


  - Andy.

What is lowest acceptable resistance we are looking for when conducting an earth electrode test.

Presuming the generator is the source of a local TN-S system, then (from memory) I think BS 7430 suggests around 20Ω.


There doesn't appear to be much maths behind that figure though - and it's the same as DNOs use for MVA systems. So perhaps it's not the end of the world if it's somewhat higher for a ditty generator.


For conventional ADS (fault L-c.p.c) the electrode won't make any difference since the earth fault loop is all metallic on a TN-S system and doesn't involve the electrode. Ditto for additional protection within the unit (presuming extraneous-conductive-parts are bonded to the MET as normal). When it comes to additional protection from direct contact where the victim is in contact with true earth (e.g. where a circuit extends outside of the unit) a high resistance electrode cuts two ways - it'll reduce the fault/shock current which might prevent a protective device from operating, but it'll also limit the current through the victim and voltage across the victim - in the extreme case where the electrode had an almost infinite resistance the system almost becomes a separated system and the shock current becomes negligible. As usual it's the middle cases that are the most troublesome.


Section 717's diagrams make some interesting reading for the various different source arrangements - often 30mA RCD protection is only required for sockets supplying things outside of the unit (not ignoring the general requirements for sockets <=32A to have additional protection).


  - Andy.