Mixed IT and TN-S Earthing Systems

411.3.1.1 does say ..."Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system..."


But it sounds to me if you have control of the source of the TN-S system as well as what's conventionally the consumer's side you'd could have a "T" that's common to both the TN-S system and the -T part of the IT system.


   - Andy.

Spot on Andy,


The requirement is for simultaneously-accessible exposed-conductive-parts to be connected to the same earthing system NOT the same system earthing arrangement.


In this case, the earthing arrangement of the IT system earth and the TN-S system earth is the MET, so no worries. In medical locations, the protective conductor in a Medical IT system is bonded, perhaps numerous times, to the TN system earthing arrangement to comply with Section 710 requirements for supplementary local equipotential bonding to achieve very low resistance and very low touch voltage in the event of a fault.


The problem comes when the two systems have different earthing arrangements. This depends on the supply conditions e.g.

• 2 separate distributor's earthing terminals can't be bonded together (542.1.3.3)

• Two TT's or TN and TT can't always be bonded together for the same reason, especially if you don't own both installations

• TN earthing terminal could be bonded to TT if you own both, but if the purpose for separating earthing arrangements is to comply with a requirement in Part 7, say prohibition of PME on caravan and camping site pitches, you wouldn't want to - in that case you'd maintain physical separation so exposed-conductive-parts connected to different earthing arrangements are not simultaneously-accessible.

Hi Gents, 

Thanks for your responses - much appreciated.

Are we saying that there is a distinction to be made between “system earthing”, “earthing system” and earthing arrangement? None of the terms are defined in 7671but the first and the last appear to be the same in that the former is  used in the first paragraph of 411.3.1.1 which refers to 411.4 to 411.6 all of which are earthing arrangements described as such in the Electrical Installation Certificate or Condition Report in Appendix 6. 

To make a distinction between “system earthing” and “earthing system” without a clear definition would be errant. May I respectfully suggest that the second paragraph of 411.3.1.1 should find a form of words that would better reflect the intention.

I guess "system earthing" (TN, TT etc) is explained (if not exactly defined in 312.2) (i.e. how the power system is earthed) while "earthing system" seems to be used consistently to mean the systems of conductors that connect exposed- and extraneous-conductive-parts to Earth. I agree some definitions would help to make it clearer though.


Having two different systems with a common Earthing arrangement isn't uncommon - e.g. on construction sites we'd likely have a RLV transformer arranged as a TN-S system on its secondary, but it's means of Earthing will usually be the c.p.c. of the 230/400V primary circuit - which is likely part of a TT system. Ditto with some PELV systems.


  - Andy.