Mixed IT and TN-S Earthing Systems

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.

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.