Touch Voltage Calculation

My point is that although the touch voltage is reduced, the additional impedance between the MET and the cut out in reality will be negligible as demonstrated above and that appears to be the only diffirence in calculation.


I see the reason why on a TT system, where the impedance of the electrode will be much higher but for other systems is it necessairy?

You're quite right that the benefits of main bonding (especially in TN system) can be quite small - and don't provide any reliable specific level of shock protection during earth faults within the installation (just a general 'probably a bit better than things would have been without it) - so if that was the only consideration there would indeed be a case for dropping main bonding from TN installations.  Main bonding does provide some protection from faults originating from outside the installation though - whether on the DNO network or other installations. Faults on the DNO network can be especially tricky as they don't follow BS 7671 rules so we can't assume things like disconnection within 5s. Broken CNE conductors are certainly one consideration but there are also things lke L-PE faults (and L-N faults on PME systems) which can raise the voltage on the incoming earth connection to easily half of the line voltage, sometimes more, for a considerable time. So main bonding on TN systems gives some (significant) shock protection from faults outside the installation. Main bonding also provides protection for other protective conductors (e.g. small c.p.c. to class 1 equipment in contact with extraneous-conductive-parts and any supplementary bonding conductors) from having to carry the large and/or long duration currents that main bonding is sized for - whether diverted N currents or fault currents flowing through parallel paths - so providing some fire as well as shock protection.

   - Andy.

My point is that although the touch voltage is reduced, the additional impedance between the MET and the cut out in reality will be negligible as demonstrated above and that appears to be the only diffirence in calculation.


I see the reason why on a TT system, where the impedance of the electrode will be much higher but for other systems is it necessairy?

You're quite right that the benefits of main bonding (especially in TN system) can be quite small - and don't provide any reliable specific level of shock protection during earth faults within the installation (just a general 'probably a bit better than things would have been without it) - so if that was the only consideration there would indeed be a case for dropping main bonding from TN installations.  Main bonding does provide some protection from faults originating from outside the installation though - whether on the DNO network or other installations. Faults on the DNO network can be especially tricky as they don't follow BS 7671 rules so we can't assume things like disconnection within 5s. Broken CNE conductors are certainly one consideration but there are also things lke L-PE faults (and L-N faults on PME systems) which can raise the voltage on the incoming earth connection to easily half of the line voltage, sometimes more, for a considerable time. So main bonding on TN systems gives some (significant) shock protection from faults outside the installation. Main bonding also provides protection for other protective conductors (e.g. small c.p.c. to class 1 equipment in contact with extraneous-conductive-parts and any supplementary bonding conductors) from having to carry the large and/or long duration currents that main bonding is sized for - whether diverted N currents or fault currents flowing through parallel paths - so providing some fire as well as shock protection.

   - Andy.