Touch Voltage Calculation

Ah do we include Ze ? well it depends on the current  split between the copper path, and how much pops into the ground and back out again at the substation. Normally the terra-firma path is a far higher impedance than the metalic one, in any kind of  TN system that is working as intended, so there is not much current in the earth path and so not a significant  voltage drop. Of course in a TT system the terra-firma path is the only one we have.

Realise that most of the voltage drop in any ground path occurs where the current is obliged to be bunched up to get on and off the electrodes themselves.

If you are brave and posses a pair of well insulated wellies you can liven up an electrode and then measure the voltages between it and the ground at a range of distances from it, you will see that the rod only pulls  up the voltage in a sort of carrot shape of earth such that on the surface the pattern is one of more or less circles of constant voltage, such that more than half the voltage drop is within one rod length of the centre of the rod.

I am inclined to say that you need to have a look in the 16th edition wiring regulations for a table and other bits that have been deleted from later editions.


Andy

The other hidden assumption in a lot of shock to ground explanations is that the substation or genset neutral is not only solidly bonded to the earth (CPC) but is somehow also perfectly bonded to earth of the terra firma kind.  Now for a large substation supplying an estate of many houses,  versus a fault impedance involving long extension leads or worse a few k ohms of human flesh as the other resistor in the voltage  divider that is formed this is a reasonable assumption.

However,  this is a less safe assumption for a small genset sitting on skids perhaps with a relatively weedy rod electrode, or indeed for a pole pig transformer supply to a  row of cottages, but with a fault path to something that may be a very good electrode,  such as a steel bodied barn with foundations forming  an array of fat electrodes all bolted together.

It may be that when the voltage is shared between the ground in the vicinity of the two sets of electrodes more of the volt drop is at the origin end, and the star point neutral is actually some way from the potential of the ground (or if you prefer true ground is pulled partway towards the voltage of the faulty phase ). In such cases until ADS operates, a   problem if it is not as prompt as it could be, then touch voltages are developed on all things that are connected to the system CPC relative to terra firma ground.

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.