PE advice by Napit (hot tubs)

Perhaps it is referring to grids, but then a figure of 20 ohms is a bit arbitrary and irrelevant, and for step voltages arguably a higher electrode resistance  is better as there is less gradient as there is less current flow. The floor above the grid is then closer to CPC voltage. It might be better if the intention was spelt out.

In extremis you could create a very good zero step voltage zone by laying down a sheet of polyethylene with aluminum foil over it, and connect that to the CPC. As an electrode connecting to terra-firma, it would be less use than the proverbial chocolate teapot, but in terms of keeping both feet at the same voltage as the supply CPC, about as good as it gets.  Until you step off it of course, for that you need to cut the plastic sheet bigger than the foil by a metre or two extra  all round.

There is another issue, not unique to PME if your local electrodes are too good, if the ones that the DNO use are comparable or higher resistance than yours. Unlikely in a city centre with many electrodes on the network, but a concern for those of us fed by pole transformer  and overhead singles to a small group of houses.

If a fault grounds the live conductor to terra-firma very well,  say with an electrode resistance of a few ohms, but the DNO neutral/cpc resistance to terra-firma earth is higher, maybe between 10 ohms and that upper limit 20 ohm figure, then 10-20A odd may flow from live to fault through earth, and back up the DNO electrodes to the transformer neutral. But this means the total 230V voltage drop is split across the two sets of electrode resistance - with as much or more across the DNOs than the users.  Now the DNO neutral and any CPC derived from it is at a dangerous voltage to terra-firma. If the fault to ground is  on a high current non RCD circuit maybe it would not be spotted for a while.

Very low electrode resistances are not always good, nor is it always safe to omit the RCD and rely on a fuse or MCB  if you have them.

Mike

Perhaps it is referring to grids, but then a figure of 20 ohms is a bit arbitrary and irrelevant, and for step voltages arguably a higher electrode resistance  is better as there is less gradient as there is less current flow. The floor above the grid is then closer to CPC voltage. It might be better if the intention was spelt out.

In extremis you could create a very good zero step voltage zone by laying down a sheet of polyethylene with aluminum foil over it, and connect that to the CPC. As an electrode connecting to terra-firma, it would be less use than the proverbial chocolate teapot, but in terms of keeping both feet at the same voltage as the supply CPC, about as good as it gets.  Until you step off it of course, for that you need to cut the plastic sheet bigger than the foil by a metre or two extra  all round.

There is another issue, not unique to PME if your local electrodes are too good, if the ones that the DNO use are comparable or higher resistance than yours. Unlikely in a city centre with many electrodes on the network, but a concern for those of us fed by pole transformer  and overhead singles to a small group of houses.

If a fault grounds the live conductor to terra-firma very well,  say with an electrode resistance of a few ohms, but the DNO neutral/cpc resistance to terra-firma earth is higher, maybe between 10 ohms and that upper limit 20 ohm figure, then 10-20A odd may flow from live to fault through earth, and back up the DNO electrodes to the transformer neutral. But this means the total 230V voltage drop is split across the two sets of electrode resistance - with as much or more across the DNOs than the users.  Now the DNO neutral and any CPC derived from it is at a dangerous voltage to terra-firma. If the fault to ground is  on a high current non RCD circuit maybe it would not be spotted for a while.

Very low electrode resistances are not always good, nor is it always safe to omit the RCD and rely on a fuse or MCB  if you have them.

Mike