Exported PME to steel floodlight columns, public tennis courts

An earth electrode should be installed and connected to the earth terminal at the point of supply. It is good practice to install and connect an earth electrode at the last or penultimate lighting column on the circuit where there are three or more columns on that circuit.

I think that comes from BS 7430 and is a precaution against broken PEN faults - which is really only going to be effective where there's a separate PME supply to the street lighting system - i.e. i broken PEN conditions the current through the electrode(s) is limited by the loads so 20Ω can pull the system down to an acceptable voltage. Where the lighting columns are supplied from a conventional installation that might have loads of tens or hundreds of amps (after the single N-PE bond) - a 20Ω electrode or two are going to provide negligible safety benefit.

   - Andy.

AJJewsbury said:

Where the lighting columns are supplied from a conventional installation that might have loads of tens or hundreds of amps (after the single N-PE bond) - a 20Ω electrode or two are going to provide negligible safety benefit.

True, but if the hundreds of amps was actually in the form of lots of floodlights each with a 20 ohm electrode, or indeed a street of houses each with an earth bond to many metres of water pipe serving as a horizontal electrode, whether or not those then meshed together to form a neutral "conductor of opportunity", then it starts to look less shaky. That is the whole PME premise, and works quite well in built up areas - and during a fault  the surface step voltages are limited by paving, and electrodes do not generally come to the surface. It is less suited to wet mud, overhead singles and small transformers whose own earth electrodes may approach that 20 ohms - it is possible in the small substation case to have a fault from live to true terra firma earth that is lower resistance than the official neutral to terra-firma electrode(s), so the neutral 'centre' of the 3 phases gets pulled significantly off earth - also not ideal but safer at the point of fault - but at the price of being more dangerous somewhere else...

There is a tendency for some to pooh-pooh TT, rather like the old ring final, as being a bit last century, and that is not really correct either.

M.

AJJewsbury said:

Where the lighting columns are supplied from a conventional installation that might have loads of tens or hundreds of amps (after the single N-PE bond) - a 20Ω electrode or two are going to provide negligible safety benefit.

True, but if the hundreds of amps was actually in the form of lots of floodlights each with a 20 ohm electrode, or indeed a street of houses each with an earth bond to many metres of water pipe serving as a horizontal electrode, whether or not those then meshed together to form a neutral "conductor of opportunity", then it starts to look less shaky. That is the whole PME premise, and works quite well in built up areas - and during a fault  the surface step voltages are limited by paving, and electrodes do not generally come to the surface. It is less suited to wet mud, overhead singles and small transformers whose own earth electrodes may approach that 20 ohms - it is possible in the small substation case to have a fault from live to true terra firma earth that is lower resistance than the official neutral to terra-firma electrode(s), so the neutral 'centre' of the 3 phases gets pulled significantly off earth - also not ideal but safer at the point of fault - but at the price of being more dangerous somewhere else...

There is a tendency for some to pooh-pooh TT, rather like the old ring final, as being a bit last century, and that is not really correct either.

M.