www.mirror.co.uk/.../man-died-after-being-electrocuted-20159577

Patrick Hammond said:

Because of this, on a new installations we would not usually export the PME outside the equipotential zone

"export"! Why export? If, as is often the case nowadays, there are no extraneous conductive parts, there is no equipotential zone.

TT-ing the lamp posts is not without risk. It depends upon satisfactory long-term operation of an RCD, which is probably less likely than a lost neutral.

When looking for justification for adding earth electrodes should this document be considered?

2.5.3 Lighting authority/asset owner private cable distribution system
A lighting authority/asset owner may design and install their own cable networks but these usually consist of a separate neutral and earth (TN-S), which is connected to a PME point of supply.
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.

www.theiet.org/.../guide-to-highway-electrical-street-furniture.pdf

It is not quite fair to say that the regs do not allow us to consider the possibility of an NE offset voltage and to do so is in someway wrong. It is only true that we are not obliged to consider it, but that is not the same at all - there may be local conditions that push toward TT islanding.

And as an aside if you are building a milking parlour or an outdoor swimming pool then in many ways the advice in the regs does oblige us to consider touch voltages and the conclusion to that deliberation is that farms in general that are not private transformer and pure TNS, are instead TT, but almost never TNC-s.

So if the lamp post was in the middle of a the yard of a dairy farm, even if the supply is a sub-main from a PME supplied  living area you may well decide it should be TT.

If however it is more like the city street light and in the middle of a tarmac pavement  of nebulous contact with the terra-firma below, then it may as well be PME and the faster fuses only ADS time is an advantage.

The question is not stupid, and the regs are not the last word in thinking.

We do not all live in urban town and city centres, and one rule does not fit all. (and in my local south English market town we have rows of houses that are all TT as well, fed from bare overhead singles - you'd certainly not want to PME from that - a tree branch bringing down the lower wire only is a very credible fault indeed.)

Mike

I should point out that the IET Guide to Highways Electrical Street Furniture was published in 2018, so predates the current edition of BS7671 and the amendments to the regulations need to be taken into consideration.

Also the additional earthing electrodes are “good practice” so may not be considered an absolute requirement.

I've been on and off the forum since I trained in 2010 and you were, and still, are a really useful contributor to this forum Andy, always polite and give briliant, easy to understand input. Thanks

chilling!

www.mirror.co.uk/.../man-died-after-being-electrocuted-20159577

Doesn't mean that RCDs are a requirement of the regulations though.  In that particular case we don't know the cause or whether an RCD would have helped at all - it might have been the result of a grumbling PEN fault (like that school incident that make the school doors and fencing live), or if the installation had been properly maintained that ordinary ADS would have done the job anyway.

   - Andy,

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 

the extract reads as it is good practice to install the additional earth rods for a TN-S lighting circuit being supplied from a TN-C-S supply, if and how you write a recommendation into an EICR is a matter for debate as there’s a “should” and a “good practice” mentioned.

”2.5.3 Lighting authority/asset owner private cable distribution system
A lighting authority/asset owner may design and install their own cable networks but these usually consist of a separate neutral and earth (TN-S), which is connected to a PME point of supply.
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.”