Street lights on PME?

[Tart83]

After conducting a recent EICR I found among other things that the Galvanised columns in the carpark area of this premises seem to be connected directly to the TNCS system. Keeping in mind the potential issues should a PEN fault occur it struck me that this was maybe not the best idea. However Googling answers on other forums the main concern seemed to be the size of the earth for the bonding of these street lights, not the idea of them being connected to PME in the first place. The IET issues some sort of guidance regarding the sizing of such earth conductors, but my natural tendency would be that these would be best on TT, does anyone have any experience/advice on this?

 

[Dave OCD]

I used to work as an Electrical Inspector for my local Council, mostly overseeing a large street lighting maintenance contract. The vast majority of streetlighting is supplied via DNO supplies here and were all PME/ TNCS. But a 10mm main bonding conductor was always connected to the MET bolt just inside the door opening.

 

[LastManOnline]

However Googling answers on other forums the main concern seemed to be the size of the earth for the bonding of these street lights..

Why do the street lights need to be bonded?

 

[LastManOnline]

But a 10mm main bonding conductor was always connected to the MET bolt just inside the door opening.

I,m curious as to why the street lights require bonding?

 

[ipf]

I used to work as an Electrical Inspector for my local Council, mostly overseeing a large street lighting maintenance contract. The vast majority of streetlighting is supplied via DNO supplies here and were all PME/ TNCS. But a 10mm main bonding conductor was always connected to the MET bolt just inside the door opening.

Sorry mate, I'm a bit baffled as to where the 10mm bond picks up..... just from incoming earth or 10mm back to source, an actual bond?

 

[Dave OCD]

It would be a 10mm straight from the link in the DNO cutout [smaller than a domestic setup] to a large brass bolt on the inside of the column. This is the installation MET so the CPC of the cable feeding the lantern/s would also connect to this with an appropriate lug, I always thought it a bit of a messy arrangement. But the 'bond' is there to negate any difference in potential between the DNO earth and column in the ground under fault conditions ?

 

[Dave OCD]

Sorry mate, I'm a bit baffled as to where the 10mm bond picks up..... just from incoming earth or 10mm back to source, an actual bond?

The incoming supply is a DNO one although often looped from the incoming side of the cutout between the lights along a road. So each supply point is effectively the source mate.

 

[ipf]

Aye, remember them now. Each column has it's own cut out.
Not 10mm main earth, though.....then again, it's just a lamp post and the worst scenario could be dead dogs after having a pee....

 

[Dave OCD]

Generally a 16mm concentric looped supply so a 10mm earth was the spec.

 

[Dave OCD]

The other thing that surprised me when I started the job was T&E being the cable of choice for the circuit , I just assumed it would be flex ? 1.5mm for columns up to 6 metre and 2.5 for anything above for 'mechanical strength'.

 

[nicebutdim]

The other thing that surprised me when I started the job was T&E being the cable of choice for the circuit , I just assumed it would be flex ? 1.5mm for columns up to 6 metre and 2.5 for anything above for 'mechanical strength'.

That is strange.