Street Light Question

[the doctor]

Hello All,

I am at college teaching street lighting apprentices. Going through the curriculum Thursday's lesson was

describe the term Protective

Multiple Earthing (PME )

describe the term Separate

Neutral Earthing (SNE / TNS) 

That was fine though I have never heard the term SNE.  Anyway as a man who has never worked on streetlighting I asked the question

                     ' Can you have a TT streetlight?'

Some seem to think they have installed various TT street furniature in rural settings with an RCD and an earth electrode.  That seems fair enough to me.

Anyway fast forward to this morning on my bicycle cycling out for breakfast and it suddenly dawned on me, and this is my question to the forum and PLEASE do not laugh.

If the street furniture item   was metal AND its value could be proved, why not use the lampost as the electrode and save the money on the electrode?

Would it comply? ( Have no regs book close at hand, it is packed in a box in Ireland....somewhere)

Best wishes 

Alan

 

[NozSpark]

So long as you have the owners permission then I can't see a problem,,, like you, I'd have to check my regs book to be certain

 

[Marvo]

By using the actual lamppost as the earth rod the lampost itself would become part of the supply rather than just an appliance receiving power. I don't know if it is permitted in your local regs but I can think of 2 possible reasons it might not be a good idea, firstly if the lamp post corrodes severely below ground level it would affect it's Ra impedance, secondly actually using the lamppost as the earth rod could cause accelerated corrosion if there's even a small IR fault.

 

[the doctor]

Thanks for the replies. Marvo makes a good point.... if the rod rotted pop in a new one, if the lamp post rots eh.... bigger job!

 

[Andy™]

chances are, the post will have a lower Ra than a rod, so the post will take most of the fault current than the rod anyway

 

[Apache]

Didn't it used to be common practice to use the metal frame of building as the 'earth'?

 

[batty]

I would imagine the street light would be painted in some way to prevent corrosion so probably would not be a very good rod. On steel buildings I have done you don't generally get a good earth of the building. Not sure if its because the structure is painted or the fact its going into concrete.

 

[m.cameron]

Modern lamp posts are galvanised steel painted inside and out with a bitumen based paint as per British standards to prevent corrosion. This would make for a very poor connection to earth.

I have recently joined my local council on street lighting, my senior techs and designers know of no TT installations in our county and would not recommend it.

 

[streetlighter]

Some very old pole bracket installations as in mounted to wooden electricity poles have no connection to Earth !

The terms CNE and SNE are what you get taught by the DNO on the ERG39 course. Most board services in new installations are TN-C-S the older installs that are usually concrete columns are mainly TN-S.

Different local authoritys have there own rules about whats acceptable on there network. Like in Cheshire on a board serviced column we would fit above the DNO cutout a charles en direct double pole isolator unit with 6mm insulated tails and a 6mm main earthing conductor and a 6mm earth to the column but you never earth the column door.

In warrington they lash the twin and earth from the lantern in the DNO cutout and have a 6mm earth to the column and earth the door too.

 

[SLIPSHOD & SLAPDASH]

This thread made me think about the feeder pillers out on the street i have to commision for Thames Water. They are all TT.