Earth Rod Needed?

OxiaUK · 12 Sep 2021

Hi

We have an old converted cast iron lamp post in our garden that has a copper head on and a 230v bulb... this is all connected via an SWA cable from the house protected by a 30ma RCBO.

Is this OK to be connected to the house earth supply (PME) or should it have a Earth rod ?

thanks

Taylortwocities · 12 Sep 2021

Assuming that the earth connection from the supply (the SWA) is connected to the earth terminal of the metal lamp post, then an earth rod is not necessary.

OxiaUK · 12 Sep 2021

Taylortwocities said:
Assuming that the earth connection from the supply (the SWA) is connected to the earth terminal of the metal lamp post, then an earth rod is not necessary.

The SWA terminates at a outdoor socket then from that goes to a weatherproof FCU then comes out in rubber flex (in conduit) as its a cast iron lamp post that was originally gas powered so there were no provisions for electrical cables

Taylortwocities · 12 Sep 2021

Fine. You need to test if the metal lamppost is connected to the earth from the supply.

Got a multimeter?

OxiaUK · 12 Sep 2021

Taylortwocities said:
Fine. You need to test if the metal lamppost is connected to the earth from the supply.

Got a multimeter?

It 100% is as the earth goes to a terminal on a brass lamp holder and its all interconnected via the head metal work to the body. I've tested it already.

Taylortwocities · 12 Sep 2021

No earth electrode required.

OxiaUK · 12 Sep 2021

Taylortwocities said:
No earth electrode required.

so its safe been connected to the house PME supply?

Taylortwocities · 12 Sep 2021

Sure, of course.

BS7671 does not seek to prohibit street furniture or lighting columns being connected to a TN-C-S earthing system.

There’s no problem unless there is earthed metalwork in the vicinity (within reach) of the lamp post.

bernardgreen · 12 Sep 2021

Taylortwocities said:
There’s no problem unless there is earthed metalwork in the vicinity (within reach) of the lamp post.

Two points of contact are required for a person to receive a significant electric shock. The lamp post is the first point, the earthed metalwork is the second point of contact.

If the second point of contact is the damp ground around the lamp post then this can be as dangerous as a piece of earthed metalwork if the lamp post does not have a local earth electrode..

Most street lighting systems have an earth rod. In a PME system this Earth electrode is just one more of the PME Earths ( Protective Multiple Earths ).

If the lamp post relied only on the PME Earth from the house then if ( or when ) the Neutral coming into the house was not at Ground potential then the lamp post would not be at true Ground potential but instead at the potential of the incoming Neutral . ( The Earth of a PME system is the Neutral of the supply )

A serious fault on the network could result in the potential on the lamp post being pulled above the potential of the ground around the lamp post and thus create a risk of electric shock to someone standing on the ground and touching the lamp post.

The current through the person from the lamp post would come from the Neutral before the RCD and would therefore not be detected by the RCD and the RCD would not trip.

OxiaUK · 12 Sep 2021

bernardgreen said:
Two points of contact are required for a person to receive a significant electric shock. The lamp post is the first point, the earthed metalwork is the second point of contact.

If the second point of contact is the damp ground around the lamp post then this can be as dangerous as a piece of earthed metalwork if the lamp post does not have a local earth electrode..

Most street lighting systems have an earth rod. In a PME system this Earth electrode is just one more of the PME Earths ( Protective Multiple Earths ).

If the lamp post relied only on the PME Earth from the house then if ( or when ) the Neutral coming into the house was not at Ground potential then the lamp post would not be at true Ground potential but instead at the potential of the incoming Neutral . ( The Earth of a PME system is the Neutral of the supply )

A serious fault on the network could result in the potential on the lamp post being pulled above the potential of the ground around the lamp post and thus create a risk of electric shock to someone standing on the ground and touching the lamp post.

The current through the person from the lamp post would come from the Neutral before the RCD and would therefore not be detected by the RCD and the RCD would not trip.

So this sounds like I should get a local rod fitted to earth / bond the lamp post?

flameport · 12 Sep 2021

OxiaUK said:
So this sounds like I should get a local rod fitted to earth / bond the lamp post?

No.

JohnD · 12 Sep 2021

old street lamps typically have a large iron or steel "root" stuck in the ground (to prevent them falling over).

if the lamp is bonded to the lamppost, I doubt your own little earth rod will be anywhere near as good.

bernardgreen · 12 Sep 2021

OxiaUK said:
So this sounds like I should get a local rod fitted to earth

I would bond the lamp post to a ground rod.

It is considered safer to NOT Earth garden equipment ( electric tools, lawn mowers etc ) from the Earth in the house

I would suggest that a lamp post has the same hazards as an electric tool / machine being used in the garden.

JohnD · 12 Sep 2021

bernardgreen said:
I would bond the lamp post to a ground rod.

why, if it already has a buried iron root?

bernardgreen · 13 Sep 2021

JohnD said:
why, if it already has a buried iron root?

If it has the iron root........but it might be bolted onto a pad of concrete or other non-conductive material

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Earth Rod Needed?

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