Street Light Earthing/Bonding Query

[lozarus]

Apologies in advance if I'm missing the obvious here or not referring to the correct section of 18th or the GN's. In honesty I think I'll get a better answer from the discussion here anyway.

So I'm about to do some head upgrades from failed SOX to LED. A reasonably starightforward like-for-like swap as such. However, as always I've done my preliminary inspection it's made me question a few things.

The poles are 1964 vintage as are some of the cutouts. It's a private system. Each bank of 3 poles is backed up by a 10a MCB via 2c PVC SWA (armour as CPC). There's 2 original cutouts (please see my post in the For Sale/Wanted section to see if you can help me locate a replacement fuse carrier for one) - they're all in reasonable condition but I'm not sure of the logic behind the earthing arrangement. It looks to be partly original as well.

As per the photograph, it's that old favourite of a section of PVC outer cut away and a BS951 clamp around the armour. Why would this have been done? Surely the all-metal cutout would naturally be bonded to the armour by the SWA gland and the luminaire's CPC could just be connected to the meteal cutout?

Second question - the CPC coming off the BS951 goes to both the luminaire, and the pole.

Earthing arrangements of the site are TN-C-S. I would have thought the pole is in effect a large earth stake (although I appreciate it may be coated in bitumen in the ground).

Is it correct that the pole is bonded to the incoming earth? I would have thought this is sort of combining two different earthing systems.

Cheers

 

[pc1966]

As per the photograph, it's that old favourite of a section of PVC outer cut away and a BS951 clamp around the armour. Why would this have been done?

Installer was an idiot?

Surely the all-metal cutout would naturally be bonded to the armour by the SWA gland and the luminaire's CPC could just be connected to the meteal cutout?

Exactly...

Second question - the CPC coming off the BS951 goes to both the luminaire, and the pole.

Earthing arrangements of the site are TN-C-S. I would have thought the pole is in effect a large earth stake (although I appreciate it may be coated in bitumen in the ground).

Is it correct that the pole is bonded to the incoming earth? I would have thought this is sort of combining two different earthing systems.

The issue here is the TN-C-S aspect and the minimum bond size of 10mm copper equivalent where such a system's CPC is connected to extraneous conductive parts. Presumable when new this was TN-S and no issues beyond meeting disconnection time on a fault?

 

[cliffed]

Street furniture requires different MPBC consideraction.
4mm bonding condutors would be sufficient

 

[lozarus]

Thanks guys. No idea what size that mess is coming off the jubilee clip.

Think I'll have another look today and do some further tests.

RE TNS/TNCS - location is a school, so documentation is non-existent - except for a recent EICR which states TNCS. I'll head to the intake room and have a look for myself. It's has an on-site substation, which backs onto the intake room.

I'll have a look at continuity and take a Zs at the cutout - should give me an idea of the current state of play.

Having slept on it, it's hit me that one way or another the post will get bonded to the incoming CPC - the new heads have a CPC which bonds to their metal enclosure, which will in turn make contact with the pole through the grub screws....

 

[timhoward]

It's has an on-site substation, which backs onto the intake room.

I've worked on a couple of schools / former schools with this arrangement.
I'd be quite surprised if it's TNCS. As the main earthing will be connected to the star point (N) of the transformer on the substation side of the wall, it would be unusual to then additionally connect N and main earthing the consumer side of the wall too and make it TNCS at that point.
Obviously in this situation the actual difference is almost irrelevant apart from a semantics/regs point of view.
There's usually a solid chunky earth bar conductor coming straight through the wall connected to up to a Kingsmill earth bar with link.

 

[pc1966]

With on-site transformer then the whole open PEN fault diverting large sustained currents to extraneous parts goes away. Then so as long as CPC is enough for fault clearing and physically sound it should be OK.