Metal lamppost in garden... TNC-S

[weevilward]

Evening all,
Need some opinions on this one please:
TNC-S house, complete new install, all going to plan, nearly finished second fix, customer then arrives with a huge metal lamppost (bought from Frinton-on-sea promenade...) and wants it hooked up in the back garden.
What is the general consensus on this - my concern is: in the event of loss is supply neutral, everything connected to MET, including that lamppost, will rise to a voltage, anyone touches it may get a shock?
Or, will the fact it's planted in the earth keep any possible voltage down?
Am I making something out of nothing here or would this be safer as a TT?

Any thoughts welcome.

 

[westward10]

What is it with this neutral loss, in the house no one bothers but go outside and there are shouts of ROD IT.

 

[weevilward]

Ok, so touching a radiator bonded in the house in no different to touching a lamppost bonded in the garden? Is that what you're getting at?

 

[weevilward]

Ok, so touching a radiator bonded in the house in no different to touching a lamppost bonded in the garden? Is that what you're getting at?

 

[Flanders]

Im think there is somthing in special location for lamp post but it may be relating to street furniture

 

[Leesparkykent]

The post will propably act as one big rod.

 

[westward10]

Most lighting columns on the highway are PME they seem okay.

 

[weevilward]

So I'm reading too much into it.
Thanks fellas. Never thought twice about domestic outside lighting before, but when something different comes along it makes you think twice...

 

[Richard Burns]

This is probably a case of minimal risk. In general you are not constantly handling a lamppost, as opposed to hand held electrical equipment, and so the risk of neutral loss fault occurring and it being a risk due to contact are low.
Bonding will have limited effect since the potential you are trying to eliminate is between earth and the supply and bonding will only limit the PD between the MET and the supply.
Within a property there should be no potential differences because the bonding connects all sources of potential difference together. Outside a property without installing conductive mats in the ground you cannot avoid being in the presence of a nominal 0V from true earth and so the risk is increased.

 

[Paul mccullagh]

Run swa to the lamp post (volt drop calculations) bonded on a 30MA rcd in the consumer unit job done!

 

[westward10]

Run swa to the lamp post (volt drop calculations) bonded on a 30MA rcd in the consumer unit job done!

Why would it require rcd protection.

 

[westward10]

The post will propably act as one big rod.

This was suggested many years ago by the ILE but was felt the column would be unreliable as an electrode due to corrosion, protection on the column to prevent corrosion and the means of bedding it in the ground.

 

[weevilward]

Why would it require rcd protection.

It doesn't need RCD but it wouldn't hurt to have it.

This was suggested many years ago by the ILE but was felt the column would be unreliable as an electrode due to corrosion, protection on the column to prevent corrosion and the means of bedding it in the ground.

Yeah this one is pretty rusty!

 

[weevilward]

This is probably a case of minimal risk. In general you are not constantly handling a lamppost, as opposed to hand held electrical equipment, and so the risk of neutral loss fault occurring and it being a risk due to contact are low.
Bonding will have limited effect since the potential you are trying to eliminate is between earth and the supply and bonding will only limit the PD between the MET and the supply.
Within a property there should be no potential differences because the bonding connects all sources of potential difference together. Outside a property without installing conductive mats in the ground you cannot avoid being in the presence of a nominal 0V from true earth and so the risk is increased.

Thanks Richard, this makes sense.

 

[Paul mccullagh]

I would personally protect this with a rcd due to someone dismantling fixtures and changing a light bulb!

 

[Deleted member 9648]

Well you cud'a knocked me down with a feather when I saw this, normally it's only sheds that bring people out in a cold sweat when TNCS is mentioned. I think I've pointed out before that those who come over all unnecessary at the mention of a shed never seem to get that queasy feeling with other outdoor installs.
So good on'yer Weevilward, shows you are thinking about what you are doing rather than just following the herd and subscribing to myths.

 

[static zap]

Is it the tallest piece of garden furniture , on a Hill Top !
(that's the other kind of rod ! )

 

[weevilward]

Ha ha, no it's not the tallest thing by a long shot...
Redirect Notice - https://goo.gl/images/RszhTX

 

[stevethesparks]

How many people think to make their outdoor socket TT, whats the difference?

 

[dmxtothemax]

If you have any concerns over safety,
then simply fit an RCD inside the house
this will not only protect the post but also the wiring to it.
you can never be too safe

 

[Ian1981]

Whilst rcds do improve safety it won't protect against an open circuit neutral on whatever the earthing system is.

 

[Midwest]

I was in a conundrum some time ago, when a customer wanted me to connect up a previously installed metal lamp post at the bottom of their driveway. It wasn't a full lamppost, but one of the smaller ones mounted on brick wall. However, it was located adjacent to the public footpath. The property had a PME supply, and a swa cable had been previously installed. The CU didn't have a spare way for additional protection by RCD or compatible new RCBO's.

I read chapter 714, BS7671, outdoor lighting installations. This chapter seems only applicable to outdoor lighting in the public domain, street lighting etc. Whilst a light in someone's garden, is more under control and constant supervision than a street light would be, it does seem strange to have some many regs and guidance specifically for street lights and none per say for domestic lighting situations?

My conundrum was the additional protection. Advice from my scheme, in these circumstances, was to provide RCD protection.

 

[Flanders]

I was in a conundrum some time ago, when a customer wanted me to connect up a previously installed metal lamp post at the bottom of their driveway. It wasn't a full lamppost, but one of the smaller ones mounted on brick wall. However, it was located adjacent to the public footpath. The property had a PME supply, and a swa cable had been previously installed. The CU didn't have a spare way for additional protection by RCD or compatible new RCBO's.

I read chapter 714, BS7671, outdoor lighting installations. This chapter seems only applicable to outdoor lighting in the public domain, street lighting etc. Whilst a light in someone's garden, is more under control and constant supervision than a street light would be, it does seem strange to have some many regs and guidance specifically for street lights and none per say for domestic lighting situations?



My conundrum was the additional protection. Advice from my scheme, in these circumstances, was to provide RCD protection.

This is what I was refering to in me post yesterday i did not have the byb with me at the time, just had a look now and as you say it refers more about street furniture but it does say it includes gardens under the scope 714.1 (i) so i would use this section of the regs for the ops installation 714.411.203 covers pme

 

[Midwest]

Not sure that 714 includes domestic gardens, more places accessible to the public. But I take your point, it doesn't include domestic situations in it's exclusions.

Lots to cover, if it does though.

 

[dmxtothemax]

just because it is not mandatory
doesn't mean you cannot do it !
If someone wants to be extra carefull ?
then fitting a RCD would do just that
provide more safety over a non rcd post.

 

[Wilko]

Whilst rcds do improve safety it won't protect against an open circuit neutral on whatever the earthing system is.

I'm struggling here - if neutral was o/c any current flow would unbalance the device and cause it to trip (?)

 

[Ian1981]

I'm struggling here - if neutral was o/c any current flow would unbalance the device and cause it to trip (?)

No imbalance as the same amount of current will flow between live and neutral in the rcd but if in pme the earth would also rise to possible mains voltage if supply neutral is broken back to the source of supply (DNO cable)
There's no imbalance so rcd won't operate

 

[Selfmade]

Is the problem with o/c neutrals not more to do with overloading the 1.0 / 1.5mm2 cpc if all the neutral current (including from shower, cooker etc) is trying to get back to the supply neutral via said cpc, rather than anything to do with shock? And isn't that why we have to run 10mm2 main bonding wherever we take power from a PME supply (because this is the smallest that would be unlikely to be overloaded in these circumstances)?

 

[Ian1981]

Is the problem with o/c neutrals not more to do with overloading the 1.0 / 1.5mm2 cpc if all the neutral current (including from shower, cooker etc) is trying to get back to the supply neutral via said cpc, rather than anything to do with shock? And isn't that why we have to run 10mm2 main bonding wherever we take power from a PME supply (because this is the smallest that would be unlikely to be overloaded in these circumstances)?

If the neutral is broken at the supply side on pme then the current will be unable to return to the source via the neutral and as there is a link in the cut out that only exists in pme between neutral and earth then the current will flow through the earthing conductor and through all connected cpcs and bonding conductors.

Yes 10mm to deal with neutral currents.

 

[Selfmade]

If the neutral is broken at the supply side on pme then the current will be unable to return to the source via the neutral and as there is a link in the cut out that only exists in pme between neutral and earth then the current will flow through the earthing conductor and through all connected cpcs and bonding conductors.

Potentially putting very high currents through very small CPCs if these are connected to earth (e.g. by this lamp post) and the PME still has connections to earth on the supply side of the break.

So surely the OP should be putting 10mm2 bonding in place to this lamp. Or else somehow insulating it from earth.