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.

 

[Deleted member 9648]

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.

The purpose of bonding is to create a zone of equal potential in the event of a fault to reduce shock risk. As the lamp post will be surrounded by the general mass of earth at or about 0v it is not possible to create a zone of equal potential so there is no point in main bonding the post.

 

[Selfmade]

The purpose of bonding is to create a zone of equal potential in the event of a fault to reduce shock risk. As the lamp post will be surrounded by the general mass of earth at or about 0v it is not possible to create a zone of equal potential so there is no point in main bonding the post.

So perhaps bonding is the wrong term. But we must surely do something to remove the risk of up to 100A going through a 1.0mm2 CPC?

 

[Ian1981]

So perhaps bonding is the wrong term. But we must surely do something to remove the risk of up to 100A going through a 1.0mm2 CPC?

Why 100amp?
I=V/R

 

[Selfmade]

Why 100amp?

Assuming it's domestic 100A is the max rating of the cutout.

 

[Ian1981]

Assuming it's domestic 100A is the max rating of the cutout.

The amount of current depends on the voltage and any resistance in the circuit.
I=V/R

 

[Selfmade]

The amount of current depends on the voltage and any resistance in the circuit.
I=V/R

Yes, point taken, so in practice it would probably never reach 100A, but surely it is theoretically possible to overload the CPC, depending on what equipment is running while the neutral is open circuit. It cannot exceed 100A (at least not substantially for very long) because of the cutout.

 

[Ian1981]

Yes, point taken, so in practice it would probably never reach 100A, but surely it is theoretically possible to overload the CPC, depending on what equipment is running while the neutral is open circuit. It cannot exceed 100A (at least not substantially for very long) because of the cutout.

But how can anything run if there's no neutral

 

[Billythekid]

Potential difference Ian.

 

[Selfmade]

But how can anything run if there's no neutral

The path I'm seeing is as follows: supply transformer, supply line conductors, cutout, meter, CU incl. MCBs, final circuit lines, loads, final circuit neutrals, CU, cutout neutral/earth, main earth conductor, CU earth bar, final circuit CPCs, earthed metal work also connected directly to earth (e.g. post lamp), mass of earth, PME earth connection on supply side of break, transformer. So we have the full installation current going through that one CPC.

 

[Billythekid]

Depending where the break in "Neutral" (PEN) is, you also have your neighbours loads to consider. Also, if a certain Entertainment provider is added to the mix it makes a hell of a mess.

 

[Selfmade]

Depending where the break in "Neutral" (PEN) is, you also have your neighbours loads to consider. Also, if a certain Entertainment provider is added to the mix it makes a hell of a mess.

So I was wrong about the 100A limit. Even worse. Not sure who the entertainment provider is?

 

[Ian1981]

So I was wrong about the 100A limit. Even worse. Not sure who the entertainment provider is?

Potential difference Ian.

potential difference is basically voltage.
If you have no neutral return path then no current can flow to make a circuit operate as there is no potential difference between the 2 points to run a circuit so nothing will work.

 

[Selfmade]

potential difference is basically voltage.
If you have no neutral return path then no current can flow to make a circuit operate as there is no potential difference between the 2 points.

But the neutral path is there - via the CPCs and earth mass to the PME earth links (see my previous post).

 

[Billythekid]

Ian you're thinking single phase, if the street/area is supplied with a 3ph+n cable and the neutral (PEN) is lost, you now have an unbalanced three phase circuit with no neutral.

HTH

 

[Ian1981]

But the neutral path is there - via the CPCs and earth mass to the PME earth links (see my previous post).

The neutral is the earth in pme after the cut out so if it's broken how do you have the general mass of earth?
Unless there's an extraneous part which is bonded.
Your lights aren't going to work put it that way.
If someone is unfortunate to touch say a gas pipe then they will create a potential difference as they are standing on the general mass of earth and may get the full mains voltage

 

[Selfmade]

The neutral is the earth in pme after the cut out so if it's broken how do you have the general mass of earth?

Via the lamp post (for e.g.).

Unless there's an extraneous part which is bonded.
Your lights aren't going to work put it that way.

I think they are.

If someone is unfortunate to touch say a gas pipe then they will create a potential difference as they are standing on the general mass of earth and may get the full mains voltage

And if the gas pipe is burried in the earth (as they often are) then current can flow to the general mass of earth without anyone having to touch it. As it can through this lamp post. And that would keep the lights on - until the gas pipe, CPC to lamp post or whatever burns out and starts a fire.

 

[davesparks]

Yes, point taken, so in practice it would probably never reach 100A, but surely it is theoretically possible to overload the CPC, depending on what equipment is running while the neutral is open circuit. It cannot exceed 100A (at least not substantially for very long) because of the cutout.

The lamppost won't be buried deep enough to achieve a particularly low resistance to earth.
I think it's fair to assume that the lamppost will be fed in SWA so the smallest possible cable size is 1.5mm, which has a ccc around 16A.
If the lamppost achieves a resistance to earth of 20ohms (which would be impressive as experience suggest you need at least a pair of 8' rods to get that low) then a direct short from the live terminal at the cutout to the lamppost will result in 12A flowing to earth via the lamppost with a 240V supply.

I think it would be more realistic to expect a lamppost to achieve a resistance to earth in the hundreds of ohms rather than the tens.

 

[davesparks]

And if the gas pipe is burried in the earth (as they often are) then current can flow to the general mass of earth without anyone having to touch it. As it can through this lamp post. And that would keep the lights on - until the gas pipe, CPC to lamp post or whatever burns out and starts a fire.

the lamppost, gas pipe or whatever will have to provide a return path to the substation of a similar impedance to that of the PEN conductor in order for the lights to stay on as you suggest. The loop impedance would have to be a couple of ohms at most for the loads in the installation to function as normal, after that the reduced voltage across the normal loads as a result of the resistances in series would become a significant factor.

As a rough calculation If the normal loads in the installation at the time of the break in the PEN occurring total 20A at 240V then their combined resistance would be 12ohms, if the loop impedance of the return path via the lamppost or gaspipe etc was also 12ohms then the potential Dropped across the installation will be 120V at which point a lot of loads will stop working.

Whichever way you look at it I don't see how the very simple science can support your ideas of melting cables and fires starting.

 

[Ian1981]

the lamppost, gas pipe or whatever will have to provide a return path to the substation of a similar impedance to that of the PEN conductor in order for the lights to stay on as you suggest. The loop impedance would have to be a couple of ohms at most for the loads in the installation to function as normal, after that the reduced voltage across the normal loads as a result of the resistances in series would become a significant factor.

As a rough calculation If the normal loads in the installation at the time of the break in the PEN occurring total 20A at 240V then their combined resistance would be 12ohms, if the loop impedance of the return path via the lamppost or gaspipe etc was also 12ohms then the potential Dropped across the installation will be 120V at which point a lot of loads will stop working.

Whichever way you look at it I don't see how the very simple science can support your ideas of melting cables and fires starting.

youve explained it better than me Dave.
A good explanation

 

[static zap]

.... I don't see how the very simple science can support your ideas of melting cables and fires starting.

More likely other way round , if lamp secured to house by 10mm
and struck by lightening ,
(I'd prefer that wiring to pop like a fuse )