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.

 

[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 )

 

[Selfmade]

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.

Well that's me told! Awesome explanation Too lazy to do the maths, me!

 

[Alex-SL]

Hi weevilward, As I understand it, your main question was: “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? ”

You can find all answers in IEC 60364-4-41 standard. There is “Protection under fault condition - Protection against indirect contact” in TN-systems for your case. According to IEC60364 you must consider two events:

Case-1 (easy to verify). Dangerous voltage potential (touch voltage on a lamppost) occurs from the loss is supply neutral. According to IEC60364 protections are protective earthing and protective equipotential bonding.

The protective equipotential bonding must connect all extraneous conductive parts within the installation, including an outdoor lighting lamppost, as part of installation. It means you must connect metal lamppost with the main potential equalization bar by a separate potential equalization conductor. Additionally you may make an equipotential bonding ring around lamppost. (for example, radius=1m, depth=0,5m). All these techniques are sufficient, but they don’t prevent you from using additional protection (rcd).

Case -2 (not easy to verify). Dangerous voltage potential (touch voltage on a lamppost) occurs from a short-circuit in a metal lamppost. In this case you must check the circuit breaker or fuse of the lamppost supply cable. According to IEC60364, disconnection time of a circuit breaker or fuse should be less than 0,2s. To check disconnection time you must calculate the minimum (phase-to- earth) short-circuit current in the lamppost. You can’t calculate it accurately by manual calculations - it’s very time-consuming. You can use any software, for example Ecodial Ecodial Advanced Calculation | Schneider Electric - https://www.schneider-electric.com/en/product-range-presentation/61013-ecodial-advanced-calculation/ . I use the MeteorCalc software MeteorCalc SL - https://meteorcalc.com/ . It’s specialized soft for outdoor lighting design.

After the finish of work, you must measure the short-circuit current in the lamppost and verify disconnection time again.

 

[telectrix]

considering where the OP's customer got the lamp post from, it might be easier to convert it back to gas, but don't forget to bond the gas pipe back to the MET.