Metal lamppost in garden... TNC-S

[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 )