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.

 

[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