Metal lamppost in garden... TNC-S

[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.