Another question about earthing with regards hot tubs?

[HappyHippyDad]

With regards hot tubs, earthing always seems to be one of the most discussed points, especially with regards PME and extending the equipotential zone.

My question is why is TNS ok to extend and PME is 'questioned' (I wont say 'not allowed' as I believe it is, it just raises questions and many (not all )electricians preferring to have a separate TT for the hot tub)?

I believe one problem is the possibility of potential between the PME earth (i.e supply neutral) and the local true earth, so client gets tingles when in contact with true earth and touching any exposed conductive part in hot tub earthed to PME (under normal conditions).

However, why is the above problem only with PME? Why would there not be a PD with TNS? With PME you have multiple earthing points, will they not be closer to the local true earth compared to the TNS system which connects to the neutral at the local source which will be further away?

I can see how under fault conditions (broken supply neutral) the TNS would still be ok compared to the PME but under normal conditions does this potential difference (between source earth and local earth) affect TNS as well as PME?

 

[Darkwood]

In a PME system the network relies on the multiple earthing along the network to provide an alternate route if the N were for any reason lost, this can then under fault conditions create a risk if somehow you became part of that route back due to a poor designed earthing arrangement of say a garage or hot-tub, really anything outside your standard equipotential zone.

 

[mattg4321]

I recently looked at a couple of hot tubs that had been recently fitted. Both PME with no rods as far as I could see.

Did a quick test between CPC terminal underneath and any metalwork exposed when in normal operation. Could find nothing at all on either hot tub.

Anybody found any exposed conductive parts on the external part of a hot tub recently?

 

[HappyHippyDad]

In a PME system the network relies on the multiple earthing along the network to provide an alternate route if the N were for any reason lost, this can then under fault conditions create a risk if somehow you became part of that route back due to a poor designed earthing arrangement of say a garage or hot-tub, really anything outside your standard equipotential zone.

I was happy with that bit darkwood, it was more with regards to normal working (non fault) conditions. I cant see any difference between TNS and PME with regards the potential difference between supply earth and local earth?

 

[HappyHippyDad]

I recently looked at a couple of hot tubs that had been recently fitted. Both PME with no rods as far as I could see.

Did a quick test between CPC terminal underneath and any metalwork exposed when in normal operation. Could find nothing at all on either hot tub.

Anybody found any exposed conductive parts on the external part of a hot tub recently?

Would not the exposed conductive parts on the tub already be part of the PME earth? It is more the PD between those parts and the actual ground which is likely to be very wet!

 

[mattg4321]

What I was getting at is that there were no exposed conductive parts at all EXCEPT behind the hatch underneath the tub.

So it would actually be pretty hard to come into contact with PME earth and true earth at the same time unless you had the hatch off and happened to grab onto the 1 or 2 things in there that were connected to CPC terminal.

 

[GEOFF S]

I was happy with that bit darkwood, it was more with regards to normal working (non fault) conditions. I cant see any difference between TNS and PME with regards the potential difference between supply earth and local earth?

Hi

Its quite simple, a TNC-S system has a CEN, so earth and neutral combined. Some voltage will be dropped across the supply neutral, therefore that voltage will also be present on the system earth. In a TN-S system this isn't the case, so the voltage dropped on the neutral wont be present on the system earth.

Cheers

 

[Lucien Nunes]

To expand upon Geoff's answer...

The neutral conductor of the DNO's cable tends to have some current flowing through it back to the substation. If single-phase, it will be collecting the entire load current; if 3-phase, different sections will have different currents as the various branches' add algebraically and the net 50Hz current might be small if the system is well balanced, but it will also carry any harmonic currents that do not sum to zero e.g. 3rd harmonic, right back to the transformer. One way or another, even on 3-phase, there will inevitably be voltage drops along the neutral.

If it is also used as a protective conductor without multiple earthing, then no point will be at the same potential as the ground net at the substation; the CPCs of connected installations will be at arbitrary voltages relative to the substation earth, maybe only a few volts but usually increasing with distance from the sub. If multiple earthing is added, then the potential between local physical earth and the CNE will be reduced around each point, plus the body of earth will be connected in parallel with the cable conductor to lower the effective total resistance. However, each earth electrode has an Ra, through which some of the CNE current may flow depending on how the voltage gradient along the cable compares with that through the body of earth. Hence even near the earthed points there is no guarantee that the PEN conductor will be at the same potential as the local physical ground.

TN-S avoids all this because no current flows through any protective conductors or physical earth under non-fault conditions.