Matt.e, hot tubs and PME

gkenyon:

Farmboy:

Q1. According to Guidance Note 7, when installing hot tubs, section 702 should be applied in full. Regulation 702.410.3.4.3 recommends that when PME is used, an mat or electrode of 20 ohms or less be connected to the supplementary bonding.

So, if no supplementary bonding is present because there are no extr/exp.c.parts, does an electrode/mat still need to be installed (anywhere on the system)?

The provision of such a mat is because neutral currents in the PME system actually travel back to the transformer through the ground as well as the supply Neutral. In fact, even in normal operation, when there is no broken neutral, ground currents and both L-N and L-PE fault currents are shared between the combined neutral and earth (CNE), the actual ground itself, and any extraneous-conductive-parts (your neighbours, even if you have no extraneous-conductive-parts). The current travelling through the ground itself can cause a "step potential" to form. The grid acts to equalize the potential, and help control the gradient of voltage difference around the edges of the area on which the pool or basin is sitting.


So, the provision of an earth mat has little or nothing to do with your electrical installation in particular, and in small curtilage properties in areas with a PME supply, is probably a good idea even if you don't export the PME from the main installation to outdoors - although of course if the grid met the requirements for an earth electrode you could use it as such. (Below, for PME, I refer to G12/4 - in section 6.2.5, where TT is used, "segregating" the two earthing systems is discussed - in smaller curtilage properties, it is not always possible to achieve this.)


Also, just to point out that such as grid is not a requirement of BS 7671 specifically - it is a recommendation in a Note. In standards speak we call this "Informative" (good practice recommendation in this case) rather than Normative (an actual requirement that shall be done).


HOWEVER, it is a requirement of G12/4 Requirements for the application of protective multiple earthing to low voltage networks - see section 6.2.5 on Pages 25 and 26. As this is part of the Distribution Code, if you do export PME and don't follow it (or any more onerous requirements of your particular DNO), they may choose not to have you connected to their PME system at least until it's made safe. G12/4 may be downloaded here: http://www.dcode.org.uk/annexes/annex-1/

 

 

Q2. A Matt.e device (https://matt-e.co.uk/single-phase-connection-units/), as used in EV charging to comply with Regulation 722.411.4.1 (iv), of Amd 1, is installed on a tub supply, on a PME system.

So, given it's a hot tub, does the electrode/mat still need to be installed to comply with Regulation 702.410.3.4.3, when:


a) supplementary bonding is present, and,


b) when no exp/extr.c.parts are present, hence no suppl bonding?

(and no, this isn't work I've done).

 

F

Unless all parts of the location of the tub are a long way (> 10 m) from all buried metalwork connected to the PME system, it is perhaps a good idea to install the grid. As discussed above, disconnecting the tub similar to an EV will have little effect on ground currents from adjacent installations.


Remember, when you get out of a hot tub, you've been immersed in water and potentially have little or nothing on your feet. It's a very different risk profile than the electric vehicle scenario - which only covers the broken neutral situation, because normally when you're wearing footwear, the currents flowing through the ground in PME systems don't really matter.

Thank you for your reply, much appreciated. I've amended my OP to reflect that the 20 ohms is in the note to the regulation.


I asked the two questions in relation to this video: https://youtu.be/l5DjBRiwCDc


Q1. was asked because someone in the comments wrote that they fed a hot tub from a PME supply but did not install a rod because no extr/exp.c.parts were present. However, they were then failed on their annual scheme inspection because no rod was present. The commenter wrote they couldn't find the requirement in the regs (should've bought GN 7) but the assessor apparently stated it's in the building regs.


So from your reply, it's useful to install an earth mat, anyway, if there's the possibility of other people's PME systems effecting the hot tub area. Would this apply even if the tub was TT'd?


Q2. was asked because the installer used PME and the Matt.e, with no mat or electrode.


So, the Matt.e does not negate the usefulness of installing an earth mat.


Note also that the load side and supply side armouring was connected together at the Matt.e, thereby negating the point of the Matt.e isolating the PME, although the load side cpc will be disconnected, so does the cross connection of the armouring present a problem?


Regards


F

 

gkenyon:

Farmboy:

Q1. According to Guidance Note 7, when installing hot tubs, section 702 should be applied in full. Regulation 702.410.3.4.3 recommends that when PME is used, an mat or electrode of 20 ohms or less be connected to the supplementary bonding.

So, if no supplementary bonding is present because there are no extr/exp.c.parts, does an electrode/mat still need to be installed (anywhere on the system)?

The provision of such a mat is because neutral currents in the PME system actually travel back to the transformer through the ground as well as the supply Neutral. In fact, even in normal operation, when there is no broken neutral, ground currents and both L-N and L-PE fault currents are shared between the combined neutral and earth (CNE), the actual ground itself, and any extraneous-conductive-parts (your neighbours, even if you have no extraneous-conductive-parts). The current travelling through the ground itself can cause a "step potential" to form. The grid acts to equalize the potential, and help control the gradient of voltage difference around the edges of the area on which the pool or basin is sitting.


So, the provision of an earth mat has little or nothing to do with your electrical installation in particular, and in small curtilage properties in areas with a PME supply, is probably a good idea even if you don't export the PME from the main installation to outdoors - although of course if the grid met the requirements for an earth electrode you could use it as such. (Below, for PME, I refer to G12/4 - in section 6.2.5, where TT is used, "segregating" the two earthing systems is discussed - in smaller curtilage properties, it is not always possible to achieve this.)


Also, just to point out that such as grid is not a requirement of BS 7671 specifically - it is a recommendation in a Note. In standards speak we call this "Informative" (good practice recommendation in this case) rather than Normative (an actual requirement that shall be done).


HOWEVER, it is a requirement of G12/4 Requirements for the application of protective multiple earthing to low voltage networks - see section 6.2.5 on Pages 25 and 26. As this is part of the Distribution Code, if you do export PME and don't follow it (or any more onerous requirements of your particular DNO), they may choose not to have you connected to their PME system at least until it's made safe. G12/4 may be downloaded here: http://www.dcode.org.uk/annexes/annex-1/

 

 

Q2. A Matt.e device (https://matt-e.co.uk/single-phase-connection-units/), as used in EV charging to comply with Regulation 722.411.4.1 (iv), of Amd 1, is installed on a tub supply, on a PME system.

So, given it's a hot tub, does the electrode/mat still need to be installed to comply with Regulation 702.410.3.4.3, when:


a) supplementary bonding is present, and,


b) when no exp/extr.c.parts are present, hence no suppl bonding?

(and no, this isn't work I've done).

 

F

Unless all parts of the location of the tub are a long way (> 10 m) from all buried metalwork connected to the PME system, it is perhaps a good idea to install the grid. As discussed above, disconnecting the tub similar to an EV will have little effect on ground currents from adjacent installations.


Remember, when you get out of a hot tub, you've been immersed in water and potentially have little or nothing on your feet. It's a very different risk profile than the electric vehicle scenario - which only covers the broken neutral situation, because normally when you're wearing footwear, the currents flowing through the ground in PME systems don't really matter.

Thank you for your reply, much appreciated. I've amended my OP to reflect that the 20 ohms is in the note to the regulation.


I asked the two questions in relation to this video: https://youtu.be/l5DjBRiwCDc


Q1. was asked because someone in the comments wrote that they fed a hot tub from a PME supply but did not install a rod because no extr/exp.c.parts were present. However, they were then failed on their annual scheme inspection because no rod was present. The commenter wrote they couldn't find the requirement in the regs (should've bought GN 7) but the assessor apparently stated it's in the building regs.


So from your reply, it's useful to install an earth mat, anyway, if there's the possibility of other people's PME systems effecting the hot tub area. Would this apply even if the tub was TT'd?


Q2. was asked because the installer used PME and the Matt.e, with no mat or electrode.


So, the Matt.e does not negate the usefulness of installing an earth mat.


Note also that the load side and supply side armouring was connected together at the Matt.e, thereby negating the point of the Matt.e isolating the PME, although the load side cpc will be disconnected, so does the cross connection of the armouring present a problem?


Regards


F