Matt.e, hot tubs and PME

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.

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.