Hot tubs and PME

vantech: 
 

Hi, 

I am planning a hot tub install, and we circle back to the question of earthing arrangements! 
 

I am thinking of 3 different approaches to this as follows: 

 

• PME to the isolator next to the tub, gland off and convert the output to TT creating an island.

 

Q: by doing this, how far away from the tub does the rod need to go? 

The position of the earth electrode relative to the tub will not necessarily help local touch voltages. The voltage gradient can be as little as 3 m or so, which means someone over the other side of the tub to the electrode might not see the same “earth” as someone next to the electrode.

 

• PME to the tub, and install a rod that is then supplementary to the PME.

 

Q: can this rod be next to the consumer unit or cut out, or does it need to go next to the tub? 

Section 702 and GN 7 talk about a “mat” - G12/4 talks about a grid around “poolside areas” - but those are swimming pools, and it depends on where your determining it's a “section 701” or a “section 702” installation.

I understand the issue of touch/step voltages, but if a rod is installed next to the consumer unit and the tub is perhaps 10 metres away, would this be sufficient? 
 

If we are talking about an electrode next to the consumer unit, then, for a maximum demand of 100 A, in a single phase installation you'd need an electrode with a resistance of less than 1 ohm to make any difference.

vantech: 
 

Hi, 

I am planning a hot tub install, and we circle back to the question of earthing arrangements! 
 

I am thinking of 3 different approaches to this as follows: 

 

• PME to the isolator next to the tub, gland off and convert the output to TT creating an island.

 

Q: by doing this, how far away from the tub does the rod need to go? 

The position of the earth electrode relative to the tub will not necessarily help local touch voltages. The voltage gradient can be as little as 3 m or so, which means someone over the other side of the tub to the electrode might not see the same “earth” as someone next to the electrode.

 

• PME to the tub, and install a rod that is then supplementary to the PME.

 

Q: can this rod be next to the consumer unit or cut out, or does it need to go next to the tub? 

Section 702 and GN 7 talk about a “mat” - G12/4 talks about a grid around “poolside areas” - but those are swimming pools, and it depends on where your determining it's a “section 701” or a “section 702” installation.

I understand the issue of touch/step voltages, but if a rod is installed next to the consumer unit and the tub is perhaps 10 metres away, would this be sufficient? 
 

If we are talking about an electrode next to the consumer unit, then, for a maximum demand of 100 A, in a single phase installation you'd need an electrode with a resistance of less than 1 ohm to make any difference.