Summer house earthing

[Hellmooth]

Hi, quick question on what earthing system to use for a summer house install I am pricing. Reading previous posts regarding the issue it seems that having the summer house wired with a TT system is recommended, I'm still unsure though.

The summer house is going to be wired in 10mm SWA from the CU in the house from non RCD side, to a garage board in the summer house, this will be RCD protected, the house is TNC-S. There is a hot tub which will be protected by a 32A mcb, a 6A lighting radial and a socket radial on a 16A, this may have a 2000-3000w heater on it.

I understand the concept of having a TT for the earthing but was wondering why I couldn't use the SWA earthing, obviously the potential difference from the SWA earth and that of Earth can cause issues, but would this not only be an issue if touching an appliance earthed from the SWA and being in contact with Earth at the same time? The sockets and lighting are all plastic.

If if going down the TT route is the best option, what is the best way to go about it? Taking the earth straight into my garage CU and doing away with the SWA earthing?

Thanks in advance.

 

[Leesparkykent]

Hi, quick question on what earthing system to use for a summer house install I am pricing. Reading previous posts regarding the issue it seems that having the summer house wired with a TT system is recommended, I'm still unsure though.

The summer house is going to be wired in 10mm SWA from the CU in the house from non RCD side, to a garage board in the summer house, this will be RCD protected, the house is TNC-S. There is a hot tub which will be protected by a 32A mcb, a 6A lighting radial and a socket radial on a 16A, this may have a 2000-3000w heater on it.

I understand the concept of having a TT for the earthing but was wondering why I couldn't use the SWA earthing, obviously the potential difference from the SWA earth and that of Earth can cause issues, but would this not only be an issue if touching an appliance earthed from the SWA and being in contact with Earth at the same time? The sockets and lighting are all plastic.

If if going down the TT route is the best option, what is the best way to go about it? Taking the earth straight into my garage CU and doing away with the SWA earthing?

Thanks in advance.

is the hot tub inside the summer house?

 

[Hellmooth]

Yes it is Lee!

 

[Hellmooth]

With regards to the heater I'm thinking it won't be the best option especially if it is mobile, will speak to the customer about this!

 

[Leesparkykent]

Personally I would TT the CCU in the summer house end due to the hot tub, even though it's being installed in a summer house so technically the person entering or exiting the hot tub would not be in contact with the general conductive mass of earth as the summer house would normally have a wooden floor.


you could use a two core SWA from the house utilising the TNC-S earth then gland the SWA at the summer house end in to a plastic adaptable box or even the CCU if it's plastic and dropping the TNCS earth at this point and then Create a TT earthing arrangement for the summer house.

 

[davesparks]

I don't see why you would want a TT system for a hot tub?
The submain is going to be 10mm so three core can be used to satisfy TNCS bonding requirements, and an indoor hot tub is no different to a bath and according to gn7 should be treated as such.
Even if the hot tub is outside there is no requirement for a non-TNCS supply, only a recommendation that an earth electrode of low Ra be connected to the MET (in addition to the TNCS earth connection, not instead of)

 

[Leesparkykent]

I don't see why you would want a TT system for a hot tub?
The submain is going to be 10mm so three core can be used to satisfy TNCS bonding requirements, and an indoor hot tub is no different to a bath and according to gn7 should be treated as such.
Even if the hot tub is outside there is no requirement for a non-TNCS supply, only a recommendation that an earth electrode of low Ra be connected to the MET (in addition to the TNCS earth connection, not instead of)

Should a discontinuity occur in the DNO supply PEN conductor (under very exceptional circumstances I know) all metalwork will rise to approximately 230 V with respect to true Earth, which will in itself mean that there could be a lethal touch voltage present between the exposed-conductive parts, extraneous-conductive-parts and true Earth. In an outdoor environment such as this there is much more likelihood that a person will come into contact with the earth of the electrical system earth and true Earth ( the hot tub could be extremely close to the doors of the summer house and kids are extremely inventive lol) a separate earth electrode connected the MET will help to minimise the potential I know.


How ever in the event of a discontinuity The touch voltage will be determined by the load connected within the installation and the value of the earth electrodes and any connections to earth that is provided by the bonded services. To reduce the voltage to a significantly low value requires a considerable earth electrode, to be affective.


Secondly, under normal operating conditions, it is possible, due to small differences in potential between the earth of the electrical system and true Earth, for a small voltage to be present. This difference is detectable, for instance, by a wet person (creating a reduction of body resistance due to being wet and no shoes, etc) a handrail and coming into contact with true Earth (I don't know how close the hot tub is to the doors) This perceived electric shock is minimised by installing the additional earth electrode/s as recommended by BS 7671, however, the most effective method of removing this risk is to provide a TT earthing arrangement and completely isolate from the TNC-S supply.


The potential for ‘tingles’ experienced from a perfectly healthy TNC-S supply system with correct protective bonding in place is cause for concern as far as customer relationships are concerned.

 

[Hellmooth]

Thanks for the comments guys, I will have a think about it and decide which option is best!

 

[davesparks]

Unless the earth electrodes are installed properly and achieve a stable Ra which allows the circuits to meet the Zs requirements for their OCPDs then the TNCS earth connection provides for a better installation than one which relies on the notoriously unreliable RCD devices.