UK to TT or not to TT

[Mfctitch]

Morning everyone
It seem I have another outside job.
this time the run is 44 metres with about 2 metres going into a chase in a concrete path so calculated for underground.
the job is to supply a double socket at the end of the garden but eventually this will be where a large shed is going to go into which they will want sockets lights and a heater
Board has one spare way but dual RCD
So thinking of putting in a 63 amp fused main switch
then of course an RCD weather proof double socket.

But I've allowed for 38 amps i.e garage unit
so as a section is underground its coming up at 10mm rather then 6mm if all over ground.
by the way i'm using 3 core SWA
That's not the problem.
I just know you like lots of info.

The question is would you TT or not?
and why would you TT or not TT?
I like watching John Ward and he's not a fan of TT so if you would TT would you test it as a ZE an the sub board or test it using the 3 pins

Thanks again for your help

 

[buzzlightyear]

what is the main system now in and any extraneous pipes going to the new shed .

 

[ChrisElectrical88]

Is the DNO supply TNS or PME? Wooden or Metal shed?

 

[Mfctitch]

what is the main system now in and any extraneous pipes going to the new shed .

Ha ha
I knew I'd leave something out TNC-S
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Is the DNO supply TNS or PME? Wooden or Metal shed?

wooden shed no metal work

 

[pc1966]

The main argument for going TT is when you have a TN-C-S supply and have some issue with bonding extraneous conductive parts. If you are using 3-core 10mm SWA then your CPC size is adequate for bonding on all system.

So really the only remaining issue would be if there was a reason to worry about the bonded metalwork presenting a hazard to true Earth if you were unlucky enough to have a PME fault?
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Final thought: make sure there is enough of the SWA feed to the socket left that it can be used to the final garage CU location. You could do a resin joint later if needed but life is much easier and more reliable if you don't have to!

 

[Mfctitch]

The main argument for going TT is when you have a TN-C-S supply and have some issue with bonding extraneous conductive parts. If you are using 3-core 10mm SWA then your CPC size is adequate for bonding on all system.

So really the only remaining issue would be if there was a reason to worry about the bonded metalwork presenting a hazard to true Earth if you were unlucky enough to have a PME fault?

very true but in that case the whole house would be affected not just the shed, so no to TT in your view?

 

[buzzlightyear]

The main argument for going TT is when you have a TN-C-S supply and have some issue with bonding extraneous conductive parts. If you are using 3-core 10mm SWA then your CPC size is adequate for bonding on all system.

lose a nuatral then your buggered ,TT IT .44 mtrs away .

 

[Mfctitch]

The main argument for going TT is when you have a TN-C-S supply and have some issue with bonding extraneous conductive parts. If you are using 3-core 10mm SWA then your CPC size is adequate for bonding on all system.

So really the only remaining issue would be if there was a reason to worry about the bonded metalwork presenting a hazard to true Earth if you were unlucky enough to have a PME fault?
[automerge]1590319120[/automerge]
Final thought: make sure there is enough of the SWA feed to the socket left that it can be used to the final garage CU location. You could do a resin joint later if needed but life is much easier and more reliable if you don't have to!

Thanks but have allowed for and extra 6 metres which will be coiled up as they don't know exactly where the shed is going

 

[bigspark17]

Ha ha
I knew I'd leave something out TNC-S
[automerge]1590318870[/automerge]

wooden shed no metal work

why would you TT this then?

 

[pc1966]

very true but in that case the whole house would be affected not just the shed, so no to TT in your view?

Generally if you have a reliable TN-derived earth then you should use it.

The issue is when you have something conductive outdoors that is on the CPC and if it were to go to ~100V under the rare PME fault condition it would be a hazard. Just now the focus has been on EV chargers for this case (with someoene washing the car when a fault occurred, etc), but in the past it was prohibited for building sites, etc.

Typical outdoor power tools like lawnmower, hedge-trimmer, etc, are not an issue as they are mostly double-insulated.