Log Cabin Electrics

[DARTHVADER]

Hi All, been a long time since I posted.

I have an swa cable (6mm 3 core) buried to a very deep depth down the edge of my garden, ready to go into a log cabin. Being delivered on the 30th Sept.

The cable run is 20m long from cu to Cabin.

I intend to use metal clad box sited next to cu, and terminate the SWA into this, and carry the earth cable (3rd conductor in the swa) to the earth block in the cu. Do I also need to carry a separate short earth wire from metal clad box to earth terminal in cu ?

I intend to take supply from a spare way in the cu from the non protected rcd side, and install an rcd protected garage type & metal clad (so as i can terminate swa straight to it) cu in the cabin. I will require approx two double sockets, and a light. Consumption will be light as only running a treadmill, small heater in the winter & possibly garden tools now and again.

The electrical supply to my house is i'm advised defo not TT, and is either TN-S or TN-C-S. This info from the supplier.

Is it OK to export the earth via the SWA, and of course utilising the 3rd core as mentioned.

The cabin is site approx 10 metres from the front of the house.

Many thanks in advance for any help.

 

[ricicle]

If you are going to export a TNC-S earth and you (more than likely) have extraneous conductive parts entering the log cabin (eg a water pipe) then the minimum size of earth to the cabin required is 10mm².
A TT set-up at the cabin sounds like it will be needed. This involves isolating/insulating the TNC-S earth in the SWA from the new TT earth (via a rod).
The armour earth will protect the cable. This is not a DIY job !

 

[DARTHVADER]

Thanks Ricicle.

I can confirm that no water pipes or any other services enter the cabin.

Will this make a difference to the earthing arrangements?

Many thanks,

 

[sparkticus]

Thanks Ricicle.

I can confirm that no water pipes or any other services enter the cabin.

Will this make a difference to the earthing arrangements?

Many thanks,

If that is definitely the case then you could export the TNC-S earth assuming you do actually have TNC-S.

Remember that even when you use an internal earth conductor you must still ensure that the outer armour is earthed via proper terminations (piranha nuts are in my opinion best)

http://www.tlc-direct.co.uk/Main_Index/Earthing_Index/Piranhna_Earth_Nuts/index.html

 

[DARTHVADER]

Hi Ricicle,

I can confirm than no other services will enter the cabin.

Thanks for the link to... (piranha nuts are in my opinion best)

tbh the supplier didn't seem too sure as to tns or tncs. They were sure it wasn't tt though.

Can you differentiate the two by looking at the incoming swa lead?

Cheers

 

[sparkticus]

Hi Ricicle,

I can confirm than no other services will enter the cabin.

Thanks for the link to... (piranha nuts are in my opinion best)

tbh the supplier didn't seem too sure as to tns or tncs. They were sure it wasn't tt though.

Can you differentiate the two by looking at the incoming swa lead?

Cheers

I am assuming you are competent with electrics and with electrical safety.

Look at the service head and identify the main earth connection and the main neutral connection. With everything off at the CU (and the main earth disconnected to the CU) do the following:

Measure across the MET and Neutral.

1) first safety check for voltage using a voltmeter.
2) measure the resistance.

If the resistance in really low < 0.05 ohm then it is probably TNC-S since the low resistance will be mostly a function of the link in the cutout.

If way above 0.05 then probably TN-S


PS, you will probably need to probe the Neutral at the CU since there is probably no opportunity to probe it at the service head.

 

[DARTHVADER]

Thanks for that sparkticus.

I do have test equipment (used to do it for a living, but many years ago)

Is there a way of visible recognition?

Will TNS or TNC-S make a difference to exporting the earth to the log cabin? i.e. is one better than the other for this.

 

[ricicle]

You cannot swap between TN-S and TNC-S so what you have is what you've got. You can only change them to TT, which, if you have TNC-S (PME) will be a requirement if the cabin is likely to have an incoming metallic service at any time in the future as your earth is only 6mm².

 

[JohnD]

the easier way to identify TNC-S is to look for a label on the incoming backboard which says "this installation has Protective Multiple Earth" or similar words. The supplier is obliged to affix such a label if you do.

Or look at/photograph the incoming service, the head may have an earth terminal on it. You can't always tell if it is TNS, TNC-S or just not connected to anything by looking, but it will give some clues, and then if need be you can do the measurements.

 

[DARTHVADER]

Thanks again ricicle,

I think I'm 99.9% there on understanding this now.

Its highly unlikely that I will add any more services, due to the work and complexity in getting them there!

Just wanted to be sure that if I export the earth (i.e. not a rod) then that could be done with both TNC-S and TNS ?

Thanks,

 

[ban-all-sheds]

I intend to take supply from a spare way in the cu from the non protected rcd side, and install an rcd protected garage type & metal clad (so as i can terminate swa straight to it) cu in the cabin. I will require approx two double sockets, and a light.

Are you aware that all of this is notifiable?

 

[ban-all-sheds]

I am assuming you are competent with electrics and with electrical safety.

On what basis?

 

[sparkticus]

Thanks for that sparkticus.

I do have test equipment (used to do it for a living, but many years ago)

Is there a way of visible recognition?

Will TNS or TNC-S make a difference to exporting the earth to the log cabin? i.e. is one better than the other for this.

With TN-S you may see a direct main earth connection to the outer sheath otherwise unless the TNC-S is labeled then no real visual way to identify.

There really is no best system.

With TNC-S the earth and neutral are one in the same which is fine unless:

1) The neutral is broken. Then the TNC-S earth potential can rise way above true earth potential causing a potential difference between the two.

2) The TNC-S earth is typically at a higher potential to that of true ground in normal service.

This simply makes it prudent to think carefully about exporting TNC-S. If you had any extraneous conductive services entering the cabin then there could well be a danger of a PD between them and the TNC-S earth you just imported into the cabin. For that reason conversion to local TT would be indicated/wise.

With TN-S the earth combines with the Neutral only at the star point of the sub-station transformer (in theory) I say in theory because you never know if someone has accidentally/or otherwise connected neutral and earth together in a nearby house.

 

[DARTHVADER]

Ban all Sheds, yes I'm aware.

Sparkticus, many thanks for comprehensive reply on the difference between the two.

 

[DARTHVADER]

thank you Johnd