supply to log cabin

[webby_58]

hello all

i have to get a supply to a log cabin and have the following arrangment :

TN-S supply in house
House CU MCB's no RCD
supply to garage CU via 16amp MCB (2.5 t/e) / garage CU has 63a 30ma RCD
approx 35mtr to garage and 50mtr to house in cabin approx 6 sockets maybe an electric heater and 1 light circuit

i propose to spilt the tails in the house into 100a henley block and take a new swa to the cabin. CU in cabin with 30ma RCD 6a MCB for lights 16a MCB for socket ring

my query is the cable size. do i have to use 10mm swa 3 core to provide the correct size earth ?

what would be the best way to do this also taking into account cost.

please help.......!!!!

thanks in advance guys.

 

[Engineer54]

I rest my case m'Lord..... lol!!!

 

[IQ Electrical]

because the fault current required to open the circuits is much lower

By that logic, we'd outlaw TN systems in favour of TT.

 

[Guest123]

because the fault current required to open the circuits is much lower

But the earth fault loop impedance is too high to enable the fuses/MCB's to operate and clear the earth fault so we always need additional protection via an RCD unless you get your Ra below 1.44 ohms as a minimum if you have any 32A circuits..

 

[Engineer54]

He's thinking RCD here, but i'm thinking he is forgetting that any TN system would also require RCD protection in this particular case.... Nothings going to trip on that TT system if the RCD fails ...but it will on a TN system... lol!!!

 

[Guest 004]

But the earth fault loop impedance is too high to enable the fuses/MCB's to operate and clear the earth fault so we always need additional protection via an RCD unless you get your Ra below 1.44 ohms as a minimum if you have any 32A circuits..

but you would also install an main 30mA ELCB which only requires 1667 ohms to trip (although I would not be happy with that)

 

[Engineer54]

But the earth fault loop impedance is too high to enable the fuses/MCB's to operate and clear the earth fault so we always need additional protection via an RCD unless you get your Ra below 1.44 ohms as a minimum if you have any 32A circuits..

Lenny, ...I would consider an RCD as the main and only form of earth fault protection on a TT system of 200 ohms Ra. It's only additional protection on TN systems to be honest!!!

 

[IQ Electrical]

It's just going round in circles.

There is no sound engineering reason not to utilise the earth from a TN-S system to an outbuilding.

I can make a case for a TT system on an outbuilding from a TN-C-S supply but mainly for economic reasons where main protective bonding is concerned.

 

[Guest123]

Well no, an RCD is defined as additional protection, end of.

How and where we use one depends entirely on regulation and circumstance and in this case one is needed due to the high EFLI.

 

[Guest 004]

I do not like exporting earths to outbuildings, as you could introduce a fault that otherwise was not there. Also section 7 of the regs special locations is a good read, and states that several sites must not have a TNCS supply.

That is IMO

If you want to export an earth from a TNS supply then do so, as there is no reg against it, but I certainly would not export an TNCS earth to an outbuilding. IMO.

 

[Engineer54]

Where's the additional protection Lenny?? What protection is it adding too, there is NO protection at 200 ohms, so the RCD can only be the main protection!! Dosn't matter what the good book says, or any other book, ....facts is facts!! This is one of my main problems with domestic TT sytems, ...the common occurance of RCD failures.

 

[Guest123]

Where's the additional protection Lenny?? What protection is it adding too, there is NO protection at 200 ohms, so the RCD can only be the main protection!! Dosn't matter what the good book says, or any other book, ....facts is facts!! This is one of my main problems with domestic TT sytems, ...the common occurance of RCD failures.

Sorry I forgot you are above and beyond the unrecognised BS 7671.

I'm out.

 

[Engineer54]

Sorry I forgot you are above and beyond the unrecognised BS 7671.

I'm out.

Well, ....if your saying that i don't treat BS7671 as if a religious bible, ...your spot on!!

 

[Hawk81]

This thread is going round in circles. Mr enigma you are not the first and probably not the last to try to argue about the exporting of earths. In the start of this thread you said that there was a reg against exporting a TNS earth then eventually you changed your mind. Then you started talking about TNCS which had nothing to do with this thred. I will tell you now there is no reg against exporting a TNCS earth. Now you keep saying IMO that is fine you are entitled to your opinion but you should not tell others that they can not do things just because you are miss informed. Engineer54 we do not alWays see eye to eye but on this subject we mostly do but I must agree with Lenny too.

 

[Deleted member 9648]

I do not like exporting earths to outbuildings, as you could introduce a fault that otherwise was not there. Also section 7 of the regs special locations is a good read, and states that several sites must not have a TNCS supply.

That is IMO

If you want to export an earth from a TNS supply then do so, as there is no reg against it, but I certainly would not export an TNCS earth to an outbuilding. IMO.

Have to say I am in full agreement with hawk.
Mr Enigma,can I ask if you use a TNCS earth to supply other external installations such as lights and sockets,or do you separate them as well?. If you do use the TNCS how do you justify those being safe but an outbuilding not?

 

[Guest 004]

Have to say I am in full agreement with hawk.
Mr Enigma,can I ask if you use a TNCS earth to supply other external installations such as lights and sockets,or do you separate them as well?. If you do use the TNCS how do you justify those being safe but an outbuilding not?

How about you go to the 'real' electrical engineers forum of IET, and do a search of exporting earths there.