Best Method For TT Dis Cct

[Gravid]

Hey up, trying to think of the best way to do a job, the incomming supply is TT and is located at the bottom of this fellas drive in a brick bunker, need to get a supply to the house ( 39m away ) and to his garage (16m away from the bunker).

Firstly can I use a 100ma rcd instead of a switch fuse at the supply end or do I have to install both?

Looks like I'll have to run 35mm 2 core SWA to the house, am i ok using the armour to export my earth or should i be using 3 core? and I take it it's ok to use the rod in the bunker as my earth for the whole site ?

Might be daft questions but my heads gone now thinking of all the different ways i could do this job.

Any advice would be great, Cheers.

 

[Gravid]

Sorted this, just needed to sleep on it.

100ma Rcd, into switch fuse, on to house. Easiest way I reckon. Overload and earth fault protection.

Can I use the armour of my 35mm 2 core ?, can't see why not.

My rod in the bunker will be sufficent for the whole site, again no reason why it shouldn't.

Confused myself with too much thinking.

 

[Sintra]

Make sure your 100mA RCD is time delayed for discrimination.

 

[caddy]

Since i assume there will be main bonding to be carried out in the house you will need to check that the CSA of the armouring will be suitable to be used as both the CPC of the submain circuit and as a main bonding conductor.

 

[Gravid]

Am I right in saying the CSA for 35mm 2 core is 62 ?

 

[caddy]

I havent looked it up but 62 sounds a little on the large side. 62 sounds more like the actual steel CSA the copper equivilant is probably half that. Just have to double check to be sure.

 

[Guest123]

I havent looked it up but 62 sounds a little on the large side. 62 sounds more like the actual steel CSA the copper equivilant is probably half that. Just have to double check to be sure.

It is.

The CU equivalent is about 29mm.

 

[steve mc]

use the swa as the cpc for the sub main and isolate it from the house,then plant a new rod at the house and take a new earthing conductor to the met in your consumer unit which creates a new tt supply soley for the house. and when your trying to achieve discrimination between the two rccbs refer to table 41.1 for your max disconnect times.0.2

 

[Gravid]

Steve thats how I was going to do it, till I started thinking do I really need to stick another earth rod in.

I havent looked it up but 62 sounds a little on the large side. 62 sounds more like the actual steel CSA the copper equivilant is probably half that. Just have to double check to be sure.

Right I take it I can use either the adiabatic equation or the table 54.7 method to work this out, is this correct ?

I thought I compared my result with the actual steel csa, am i barking up the wrong tree here or what?

When you say look it up, where do you find this information?

 

[Sintra]

Have a look here gives info on swa armouring sizes.

http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf

 

[steve mc]

The adiabatic equation can be used to size the cpc, and main earthing conductor, but not the main protective conductors these have to-be sized from table 54.8 and copper is used,or another material affording equivalent conductance.

to work out the copper equivalent of a steel wire armour you first have to find the manufactures data for the cable
find the
ohms/km for the copper conductor divided by the ohms/km for the armour.
multiply the answer by the copper conductor size and that will give you the copper equivalent of the swa
then multiply the copper equivilant by 8.8 and that will give you the swa c.s.a