No discrimination, no problem???

[Lazlo]

Had a discussion with a mate about this upcoming job, scenario is as follows -
TNCS to dwelling, 40a MCB @ DB1 (not RCD protected/not required) feeding 6mm2 3core SWA running through loft to outside wall (not buried in walls or ground anywhere) to DB2 (lean to timber shed attached to dwelling), DB2 - 100A DP Isolator with 2no. 30mA RCBO's, one supplying outside sockets in lean to, one supplying DB3 (outbuilding 20m from dwelling with extraneous conductive parts) supply to DB3 in 2core SWA buried @ 600mm in sand bed with warning tape to outbuilding, armour earthed at DB2 to TNCS, SWA armour insulated at DB3. 30ma DP RCD main switch at DB3, TT for DB3. So you have a 30ma RCBO protecting the 2 core SWA run, then a 30ma RCD at DB3. The issue being there is no discrimination between these two but does that matter as only DB3 supply will be affected, faults would trigger either the RBCO at DB2 or the RCD at DB3 but the same circuits will be affected (only those fed by DB3) Would a time delay 100ma RCD @ DB2 and an MCB be a better option......Thoughts please.....

 

[Lazlo]

Sod it probably just get some flex and stick it in some hosepipe. Proper job........

 

[bigspark17]

There is no requirement to rcd the swa to db3.! But there's nothing wrong with the two rcds just no discrimination.! If it gives you peace of mind and the customer is happy to pay for the extra protection carry on. What earth electrode was in place at db3 through curiosity?

 

[Leesparkykent]

Sod it probably just get some flex and stick it in some hosepipe. Proper job........

What size SWA are you running to DB3?

 

[Lazlo]

Not sure yet, try and get a two piece 8ft jobby in but if not one of those 4ft tent pegs I expect. See what Ra we gets.....

 

[Lazlo]

6mm2 to DB3

 

[Leesparkykent]

6mm2 to DB3

Are you also installing the SWA from db1 to db2?

 

[Lazlo]

Yes, 6mm2 as well.

 

[Leesparkykent]

It wouldn't cost that more then to upgrade to 10mm 3core, then you've got a decent earth also will be of adequate size for the bonding at the outbuilding and you wouldn't have to bother installing rods, etc.

 

[bigspark17]

I prefer your original post, tt the outbuilding with extraneous conductive parts

 

[shanky887614]

I prefer your original post, tt the outbuilding with extraneous conductive parts

there installing swa from db1 to db2.

it would be quicker and easier to use 10mm 3c like Lee sugested

even if you run a seperate earth cable from board to db1 or replace feed

 

[bigspark17]

Only tt db3 he posted.! I would go about it this way too

 

[Lazlo]

Yeah true still more expensive than 2 core and rod hence original design. Splitting hairs but 6mm2 2core easier/quicker to work with than the 10mm2.....and extraneous conducive parts....
Don't want to open the exporting earth debate. Thanks for the comments, I've got the info I needed, will go through it with matey tomorrow.

 

[IzzyS]

Because in the event of a neutral earth fault its a pain in the backside and how are you going to carry out the appropriate RCD tests when your tester will be set to 30ma? How will you know what RCD your readings are from when testing when both have tripped?

By connecting the "live" test probe to the outgoing "L", and the other to the incoming "N"? That's what I do when there's an upstream RCD as it doesn't cause an out of balance line / neutral current for the first RCD.