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.....

 

[Leesparkykent]

Why not just change the OCPD feeding DB3 to a MCB? Why would you need a 100ma S type in DB2?

 

[shanky887614]

Why not just change the OCPD feeding DB3 to a MCB? Why would you need a 100ma S type in DB2?

because he said he wants to
TT
db3, i dont understand why you would, your feeding db 2 and 3 via swa so you dont need an rcd

 

[Lazlo]

Want to RCD protect the buried cable. I know it's not a requirement as SWA offers additional protection 522.6.204 but I also note 415.1.2 'or carelessness by users' ie driving a metal fence post through it (these go deeper than 600mm and are conductive), it is a garden after all. Does the armour on SWA really offer substantial enough mechanical protection for it's intended environment, under a garden?? If not then 30ma RCD is required? Ignore the 100ma SType for now.....

 

[shanky887614]

Want to RCD protect the buried cable. I know it's not a requirement as SWA offers additional protection 522.6.204 but I also note 415.1.2 'or carelessness by users' ie driving a metal fence post through it (these go deeper than 600mm and are conductive), it is a garden after all. Does the armour on SWA really offer substantial enough mechanical protection for it's intended environment, under a garden?? If not then 30ma RCD is required? Ignore the 100ma SType for now.....

think about what your asking.

it would be difficult to put a spade or post through it without hitting the earthed steel armour first

 

[Lazlo]

OK but if the sheath was to become damaged by putting in fence posts on year 1, armour open to elements then rusts, fence posts or excavation for greenhouse year 3.....Bang.

Is less protection better than more?

 

[shanky887614]

OK but if the sheath was to become damaged by putting in fence posts on year 1, armour open to elements then rusts, fence posts or excavation for greenhouse year 3.....Bang.

then no warning tape?

 

[Leesparkykent]

because he said he wants to
TT
db3, i dont understand why you would, your feeding db 2 and 3 via swa so you dont need an rcd

The OP says the outbuilding where db3 is has extraneous conductive parts, the earthing arrangement is TNC-S and the SWA feeding DB2 is 6mm.

So the SWA feeding DB2 and the SWA from DB2 feeding DB3 would both have to be a minimum of 10mm or an appropriate sized bonding conductor would have to be installed. If these aren't an option then you are left with making DB3 a TT system due to the extraneous conductive parts.

As the cable feeding DB3 is SWA in can be placed on an MCB and the TNC-S earth can be isolated at DB3.

 

[Lazlo]

Fence posts are driven in not dug so warning tape wouldn't make any difference, it is going to be used as per my post.

Surely more protection is better than less for the sake of £30??

 

[Leesparkykent]

OK but if the sheath was to become damaged by putting in fence posts on year 1, armour open to elements then rusts, fence posts or excavation for greenhouse year 3.....Bang.

Is less protection better than more?

Put the SWA in a duct so if it ever needs to be replaced it can easily be done.

 

[shanky887614]

The OP says the outbuilding where db3 is has extraneous conductive parts, the earthing arrangement is TNC-S and the SWA feeding DB2 is 6mm.

So the SWA feeding DB2 and the SWA from DB2 feeding DB3 would both have to be a minimum of 10mm or an appropriate sized bonding conductor would have to be installed. If these aren't an option then you are left with making DB3 a TT system due to the extraneous conductive parts.

As the cable feeding DB3 is SWA in can be placed on an MCB and the TNC-S earth can be isolated at DB3.

as long as swa armour is isolates from board at db3 then no need to earth supply like you say then tt at db3.

 

[Lazlo]

Ducting is a good option, no need for sand bed either. Still would like to RCD protect that SWA. Anyway the original question is what is the problem with having the two RCDs and no discrimination for this leg if only DB3 is affected?

 

[Leesparkykent]

Ducting is a good option, no need for sand bed either. Still would like to RCD protect that SWA. Anyway the original question is what is the problem with having the two RCDs and no discrimination for this leg if only DB3 is affected?

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?

 

[Lazlo]

Test the rcbo at DB2, put 6mm 2 core into MCB (for test only) at DB2, test DB3, swap out MCB for tested RCBO @ DB2.

 

[Leesparkykent]

Test the rcbo at DB2, put 6mm 2 core into MCB (for test only) at DB2, test DB3, swap out MCB for tested RCBO @ DB2.

Happy days then!