Separate main bonding to outbuilding

Is main protective bonding actually required, you have not answered the question? You need to FULLY understand 411.3.1.2 etc. to decide.

Yes bonding is required in the second building.

KS

Ok, you need bonding. Now what are your extraneous parts and could they contribute diverted neutral current? Are they external metal service pipes, water or gas or iron sewers, or are they odd items with a significant resistance to Earth? The size of the main bonding required needs some consideration, as if all these have a resistance of say 100 Ohms to Earth, the diverted current is limited to a low value. Main bonding conductor size is directly related to the potential current. You say the supply is TN-S, this is no longer a reasonable assumption due to DNO policies, repairs to the distribution network may change this at the drop of a hat. What size is your submain cable? You may like to refer to table 54.8, in all cases the SWA armour is large enough for main bonding up to 95mm2. Remember the main bonding conductor(s) never need to be larger than the supply neutral even with PME, and for 100A supplies 10mm2 is normally considered adequate. I am afraid that somehow you have gained the wrong information here, many people do not understand main bonding.

Thanks for that David, I will go away and do some more investigation.


KS

I must admit that I have pondered once or twice that if both cpc and bond required then are they additive or just the greatest one required..


The often mentioned "fulfils both" does not make it clear

As they are extremely unlikely to carry both currents at once, the largest size is enough. Failure due to fault current is very unlikely anyway unless this is a continuous diverted neutral current. You can immediately understand this in practice as you will almost certainly never have seen an overheated main bond, and it is unlikely even if there has been a neutral "incident". Making the bond larger than the incoming neutral is obviously unnecessary, the cable enclosed neutral will melt first whatever the bond does. One needs to be careful of "what if's" as most of them have never been seen or happened.

 You may like to refer to table 54.8, in all cases the SWA armour is large enough for main bonding up to 95mm2. Remember the main bonding conductor(s) never need to be larger than the supply neutral even with PME, and for 100A supplies 10mm2 is normally considered adequate.


But consider if the incoming supply neutral is say 120 mm2 and you're taking a 25 mm2 4c SWA out to a building that requires main bonding.....................


Regards


BOD

That is nowhere near a domestic supply BOD. But you are trying to wind me up! We need to know what these extraneous parts are and what the Re is before a difinitive reply is possible. Perhaps you know?


Kind regards

David

That is nowhere near a domestic supply BOD. But you are trying to wind me up! 


Not sure where domestic supply gets a mention in the OP though..............


Admittedly my real life example is indeed a domestic albeit a large house where the DNO took the incoming service straight into the cellar into a 400 A cut-out and downrated the fuses. The installer assumed that as 100 A fuses, 10 mm2 would do. Unfortunately for him the Reg refers to the incoming neutral size, not that of the cut-out fuse. Quite an expensive job for him to rectify. 


I did suggest that he could ask the DNO to do what they normally do and terminate their 185 mm2 just outside the property and joint to 35 mm2 and take into the property to a 100 A cut-out. Unfortunately the owner had just spent a fortune on red tarmac so he wasn't going to allow a patch up job in it. Besides which he paid a lot of money to get the HV line and Tx moved out of view, hence the need for 185 mm2 due to distance. Plus he didn't want his £500k+ of wine/port disturbed.


Whilst I note that 10 mm2 would have been adequate with the second option and compliant, the "as installed" did not comply. As to the difference in reality could be argued at length, it is a Reg and Table 54.8 still applies. 


No amount of experience or arguments can alter that Reg requirement...................... Unless you were to change the earthing arrangement and go TT of course............


Regards


BOD

Besides which he paid a lot of money to get the HV line and Tx moved out of view,

Presumably he wasn't the only consumer on that Tx then - otherwise it might have been PNB rather than PME - then table 54.8 could have been neatly side-stepped.

   - Andy.