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Bonding Question

me1899
Hi All


Hope you are well - just looking for a bit of guidance on bonding on a job - schematic picture attached.


Basically, 200A head to busbar chamber (TNCS)


63A SPN switch fuse to Building 1 - a big metal shed - 35mm buried SWA 3 core. For main bonding, I understand I have to put 35mm bonds to metalwork. Incoming pipes are plastic. Will I need to bond the incoming BT? Single copper line off the road.


200A isolator to central plant room - 185m away with a 200A MCCB board. This is to feed 5 buildings inc. a 3 phase. Plant room will have an MCB board also for external lighting and air con units which will serve 2 buildings via buried refrigerant pipe. Also a booster pump with metal pipework which is plastic as it leaves/enters the room. Also a central TV/radio system to feed coax out to the 5 buildings fed from here.


As I understand, I need to do 35mm bonds for structure, air con, and also BT incomer and aerial. Believe latter two to be 6mm? Also supplementary 2.5mm bonds for pipes, pumps, metal door frames.


All 5 properties fed from plant room have 35 or 50mm feeds and I understand these all need 35 main bonds for structure, AC pipes (where fed from ac outdoor units at plant room). For coax and BT cables - 6mm bonds? 


In a nutshell - am I correct in my understanding of the bonding sizes, in particular  telecoms and BT feeds? Using combined CPC and bonding conductors. I tend to see 6mm for comms and TV stuff.


A sanity check would be very welcome.


Regards


Muhammad

 
 

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  • 0
    There is little point in bonding to antennas and telecom equipment in a cable size that is much larger than the telecom or antenna cable itself - all that happens if a large current really can flow is  that the thinner cable acts as a fuse to protect the system.


    BS7671 does not handle the (quite likely) case of  earthy metalwork that is not going to be neutral bonded off-site particularly well -  an antenna mast that looks like a pylon with deep footings may  be earthed as well as the local substation, and so be able to carry a substantial fault current, but pipes out to an oil tank on a concrete slab or wires to a TV antenna that goes on wall or chimney and is earthed by wet brickwork is not really in the same league. Sadly the regs see no difference.


    Do the pipes in the out buildings come from the water supply that is already bonded in the first building, or are they external.? You mention plastic - if there is no metallic path to the outside for the pipes then the bonding can be relaxed.


    For mechanical robustness reasons 2.5mm is usually seen as the smallest size cable and if the cable is exposed then 4mm.

    The 35mm sounds like you  need where there is a potential low impedance path that may carry more or less the full  supply neutral current - so from the incoming service head to any metal pipes that go off site, as those metal pipes can be assumed to be bonded to  supply neutral at other sites that share the same substation.




  • 0
    Unfortunately I can't quite read your diagram.


    Double check the bonding size needed - I would have expected a 200A 3-phase supply to arrive on something like a 95mm² cable and presuming the CNE is the same size that would imply a more workable 25mm² bond according to table 54.8 (yours might validly differ though, or have more onerous network conditions).


    I don't every recall seeing a BT line being bonded - many (esp. overhead ones) won't have anything to bond and where an underground cable does have a metallic armour, I think it's more usual just to ensure that armour is covered/insulated so it can't be touched or introduce a potential into the installation, rather than bond it. You'd need the explicit consent of the cable's owner in any event (see reg 411.3.1.2) - which I suspect might not be forthcoming.


    I'm not sure where you get 6mm² bond size from - that's the minimum bonding size for non-PME systems or for connection of type 2 SPDs near the origin. The general minima for individual  protective conductors is 2.5mm² where mechanically protected or 4mm² otherwise - except that main bonding conductors will have to meet table 54.8 of course.


    Bonding metal door frames sounds suspect  - they're unlikely to be exposed-conductive-parts as defined (i.e. introducing a potential) and bonding them creates a significant risk to people just outside the building, especially during faults or open CNE conditions.


      - Andy.
  • 0
    Thanks gents


    Mapj1 - pipes entering and exiting are plastic, metal is just in the room. 


    The water pipes are all plastic coming into or leaving buildings - only metal internal to the central plant room. 


    Andy


    I get where you are coming from - the 6mm is basically something I have seen on old drawings in various practices in the past. These used to show main bonds for everything else as the 54.8 sizes, but only with connections to TV/radio  and BT incoming distribution point being smaller at 6mm - presumably following the principle in Mapj1's post.


    For the outbuildings will be fibre for data and tv now anyway. Just copper coming into the central plant room


    Regards


    Muhammad

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