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Bonding both sides of an swa cable

MrJack96
Is it necessary to bond both sides of an armoured cable if it’s not being used as a CPC and if you bond it at the supply side of an electrical motor starter panel is there anything in the regs where it should be bonded too or can it just be bolted to the chassis stud using a 6mm fly lead off the gland. 


thanks for your help in advance guys.
  • 0
    are you relying on the earthed armour to avoid having to have an RCD at the origin, and or conduit or a visible route?


    If so it should be earthed at the supply end.  It need not be bonded at the load end, but unless you are creating a TT island you normally would connect to the CPC at both ends, it just lowers the effective CPC resistance a bit, and it is also good practice from an EMC point of view.

    Regs do not oblige you to earth at one end or the other or indeed in the middle, but requiring ADS to work normally means you do.
  • Former Community Member
    0 Former Community Member
    It should be earthed at least at the source end, as it is clearly an exposed conductive part.


    Most calculation methodologies and much tabulated cable data assumes that the armour is reliably connected to earth at both ends.


    If the switchgear, MCC's etc is metalclad, then you achieve this simply by the use of the correct cable gland, banjo and bolt (as you describe with a connection from the "chassis stud"). Whether you chose to connect the banjo to the switchgear earth bar in addition to your core CPC is up to you, but good practice says that you should as either the core, or the armour or both may need to carry fault current . It ultimately depends on how the gland plate is fitted (or is integral to the metal enclosure ?)


    Regards


    OMS
  • 0
    Previous posts are correct from a safety perspective.


    But Mike also mentioned EMC. If the motor is running from a VSD, then it's advisable to bond both ends, using glands that provide a 360-degree bond to the enclosure or motor.


    However, check the VSD and motor manufacturer's installation instructions - as a result of the EMC testing they carried out, they may recommend a higher level of screening provided by:

     - Metal conduit

     - SY or CY cable (but in that case, full 360-degree SY/CY glands are recommended, and see section 7.9 of the O-Site Guide (Page 90 in the latest 2018 edition) regarding selection of cables.
  • 0
    Very common to see the armour not earthed. In fact was doing an inspection yesterday on a large golf club where some of the smaller outbuildings were wired in 3-core swa with one of the cores used as earth. However, the armour was cut back at both ends but the wire ends were taped over. That may mean that the armour does not meet the definition of exposed conductive part.
  • 0
    OK, but if the armour is not earthed at all, at the risk of stating the obvious,  the cable does not meet the regs requirement for a cable with an earthed armour, and is then to be treated as you would perhaps twin and earth, in terms of safe zones where you can route it and if it needs an RCD at the origin in case it is damaged.


    Given where and how SWA is normally used,  there are few cases where electrically floating the armour makes any sense - if you do not need an earthed armour,  then use a cheaper cable. If you do need it, then earth it


    I have seen the armour terminated not in glands but combed as a 'tail' into large terminal blocks, in much the same way that concentric cables are terminated at DNO fuses etc. It does not provide the mechanical anchor like  a normal gland,  but n those odd  cases where that is not needed, electrically such a termination  can be very satisfactory. (though I like to see heatshrink and there almost never is.) Certainly a tail is better than no connection.
  • 0
    mapj1:

    OK, but if the armour is not earthed at all, at the risk of stating the obvious,  the cable does not meet the regs requirement for a cable with an earthed armour, and is then to be treated as you would perhaps twin and earth, in terms of safe zones where you can route it and if it needs an RCD at the origin in case it is damaged.


    Given where and how SWA is normally used,  there are few cases where electrically floating the armour makes any sense - if you do not need an earthed armour,  then use a cheaper cable. If you do need it, then earth it


    I have seen the armour terminated not in glands but combed as a 'tail' into large terminal blocks, in much the same way that concentric cables are terminated at DNO fuses etc. It does not provide the mechanical anchor like  a normal gland,  but n those odd  cases where that is not needed, electrically such a termination  can be very satisfactory. (though I like to see heatshrink and there almost never is.) Certainly a tail is better than no connection.


    Agree with Mike.


    If buried, Regulation 522.8.10 applies even for armour of ELV cables.


    Taking what Mike says a little further, also if not buried, then we need to consider whether the armour is or is not an extraneous-conductive-part.


    We all know that Insulation + Sheath cables are treated as double or reinforced insulation, but what separates the armour from the live conductors? Strictly, according to the cable construction standards, it's Insulation + Filler.


    As the plastic around the insulated conductors is not intended as a sheath, but a filler (the sheath being over the armour, and not under it), we must assume that the armour is an extraneous-conductive-part in LV and FELV armoured cables.


    I suppose you could consider insulating the armour to avoid any contact (that would including during maintenance and testing activities), and it couldn't be terminated in an SWA gland - but that insulation would have to be done by a method that requires destruction (e.g. properly insulated using cold shrink sleeving or similar, not simply wrapping with tape).


  • 0
    Well I would not disagree chaps, firmly with you that it should be earthed. Take the golf club example. The swa cable is in a Wavin duct underground for the full route so 522.8.10 does not apply. The wire armour is not earthed but is taped up albeit inappropriately. Quoting regulations that are infringed is, of course, important, but an inspector carrying out a periodic inspection has to form a view on whether or not danger arises. If this was a new installation, the contractor would be back but I am inclined to consider the risk as tolerable and will only mention it in the interest of ensuring that any further installation activity is appropriately compliant.

  • Former Community Member
    0 Former Community Member
    Potentially if this is feeding a motor of a machine then BS 7671 is irrelevant. (110.2,xi)

    If this is on a VSD, then the SWA will be probably useless as a screen.

    Most we have tested are worse than un-screened.

    Many drive OEM's forbid SWA to the motor for this very reason.
  • 0
    So if the gland plate is bonded to the chassis of the MCC would you again run a separate flylead to the earth bar.
  • 0
    So if the gland plate is bonded to the chassis of the MCC would you again run a separate flylead to the earth bar.

    The regs aren't as specific as that - the requirement is to have a suitable and reliable connection - but there are many ways to achieve that and choosing which is up to you (or occasionally customer specifications).


    If you have a thin gland plate that's covered in paint and held onto the rest of the enclosure by a few feeble screws and don't bother removing the paint from either under the glands or from around the fixing screw, then you'd probably want a fly lead from each gland, sized appropriately for the circuit, to a good earthing point or something similar. If on the other hand you had good solid gland plate, fixed by substantial bolts and with all contact areas clean bare metal but protected from corrosion and with a c.s.a. that was more than adequate for use as as c.p.c. in those materials then you might not need anything extra at all.


       - Andy.

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