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Flex cable ties to uni strut.

MrJack96
Should the unistrut be bonded to the installations metal work. A measurement from the Starter Panel to the metal unistrut reads 0.12megaohms but as the flex is cable tied to it is it a requirement? The flex is ran from a plastic junction box mounted on the unistrut, the flex is ran from the JB to a 110v solinoid approx 1 meter away. Thanks for your help guys.
  • 0
    For protection against electric shock you only need to bond extraneous-conductive-parts (and earth exposed-conductive-parts).


    Unless the unistrut can introduce a potential into the installation (e.g. from true earth or a fault in another installation) it's not going to be an extraneous-conductive-part.


    If the cables fixed to it are sheathed and suitable for the environment then they should already give adequate protection from shock, and any metal they're in contact with isn't going to be made live by a simple failure of basic insulation - so won't be an exposed-conductive-part.


    Some customers might still prefer lots of metalwork to be bonded even though it's not strictly speaking an exposed- or extraneous-conductive-part (sometimes for valid EMI or functional reasons) - but there's no fundamental BS 7671 demand to do so, at least from a shock point of view.


       - Andy.
  • 0
    Andy you’re the man. Thank you. I take it in my case it does not need a connection to earth as it doesn’t present an earth potential. If it did present an earth potential is the easiest way to get this to comply by just linking the earth ring of the gland closest to the unistrut and connecting it to earth that way through the armour of the cable?
  • 0
    If it did present an earth potential is the easiest way to get this to comply by just linking the earth ring of the gland closest to the unistrut and connecting it to earth that way through the armour of the cable?

    If it was an extraeous-conductive-part as far as the installation was concerned it would need main bonding - i.e. full sized bonding conductor all the way back to the MET (e.g. 10mm² copper for a typical domestic, probably a lot larger for a industrial) - that could indeed be 'borrowed' from a more local circuit's c.p.c. but the size requirement all the way back to the MET remains, so it's only usually practical for submains to local DBs rather than small final circuits as their c.p.c.s would be too small. With steel armour used as the c.p.c. and PME earthing it's even harder as the requirement for equivalent conductance to copper pushes the required c.s.a. up by a factor of 8 or 9.


    If for some reason you needed supplementary bonding (e.g. it was some kind of special location and the metalwork may introduce a potential from elsewhere in the installation but not directly from outside it), then bonding to local c.p.c.s would be more much more usual.


       - Andy.
  • 0
    It’s industrial I work for a water company. the main conveyor is main bonded back to the starter panel and all the cable tray has been supplementary bonded to the conveyer. There was just the uni strut that wasn’t. But if it was measured as having an earth potential I’d have to run a main bonding conductor back to the panel? I’ve seen it before on sites where the cable tray and uni strut are bonded together and sometimes have been linked to hand railing with an earth strap?
  • 0
    "Some customers might still want metalwork to be bonded"

    Its always best to bond scenarios like this, industrial, saves problems later, we always expect to see bonding. There is a "guide" type of book, I cant remember the title, which discourages electricians to bond anything, although they have been bonding for 50 years. For example, the anonymous author demands a steel fuse board is unearthed. As the years go by, cables will heat, come loose and short to the unearthed frame. 


    Regards, UKPNZap
  • 0

    Let's look at the facts, rather than making assumptions and firing assertions off in places they don't belong.


    A conveyor system is very likely to be "Machinery" as defined. It's probably not even within the scope of BS 7671.


    If it is, BS EN 60204-1 applies.


    Unlike BS 7671 (or any "guide" book for general electricians using that standard), BS EN 60204-1 has a requirement for the majority of structural parts of the machinery and/or electrical equipment to be bonded (Clause 8.2.1), except in special cases where Clause 8.2.5 applies.



    So it might hinge on whether you class the unistrut as a structural element of the machinery or the electrical equipment of the machine.


  • Former Community Member
    0 Former Community Member

    MrJack96:

    It’s industrial I work for a water company. the main conveyor is main bonded back to the starter panel and all the cable tray has been supplementary bonded to the conveyer. There was just the uni strut that wasn’t. But if it was measured as having an earth potential I’d have to run a main bonding conductor back to the panel? I’ve seen it before on sites where the cable tray and uni strut are bonded together and sometimes have been linked to hand railing with an earth strap? 


    I would have thought that WIMES would cover this. Whilst probably not necessary for the reasons given above, you water chaps do like your bonding so I'd bet someone will pick it up and ask why they can't see a bond.


    Regards


    OMS


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