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Cable routing/eddy currentso

Baldyhugh
Hi


I have a query regarding a cable route for  a job I’m currently on (when lockdown rules allow me back to it!). I have to supply a feed motor on a farm, the contactor feeding it is at one end of the building in a plant room. There is an existing steel trunking running the length of the building and I plan to come out of this close to the motor with pvc conduit. However there is a paddle switch in the meal hopper beside the motor that will control the contactor. This needs a simple two wire connection. My question is can I run these two cables out of the trunking in the same conduit as the cables to the motor. I plan to put a wiska box or similar at the other end of the conduit so that I can split off to the motor and paddle switch separately, and there will be an isolator before the motor. 

The question however remains regarding the ‘control cables’ in the same conduit as the motor supply. They are from the same circuit and the same voltage but was concerned about eddy currents. 

Thanks in advance
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    Is not the trunking the CPC ?  What gets removed later cannot sensibly be your responsibility.

  • 0
    Hi Mike


    No the trunking is earthed but is not the cpc. The switches and conduit in question are plastic. Your answer kind of echo’s my thoughts as regards leaving an earth cable in a plastic accessory for a future time when someone may change a plastic switch to a metal one. As has been made clear there is a regulation stating this but it doesn’t lessen my annoyance that we have to prepare an installation for what someone may or may not do in the future.
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    I'd be tempted to suggest, that for the purpose of the switch, an earth may be as simple as a short ink to the trunking. It is then not removed with the switch wiring. It is always possible I have the wrong mental picture of course.
  • 0
    Hi Mike


    Yes that would be an option to avoid the possibility of leaving the switch without an earth. I’m still considering both options of just lines and earth to the switch position or L/N/E in and out of each switch position. The latter would leave it ready for any eventuality with having a neutral available and the earth would satisfy the regs previously mentioned.
  • 0

    If I run an earth cable to the switch from one of the lighting circuits, would I also need to run one from the other lighting circuit incase at some point later on the first lighting circuit is removed for some reason, leaving the other circuit live but with no earth at the switch?



    BS 7671 permits two or more circuits to share a c.p.c.(*) - so there's no regulatory demand to do so. (But equally nothing stopping you if you really want to either.) There is an implication that the c.p.c. should be run through the same holes in steelwork etc as the live conductors, which means that the two circuits will have to (mostly) share a common route upstream of the switch, but that's probably not difficult in practice.


    *OK  BS 7671 doesn't say so plainly, but given regulation 543.1.2 describes how to calculate the c.p.c. size common to two or more circuits, it's seems a sensible deduction.


       - Andy.
  • 0
    So just to clarify either of these methods below are acceptable when coming out of a steel trunking in a single hole in plastic conduit to a plastic box and switch;

    Just line/switch line and earth to switch position for both circuits

    Line/neutral/earth in and out of switch position for both circuits


    Thanks in advance

  • 0

    Baldyhugh:

    So just to clarify either of these methods below are acceptable when coming out of a steel trunking in a single hole in plastic conduit to a plastic box and switch;

    Just line/switch line and earth to switch position for both circuits

    Line/neutral/earth in and out of switch position for both circuits


    Thanks in advance

     




    Yes both methods are o.k. 1. Line into switch and switched line out to lights through the same hole in the metal trunking, with suitable mechanical protection to avoid damage to insulation of course, and a single earth terminated in the plastic box. Note. A single line lighting feed and two switched lines out  can also be used. The supply line being common to both lighting circuits and linked at the switch. So one supply line in and two switched lines out through the one hole is o.k. with a two gang switch.


    or.....


    2. Supply line in to switch+neutral feed in to switch box, then switched line(s) out to lights and unswitched neutral out to lights. Earth (C.P.C.) in and out as well.


    Method 1 is easier. Method 2 is perhaps more suited to domestic or office installations where a switch upgrade to an electronic control needing a N. may happen at a later date.


    Z.




     

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    There sometimes is a possible benefit from having earths from both circuits connected.


    1/ Effective R2 reduced as via parallel paths on at least some of the earth path.

    2/ Redundancy - a break in cpc somewhere on one circuit might possibly be mitigated to some extent.


    On domestic rewires I tend to link all the earths on lighting circuits and that is usually acheived on two way lighting - ganging switches so at least two circuits present in switch backbox , often at stairway lighting. Of course anyone woprking on such switches would need to isolate two or more circuits before proceeding into the switchbox (We all always isolate before opening up dont we? ? )


    PS - One thing to note - any R2 testing must be done on each circuit separately to omit para paths to prove integrity of each circuit E
  • 0

    mapj1:

    I'd be tempted to suggest, that for the purpose of the switch, an earth may be as simple as a short ink to the trunking. It is then not removed with the switch wiring. It is always possible I have the wrong mental picture of course.




    Indeed Mike. I expect that you were thinking of 543.2.7.


    "Where the protective conductor is formed by metal conduit, trunking or ducting or the metal sheath and/or armour of a cable, the earthing terminal of each accessory shall be connected by a separate protective conductor to an earthing terminal incorporated in the associated box or other enclosure."


    Also 543.2.5.


    Z.

  • 0
    Thanks Z


    Method 2 can all go through a single hole Containing both circuits if I’m correct or separate holes for each circuit is also acceptable?

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