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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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    My question was enquiring about different methods but also if an earth was required along with the live and switched live for eddy current reasons



    As long as the total currents balance you're fine. L will be balanced by SL (or Lin by Lout and SL if L is looped) in normal conditions (and L by PE if there's a L-PE fault). Looping a N in and out that's not connected to a load in the switch will still balance just the same - just with the N current added to both sides as it were.

       - Andy.
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    Baldyhugh:

    Thanks Z


    I don’t think that my reading of 542.4.1 says that an earth terminal must be provided at a plastic accessory incase future alterations require an earth terminal, for example, changing a plastic light switch to a metal. 




    Please see 411.3.1.1


    Z.

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    Thanks Andy


    So in my example of plastic conduit and plastic switch and box coming from a steel trunking, it is actually a two gang switch on two different circuits, it is acceptable to have (in this example) just a live and switch live for each circuit down to the switch but an earth isn’t required as regards eddy current considerations as there can be no possibility of an earth fault at the switch due to its plastic construction?


    Thanks again
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    Thanks Z


    i suspect that this regulation is in anticipation of someone changing the accessory, switch in my case, to a metal one for example. I couldn’t see this happening in my case but I’m grateful for you pointing it out


    Peter
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    So in my example of plastic conduit and plastic switch and box coming from a steel trunking, it is actually a two gang switch on two different circuits, it is acceptable to have (in this example) just a live and switch live for each circuit down to the switch but an earth isn’t required as regards eddy current considerations as there can be no possibility of an earth fault at the switch due to its plastic construction?



    That's about the size of it!

     

    Please see 411.3.1.1



    I'd suggest otherwise.


    If it's an all-insulated switch then protection against electric shock is by double or reinforced insulation rather than ADS (how could ADS operate without any earthed metalwork?) Thus it's not section 411 that applies but 412. It's therefore the (not greatly dissimilar) regulation 412.2.3.2 we should be looking at. 412.2.3.2 does have a get-out though (via 412.1.2) where the system is controlled so that unauthorised changes (such as the introduction of class I accessories) can't be made.


       - Andy.
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    AJJewsbury:




    So in my example of plastic conduit and plastic switch and box coming from a steel trunking, it is actually a two gang switch on two different circuits, it is acceptable to have (in this example) just a live and switch live for each circuit down to the switch but an earth isn’t required as regards eddy current considerations as there can be no possibility of an earth fault at the switch due to its plastic construction?



    That's about the size of it!

     

    Please see 411.3.1.1



    I'd suggest otherwise.


    If it's an all-insulated switch then protection against electric shock is by double or reinforced insulation rather than ADS (how could ADS operate without any earthed metalwork?) Thus it's not section 411 that applies but 412. It's therefore the (not greatly dissimilar) regulation 412.2.3.2 we should be looking at. 412.2.3.2 does have a get-out though (via 412.1.2) where the system is controlled so that unauthorised changes (such as the introduction of class I accessories) can't be made.


       - Andy.

     




    The O.P said........ "Reason I ask is that I have seen different methods of only lives to the switch and then some as L/N/E. Obviously a metal switch would require an earth cable ran to it and in domestic settings it is becoming more common to have a neutral in the switch box for a variety of reasons, but in my example of trunking/conduit coming out of a metal trunking to a plastic switch I was curious to other peoples methods"


    The O.P. refers to light switches in "domestic settings" and I replied with reference to domestic settings saying that a C.P.C. will be required at all electrical "points". Why wouldn't they? I think that you are muddying the water unnecessarily saying that a C.P.C. is not required. This could cause confusion. 411.3.1.1 is a coverall requirement in most domestic and many other cases. Running a C.P.C. to a plastic box is no great difficulty and will cause no harm. In fact it will be good to have if needed in the future. In a domestic situation the householder may change a plastic light switch for a metal plated switch that requires earthing. A readily available C.P.C. will make this upgrade easy and safe.


    I would not rely upon the 412.1.2 requirement that effective measures such as by adequate supervision will be reliably in place here at the farm,  so that no change can be made that would impair the effectiveness of the protective measure of Double or Reinforced insulation. Any old body could alter the installation. "Oh, I'll put a metal switch here as the old plastic one is cracked. A metal one will be stronger safer. Oh, there is no earth. I'll just have leave the earthing then."


    Can we really be sure that changes won't be made by a user without authorisation? 412.1.2.


    Z.

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    I do like the application 412.2.3.2. in this example at the farm. I don't like the 412.1.2 get out clause for any installation that is not meticulously run and supervised.


    412.2.3.2  " Except where 412.1.2 applies, a circuit supplying one or more items of Class II equipment shall have a circuit protective conductor run to and terminated at each point in wiring and at each accessory. NOTE: This requirement is intended to take account of the replacement by the user of Class II equipment by Class I equipment."


    Z.
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    Thanks both for your comments on this post. Just another point, my example involves a two gang switch on two different circuits so there will be a two lines from the db, one to each switch and two switch lines, one leaving each switch. What are your views on a single conduit containing different circuits?
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    my example involves a two gang switch on two different circuits so there will be a two lines from the db, one to each switch and two switch lines, one leaving each switch. What are your views on a single conduit containing different circuits?



    No problem at all - L & SL for circuit 1 will balance, likewise L & SL for circuit two - combined together they still balance. So you can have one conduit for all 4 wires, or two conduits one for L & SL for circuit 1 and the other for L & SL for circuit 2. The only thing to avoid would be to have two conduits with both Ls in one and both SLs in the other. (c.p.c.s omitted for clarity ;-) )

       - Andy.
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    Thanks for your replies. Another point has come to me. 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?

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