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Cable routing/eddy currentso

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



  • 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 




    Are you concerned about the motor supply cables inducing currents into the paddle switch cables and causing misoperation? If all of the cables are mains rated and originate from the same supply and are correctly fused for A.D.S. fault protection I can see no problems arising. Does the paddle switch operate a relay or does it carry small currents that operate electronic switching? If the motor is not running then motor currents will not affect the parallel run paddle switch cables. Perhaps when the motor is switched off induced current may cause a problem. The proof of the pudding etc.


    Yes you can run all of the circuit cables through the same hole in the steel trunking (motor and paddle switch) as the magnetic effects of the currents will cancel out. I can see no problem with that.


    Z.




     

  • Hi Z


    No I’m not concerned about cables inducing current into other cables. The paddle switch simply operates the contactor, there are no electronics involved. My query was around the two cables to the paddle switch exiting the steel trunking in the same hole as the motor cables or would the paddle switch cables need to exit in a separate hole? My mind tells me that if they are all in the same hole then any eddy currents should cancel out but wasn’t sure

  • Baldyhugh:

    Hi Z


    No I’m not concerned about cables inducing current into other cables. The paddle switch simply operates the contactor, there are no electronics involved. My query was around the two cables to the paddle switch exiting the steel trunking in the same hole as the motor cables or would the paddle switch cables need to exit in a separate hole? My mind tells me that if they are all in the same hole then any eddy currents should cancel out but wasn’t sure




    If all of the circuit cables run through just one hole in the steel trunking there will be no Eddy currents induced in the metal of the trunking. This includes all of the motor supply cables and the two paddle switch cables. So there is nothing to worry about. 521.5.1.


    Z.


     

  • A John Ward clip....

    https://www.youtube.com/watch?v=hg5eZkq2KgE



    Z.
  • Thanks for your assistance in this. I have the same situation at the other end in the plant room where the supply from the dist board to the contactor, the supply from the contactor to the motor and the two cables to the paddle switch need to exit the steel trunking and enter the contactor. I plan to have one pvc conduit from the steel trunking to the contactor containing all these cables. My reasoning would be that all eddy currents will cancel out if all cables are in the same hole
  • Yes that is correct. If all cables run through the same hole the magnetic effects will be zero so no large heating can occur in the steel trunking. In practice though the heating effect would be pretty small as the load is pretty small, and the coil paddle switch current is tiny, so any heating effect will be negligible. BUT to comply with B.S. 7671 all load carrying cables should definitely run through the same hole.


    Is the supply earthing TT as it is a farm? Are there animals about?


    Z.
  • Hi Z


    It is a farm but I got a new supply in recently for the customer recently and convinced them to give them TN-S for the well discussed earthing/bonding reasons. 

    On another note but related to the initial question, if coming out of a steel trunking   In pvc conduit to a plastic light switch with a plastic back box would you run the live/neutral/earth down to the switch and back out or just the lives. 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


    thanks


    Peter

  • In pvc conduit to a plastic light switch with a plastic back box would you run the live/neutral/earth down to the switch and back out or just the lives.



    Probably just line/switch line & c.p.c. for me. (You can omit the c.p.c. if the switch doesn't need it and it's an installation where unauthorised changes aren't going to be made).


       - Andy.
  • With the older three plate wiring a pair used to run down to light switches from the ceiling rose. A L and switched L. Sometimes on older installations there is no E. These days there should always be an E. terminal connected to a C.P.C. even at plastic switches or boxes, as the E may be needed in the future if a metal switch is installed. These days some wall mounted devices/accessories need a N so one in run down to light switch positions. A C.P.C. should always be run and a E terminal used even for plastic switches. 542.4.1. 


    A C.P.C. shall be run to each point and terminated in an installation. 411.3.1.1


    Z.
  • 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. I’ll agree that it could be beneficial for future use but don’t see it as a requirement. 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