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Thread: switching neutrals

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    switching neutrals

    having a "debate" with engineers ,consultants+electrician . its a new factory installation incorporating lots of motors,conveyors,plcs etc. they want me to inspect and issue the coc.the engineer designed the system where the 230 volt contactors/relays have most of the switching on the live side of the relay,but the limit switches are all switching the neutral side.
    i failed it. they reckon its safer as you only get a tickle instead of the full 230 volts as the coil of the relay will drop the voltage. i say if the cable is damaged to earth then the limit switch is bypassed and the relay is energised(from the 1 side anyway)
    any thoughts?

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    Diamond Member AndyD's Avatar
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    Why does the control panel and the extraneous devices like limit switches fall under the COC?

    I can see what they're saying, if a user contacts a live that's limited by a resistor then the fault current would be less. There's two holes in that arguement that I can immediately see;

    1 Limiting an earth leakage fault at higher than 30mA is a moot arguement as the 30mA figure is the maximum fault that ought not cause defibrilation and death of the recipient. If the fault current could possibly exceed 30mA then it's not a valid form or user protection and using a contactor coil (less than 7333ohms) as a limiter you couldn't predict the size of the fault current.

    2 A coil does not have a linear resistance, as the current decreases so does its impedance. It relys on inductance to limit the current. When you first apply power to a coil it draws a large current until the ferrous core it's wrapped around becomes saturated, similar to a motor start current. This initial start-up is the leakage current that would flow through the user in the case of a fault. It would be almost impossible to guarantee that this current would be less than 30mA due to the number of unknown variables in the circuit.

    I'm not buying the fact that it's safer to switch the neutrals as such although in certain fault conditions it may well be. If there was an isolating transformer set up then this would make it safer. That said it's not uncommon practice to switch neutrals withing machine control systems, it's more uncommon that the machine controls would need to comply to the domestic installation regs.
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    thanks for the help andy.
    normally a "machine" is not part of the coc,but where the different parts that make up the machine is in such a big scale that it takes up a whole building then i would say it would need inspecting. i wont be checking the process control,etc.
    if there was an isolating transformer ,there wouldnt be a live or neutral( 2 lives?)
    i think if the engineer who designs the design part of the coc then i will accept it. otherwise not.

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