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reduced cable

MrJack96
Hi guys just wanted to ask what you think of this. 


there is a bit of temporary kit which runs a 22KW motor. the FLC is 41 amps the cable supplying the motor is 10mm flex this then goes into a JB which someone has connected 6mm YY cable (4 core Lapp 130H) to extend the cable to a local isolater. the local isolator where the supply has been picked up from is a 100amp supply fed from a MCC approx 50 meters away. the method of starting is by soft start. 


I spoke to another Electrician who told me its fine as it is as the overload has been switched down. my thoughts were the overload protects the motor not the cable. I know the maximum capacity of the cable is 44amps so i can understand the overload should trip but is there an issue just relying on the overload. my suggestion was to down rate the fuses after the soft start to 40amps  i know its close but for a temporary measure, the motor runs at 32 amps normally.
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  • 0
    Uh-huh.

    a 100a MCB will not protect a 6mm cable from heat death, if you can trick the load into drawing more than about 50A but less than 100. But, the motor overload relay will respond  essentially like the slow thermal part of an MCB and if it is set to something in the range  40-50A the motor will run and that long duration overload will be impossible, and the cable will last for the duration. (*)

    The second question is now only if the cable gets spiked (an accidental damage sort of short circuit) between the 100A MCCB and the overload relay, is the current enough to get the 100A trip to do its duty before the cable melts, and that is a Zs/pssc trip time question, where you need data on that 100A breaker.

    Assuming a TN supply, and that you do not have a voltage drop problem at full load, then that prompt clearing case will also be met.

    If the Ze is high enough that the 100A may not fire instantly under all conditions,  then it may need more thought.


    Mike


    *And note that  a cable routed in a way that is rated at XX amps does not mean it will burst into flames if it passes XX + 10% all day, nor if it passes twice XX for a few minutes and then has time to cool off. If it happens a lot, it may become  de-plasticised and stiff, and be so much expensive scrap when you wind it in, but it won't fail in an immediate and dangerous way - though it may get painfully hot to the touch.


    It is quite fun to look at the automotive ratings for cables, compared to BS 7671 as they are based on a much hotter, and rather shorter, life  and see that your 1mm2 cable is now good for the  best part of 25A...


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  • 0
    Uh-huh.

    a 100a MCB will not protect a 6mm cable from heat death, if you can trick the load into drawing more than about 50A but less than 100. But, the motor overload relay will respond  essentially like the slow thermal part of an MCB and if it is set to something in the range  40-50A the motor will run and that long duration overload will be impossible, and the cable will last for the duration. (*)

    The second question is now only if the cable gets spiked (an accidental damage sort of short circuit) between the 100A MCCB and the overload relay, is the current enough to get the 100A trip to do its duty before the cable melts, and that is a Zs/pssc trip time question, where you need data on that 100A breaker.

    Assuming a TN supply, and that you do not have a voltage drop problem at full load, then that prompt clearing case will also be met.

    If the Ze is high enough that the 100A may not fire instantly under all conditions,  then it may need more thought.


    Mike


    *And note that  a cable routed in a way that is rated at XX amps does not mean it will burst into flames if it passes XX + 10% all day, nor if it passes twice XX for a few minutes and then has time to cool off. If it happens a lot, it may become  de-plasticised and stiff, and be so much expensive scrap when you wind it in, but it won't fail in an immediate and dangerous way - though it may get painfully hot to the touch.


    It is quite fun to look at the automotive ratings for cables, compared to BS 7671 as they are based on a much hotter, and rather shorter, life  and see that your 1mm2 cable is now good for the  best part of 25A...


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