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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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    Why would you do any of these things? Really you have asked the wrong question, presumably, you have no understanding of circuit protection. The motor overload protects the whole circuit, as does the CPD. The sustained circuit current is set by the overload to a satisfactory figure for the cables you have described. The question is "would the 100A CPD open before the 4mm cables under short circuit conditions?". The answer is to use the adiabatic formula and you will find out if it is satisfactory. The circuit PSCC is important as if (it almost certainly isn't) it is quite low the 100A fuse may go outside the expected fusing time and cause the cable to overheat seriously outside the adiabatic time limit. This is probably not particularly important as it is a temporary arrangement. The soft starter fuses are probably 63A and will probably fail first anyway. There is no reason at all to suggest that the overload may not open under overload conditions, they are very reliable and the most likely fault would be that the soft starter does not disconnect the load.


    The circuit is therefore protected against short circuits, sustained overload, and motor faults in a satisfactory way. If you fit 40A fuses it is possible that you will get a failure, the loss of a phase, and possible motor damage. This would be stupid and expensive, and more importantly completely unnecessary.
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    Why would you do any of these things? Really you have asked the wrong question, presumably, you have no understanding of circuit protection. The motor overload protects the whole circuit, as does the CPD. The sustained circuit current is set by the overload to a satisfactory figure for the cables you have described. The question is "would the 100A CPD open before the 4mm cables under short circuit conditions?". The answer is to use the adiabatic formula and you will find out if it is satisfactory. The circuit PSCC is important as if (it almost certainly isn't) it is quite low the 100A fuse may go outside the expected fusing time and cause the cable to overheat seriously outside the adiabatic time limit. This is probably not particularly important as it is a temporary arrangement. The soft starter fuses are probably 63A and will probably fail first anyway. There is no reason at all to suggest that the overload may not open under overload conditions, they are very reliable and the most likely fault would be that the soft starter does not disconnect the load.


    The circuit is therefore protected against short circuits, sustained overload, and motor faults in a satisfactory way. If you fit 40A fuses it is possible that you will get a failure, the loss of a phase, and possible motor damage. This would be stupid and expensive, and more importantly completely unnecessary.
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