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Overloaded fused spur, opinion please

I was an industrial electrician and have now come across a domestic wiring issue where nothing appeared wrong with the installation.  Those with more experience will have probably have come across this many times.


The kitchen is supplied by a 2.5mm T&E ring fed from a B32 circuit breaker.  On this ring there is a 13A fused switched spur above the worktop feeding a double socket underneath via 2.5 T&E.  There is a washing machine and a dishwasher plugged into this double socket.  Every so often, I assume when both appliances are heating at the same time, the 13A fuse in the spur blows.  The cable supplying this double socket is in the wall so the current-carrying capacity appears to be 18.5A so this was probably being overloaded as well.


I understand that in the regs diversity covers some aspects of this situation, but this specific situation must occur often surely?  The switched spur gives the ability to switch off the appliance easily without having to pull it out in order to reach the switch, so is surely desirable?


In this case I have installed a second switched fused spur feeding a single socket and converted the other to a single.


Is this a common problem in kitchens?
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  • davezawadi (David Stone):

    Graham, I think that is wrong and misleading. The only thing which affects the slip in an induction motor is the LOAD, not the supply voltage. A fuse that lost 1V at 13A wouldn't last long, 13W is quite sufficient to blow a 100A BS88 fuse quite quickly. If the motor is electronically controlled, it is probably not an induction motor and so there is effectively no slip. This is not a mechanism of induction motor failure, mechanical overload is due to increased motor current and therefore winding resistance loss, not due to excessive rotor (squirrel cage) current which is entirely caused by the level of "slip". A normal 13A fuse dissipates around 500mW- 1W under normal maximum current, which is less than 500 mV drop. Note that supply voltage variation between 208 and 253 V (phase to neutral) does not cause failure!


    My experience tells me otherwise.


    The issue is actually to do with the capacitor voltage of a capacitor-run motor, and associated overheating (over time) of the start winding. We are talking about single-phase variant only here.


    Unfortunately, it's [again] to do with penny-pinching in appliance manufacturer, but a problem all the same ... and not a new one, but something that appliance repair specialists have noted over a period of over 30 years.


    Now, yes you are correct that most electronically controlled motors are not induction motors, but that's not always true ... the first appliance manufacturer's technical department that I am aware of noticing this problem with single-phase induction motors was actually a washing machine manufacturer, and they had speed control, but more rudimentary than now available.


    I've not looked into whether it's a potential problem with some of the newer permanent magnet single-phase motors being installed on some higher-priced washing machines, because I'm pretty sure that will be addressed by far more modern control via VSD-type technology.


    However, the problem remains very real for tumble dryers - particularly the cheaper ones.


Reply
  • davezawadi (David Stone):

    Graham, I think that is wrong and misleading. The only thing which affects the slip in an induction motor is the LOAD, not the supply voltage. A fuse that lost 1V at 13A wouldn't last long, 13W is quite sufficient to blow a 100A BS88 fuse quite quickly. If the motor is electronically controlled, it is probably not an induction motor and so there is effectively no slip. This is not a mechanism of induction motor failure, mechanical overload is due to increased motor current and therefore winding resistance loss, not due to excessive rotor (squirrel cage) current which is entirely caused by the level of "slip". A normal 13A fuse dissipates around 500mW- 1W under normal maximum current, which is less than 500 mV drop. Note that supply voltage variation between 208 and 253 V (phase to neutral) does not cause failure!


    My experience tells me otherwise.


    The issue is actually to do with the capacitor voltage of a capacitor-run motor, and associated overheating (over time) of the start winding. We are talking about single-phase variant only here.


    Unfortunately, it's [again] to do with penny-pinching in appliance manufacturer, but a problem all the same ... and not a new one, but something that appliance repair specialists have noted over a period of over 30 years.


    Now, yes you are correct that most electronically controlled motors are not induction motors, but that's not always true ... the first appliance manufacturer's technical department that I am aware of noticing this problem with single-phase induction motors was actually a washing machine manufacturer, and they had speed control, but more rudimentary than now available.


    I've not looked into whether it's a potential problem with some of the newer permanent magnet single-phase motors being installed on some higher-priced washing machines, because I'm pretty sure that will be addressed by far more modern control via VSD-type technology.


    However, the problem remains very real for tumble dryers - particularly the cheaper ones.


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