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Overloaded fused spur, opinion please

Foffer
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?
Parents
  • 0
    Yes, Graham, that is a reasonable failure mechanism, and quite understandable. It is usual to use motor capacitors with a working voltage of 350 or 400V AC (RMS) which usually last very well. Obviously, this was a capacitor run motor, with as you say an under-rated capacitor. It is necessary to have considerable working voltage as the phase shift is produced by resonating the winding inductance with the capacitance, and depending on the Q achieved the voltage is fairly unconstrained. The reason why the new capacitors don't last is interesting, are they identical to the original or just "replacements" It could be that the winding has developed a shorted turn, but this would have a big effect on motor performance. I think it may be because the replacement capacitor is unsuitable for the current through it, and gets heated, as a shorted turn would make the winding current very significantly higher due to the reduced inductance. Certainly an interesting problem for the student, and probably a serious one for the manufacturer too in terms of guarantees. Whether it is due to the fuse volt drop (or circuit drop, or mains voltage or whatever) is rather open to "FI", but I suspect that the capacitor specification is inadequate, and therefore it is really a design problem.


    Edit: Another problem is that self healing polypropelene capacitors suffer continuous capacitance drop over life if anything goes wrong and this will mean that the phase shift becomes less, which will cause much more heat in the motor, as well as less torque. This could well lead to partial stalling when loaded heavily and then failure of the whole motor is inevitable due to overheating, as operating with excessive slip increases the current hugely due to reduced back EMF. In fact if the failures are due to excessive motor heat I think this is the most likely mechanism.
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  • 0
    Yes, Graham, that is a reasonable failure mechanism, and quite understandable. It is usual to use motor capacitors with a working voltage of 350 or 400V AC (RMS) which usually last very well. Obviously, this was a capacitor run motor, with as you say an under-rated capacitor. It is necessary to have considerable working voltage as the phase shift is produced by resonating the winding inductance with the capacitance, and depending on the Q achieved the voltage is fairly unconstrained. The reason why the new capacitors don't last is interesting, are they identical to the original or just "replacements" It could be that the winding has developed a shorted turn, but this would have a big effect on motor performance. I think it may be because the replacement capacitor is unsuitable for the current through it, and gets heated, as a shorted turn would make the winding current very significantly higher due to the reduced inductance. Certainly an interesting problem for the student, and probably a serious one for the manufacturer too in terms of guarantees. Whether it is due to the fuse volt drop (or circuit drop, or mains voltage or whatever) is rather open to "FI", but I suspect that the capacitor specification is inadequate, and therefore it is really a design problem.


    Edit: Another problem is that self healing polypropelene capacitors suffer continuous capacitance drop over life if anything goes wrong and this will mean that the phase shift becomes less, which will cause much more heat in the motor, as well as less torque. This could well lead to partial stalling when loaded heavily and then failure of the whole motor is inevitable due to overheating, as operating with excessive slip increases the current hugely due to reduced back EMF. In fact if the failures are due to excessive motor heat I think this is the most likely mechanism.
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