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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?
  • 0
    it is not a good idea to have two heavy loads plugged into one twin socket and you have resolved this by putting single sockets in place. twin sockets are still rated at 13amp total load just like a single socket is. however, a twin socket is type tested for a certain time with 13a on one side and 7a on the other. Two "heavyish" loads such as this only usually occurs in a kitchen and you`ve found one way of avoiding it
  • 0
    Thanks ebee.  I think you've given me the answer to this, something that I had overlooked, the double socket is still only rated at 13A.  This makes the installation logical now in my mind, albeit not desirable.
  • 0
    May I suggest that you have increased the likelihood of the ring being overloaded?


    In nominal terms, of the 32 A available, only 13 could supply the two appliances with 19 left over. Now they can draw 26 A with only 6 left over.


    In real world terms, a 13 A BS 1362 fuse will sit there warming up an FCU or plug all day long at 20 A so the "nuisance tripping" would have been around, say 24 A. Similarly, that 32 A MCB will only start thinking about tripping at 37 A. The CCC (27 A for 2.5 mm² T&E in plaster) takes all of this into consideration. So whilst the clothes and dishes are being washed, you decide to have tea and toast. The MCB will cope with the extra 16 A or so for a few minutes. (Or is this where DZ's 500 W energy saving kettle becomes useful?)


    In answer to the question, spurs seem to proliferate as a house gets older. What I really don't like is spurs into the kitchen fed from an adjacent circuit.
  • 0
    It's also worth bearing in mind that some appliance motors (such as tumble dryers, but perhaps also other appliances motors that have no carbon brushes) can be damaged over time by this arrangement, because of the increased slip in the motor from the additional fuse resistance.


    Whilst double socket-outlets used to be rated at 20 A, for many many years now, double socket-outlets to BS 1363-2 are rated at 13 A max for the entire accessory, that is, both socket-outlets. I understand it could be argued that the type testing at 20 A shows temperature rise shouldn't be an issue, note that this is carried out on a brand new device, conditioned as per the standard, and is for a limited time only - so is not indicative of long-term overloading over the product's life.
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    I am not sure but a fused spur off a ring has cartain conditions , and i think one of them is ,it must be a single socket. But useful thread I didnt know twins are rated 13A combined , so logic must be you should never overload a twin beyond 13A.

    As a learner in electrical installation , for a long time I thought rings were a really good idea , and I guess in the days when electrical goods werent efficienct and could draw high currents , rings were the solution , but with goods having electrical efficiency , I think installtion thinking might be better off with radials in part because I can see electrical heating comming along , I have done my own house (a not very thermal efficient one) to fullyy electric and it can come with some eye watering winter bills , but a well designed new build should easily be managable on electricity.
  • 0
    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!
  • 0
    Helios, a spur without fuse and 2.5mm cable may feed either one double or one single socket, from the 18th edition. Your situation may be incorrect in that dishwashers usually have smaller heaters than 3kW, often 1.5 kW. 4.5 kW will usually blow a 13A BS1362 fuse in a few minutes as you have observed. There is no need to have the FSU at all, then you have no problem with anything and the circuit is compliant, and very unlikely to suffer from overheating as the cable will take at least 1/2 an hour to get to 70C, by which time the water in both appliances will be ata operating temperature. Please buy at least an Onsite Guide, and preferably take the 18th edition exam if you are professionally engaged in electrics.


    Kind regards

    David
  • 0
    davezawadi (David Stone):

    Helios, a spur without fuse and 2.5mm cable may feed either one double or one single socket, from the 18th edition. Your situation may be incorrect in that dishwashers usually have smaller heaters than 3kW, often 1.5 kW. 4.5 kW will usually blow a 13A BS1362 fuse in a few minutes as you have observed. There is no need to have the FSU at all, then you have no problem with anything and the circuit is compliant, and very unlikely to suffer from overheating as the cable will take at least 1/2 an hour to get to 70C, by which time the water in both appliances will be ata operating temperature. Please buy at least an Onsite Guide, and preferably take the 18th edition exam if you are professionally engaged in electrics.


    Kind regards

    David


    I am retired, not professionally engaged in electrics.  I have C&G2382-10 and 2391-01 but as I have said have very limited experience in house wiring.  The spur socket single 2.5mm T&E cable was installed in a groove cut within a thermalite block wall covered with metal.  I took this as being Reference Method A which means the CCC is 18.5A which gave rise to my question.  Your statement about the spur being ok without a fuse assumes Reference Method C


  • 0
    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.


  • 0
    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. 


    I'm afraid I can't agree with this either.


    The torque developed by an induction motor is a function of the applied voltage. If the voltage is lower than the design voltage, the torque will be lower than design, as a result the slip will increase & hence the current.


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