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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
    No Adrian the slip is a secondary effect. The magnetic field in the rotor is developed by the slip and is related to the torque, BUT the motor current is directly controlled by the load torque. The change in slip up to the design maximum load is quite small. The voltage change Graham is talking about due to a fuse is tiny, and even if it had a 1% effect on the running current would not make much difference to the heat. However, with a single-phase motor things are a bit different because the capacitor in one of the windings provides phase shift, and in capacitor start motors the phase shift is much more complex and not very efficient. In order to run they have windings and poles which are not symmetric to provide an artificial second phase shift and this is a problem. Similarly, in capacitor run motors, the capacitor phase shift depends on running current which is also problematic in terms of efficiency. However, the fuse in the circuit has a tiny effect, which is the point I was making.  Yes, domestic appliances are run close to the limits by design because of cost considerations, but this should not cause direct failure unless the prototypes are not tested fully (which is common). Once a few manufacturing tolerances are added it is very easy to make an unreliable product, but not because of the fuse volt drop! In all of these cases it is very easy to blame all kinds of things and manufacturers Engineers often do, because it reduces blame, and the bosses tend to believe them for some reason I don't understand, although I have been there.
  • 0
    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.

    No, the number of sockets permitted on a fused spur is not limited (see appendix 15 of BS 7671) - it's just left to the designer to ensure that the arrangement is suitable for the intended loads (including diversity).


    An unfused spur is normally limited to one single or one twin socket (presumably a triple BS 1363 socket is also acceptable as they have an in-built 13A fuse).


       - Andy.
  • 0
    davezawadi (David Stone):

    No Adrian the slip is a secondary effect. The magnetic field in the rotor is developed by the slip and is related to the torque, BUT the motor current is directly controlled by the load torque. The change in slip up to the design maximum load is quite small. The voltage change Graham is talking about due to a fuse is tiny, and even if it had a 1% effect on the running current would not make much difference to the heat. However, with a single-phase motor things are a bit different because the capacitor in one of the windings provides phase shift, and in capacitor start motors the phase shift is much more complex and not very efficient. In order to run they have windings and poles which are not symmetric to provide an artificial second phase shift and this is a problem. Similarly, in capacitor run motors, the capacitor phase shift depends on running current which is also problematic in terms of efficiency. However, the fuse in the circuit has a tiny effect, which is the point I was making.  Yes, domestic appliances are run close to the limits by design because of cost considerations, but this should not cause direct failure unless the prototypes are not tested fully (which is common). Once a few manufacturing tolerances are added it is very easy to make an unreliable product, but not because of the fuse volt drop! In all of these cases it is very easy to blame all kinds of things and manufacturers Engineers often do, because it reduces blame, and the bosses tend to believe them for some reason I don't understand, although I have been there.


    David,


    Consider the following scenario and you will understand the failure mechanism at play. It's very simple, and due to a lack of tolerance in design (for cost purposes).


    The voltage across the capacitor in series with the "start" winding is very high as a result of the phase shift - usually well over 350 V, but can be more. Sometimes, the voltage (with the machine loaded - say full of wet washing in the case of a tumble dryer) is greater - sometimes getting close to the rated working voltage of the capacitor. At this point, a sub 1 % change in VD is sufficient to tip the voltage over the rated capacitor voltage.


    Now, the capacitor is self-healing, but over time the capacitance drops. This causes repeated overheating of the windings - typically the "start" winding. This in turn causes higher resistance over time due to bad connections etc.


    So, the capacitor eventually pops, over a period of say 12-18 months. If it is simply replaced, you find the new capacitor only lasts about 3 months because of the increased resistance posed by the "start" winding having been damaged by heat.


  • 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.
  • 0
    Yes David.


    So, how do I know increased resistance can tip things over the edge?


    As you'll be aware, it isn't unknown for the wiring in a detached garage to be spurred off a kitchen ring, using [cynical moment coming up] 2.5 T&E clipped to a fence.


    The number of failures of this type, where replacement capacitor lasts only a short time, was observed to happen far more frequently with tumble dryers used in this situation, or also off a "4-way" with other appliances. And this is compared to very few observed failures in garages supplied using SWA in a more conventional [compliant] manner, from their own way on the primary CU.


    I'm guessing a combination of primary voltage drop on the kitchen ring, plus additional through the FCU to the garage etc, is the real issue, and similar for running the tumble dryer off a 4-way extension in a kitchen.


    This won't be too different to an FCU feeding a socket outlet for a tumble dryer in the same kitchen - hence my initial comment on this. If you want to provide isolation "above the unit" for under-counter appliances, use a 20 A DP switch to supply the under-counter socket-outlet ...  and not an SFCU.
  • Former Community Member
    0 Former Community Member
     use a 20 A DP switch to supply the under-counter socket-outlet ...  and not an SFCU.


    If a 20 A accessory is suitable for use on a 32 A RFC, why do people use a 30/32 A JB on a RFC?


    Regards


    BAD
  • 0
    perspicacious:
     use a 20 A DP switch to supply the under-counter socket-outlet ...  and not an SFCU.


    If a 20 A accessory is suitable for use on a 32 A RFC, why do people use a 30/32 A JB on a RFC?


    Regards


    BAD


    BS 1363 socket-outlets are rated 13 A, as are FCUs, so I don't follow the argument fully ... although I guess it supports the query.


    The only answer I can give is "always been done that way" - and of course, permits an unfused "spur" to be connected into the ring, whereas the terminals in a 20 A JB would be overloaded by 3x2.5 ?


    Would we be happy extending 2.5 sq mm conductors using Wago 773's on an RFC?


  • Former Community Member
    0 Former Community Member
    The only answer I can give is "always been done that way" - and of course, permits an unfused "spur" to be connected into the ring, whereas the terminals in a 20 A JB would be overloaded by 3x2.5 ?


    If the terminals in a 20 A JB would be overloaded by 3x2.5, why aren't the INPUT terminals in a BS1363 accessory overloaded then?


    Does BS1363 cover 20 A DP switches or are they another BS?


    Does 433.1.204 permit the connection of 20 A DP switches?


    Regards


    BOD



  • 0
    I'm on about mechanical overloading - BS 1363  specifically requires that the input terminals of socket-outlets and FCUs for sol (Class I and Class II stranded conductors) are suitable for 3 no. 2.5 sq mm conductors ...


    BS 5733 if memory serves for the 20 A DP switch.


    433.1.204 is permissive, but doesn't prohibit other arrangements .. if the 20 A DP switch supplies only a single socket-outlet, surely overload current protection is not necessary as per 433.3.1 (ii).


    Surely, we don't want to tie ourselves in knots and conclude that it's not feasible to supply a tumble dryer in a kitchen from an RFC, in one of our devolved nations, unless the socket-outlet that it is connected into is located above the counter? Or do we?
  • Former Community Member
    0 Former Community Member
     if the 20 A DP switch supplies only a single socket-outlet, surely overload current protection is not necessary as per 433.3.1 (ii).


    Simply, are the INPUT/loop in terminals of a 20 A DP switch suitable for connection to a 32 A RFC irrespective of whatever is on the OUT terminals?


    Regards


    BOD

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