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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
    I have read the rest of this thread and we seem to be back to this ridiculous idea that currents of any duration are exactly the same as far as effects on parts of the circuit are concerned. This is not, and never has been the case. Taking the limiting case, a short circuit trips the CPD, at 50 x In. Has anything been "overloaded"? Is anything damaged by this large current, which exists for a short period? I would expect not, even if the fault was made by means of a switch. People here and probably elsewhere often worry about cable ratings, and exact compliance with the tables in the "Book" to the last amp. This is particulary common when ring circuits are discussed, for example the aparent 21A rating of 2.5mm cable above. In some way these tables would be better re-written with current ratings in Ampere-minutes, or something actually connected to the temperature rise caused by loading. This would make understanding of diversity much easier, and final cable temperature in use better defined. It takes a 2.5mm T&E cable something like 1/2 hour or more to reach a final temperature at the tabulated maximum current, and even longer for large cables like 300mm. If you have a short duration load, like a washing machine and dishwasher which have thermostats and water heaters, one can see that the cable heating is not a problem, even if the current is somewhat high for a short period, say 15 minutes. The motors take much less current and the cable will gradually cool. The danger comes from "long and small" overloads (perhaps 25%), which do not trip the CPD but do heat the cable over a long period, typically many hours. It is often suggested that two electric fires on a ring spur is unsatisfactory, but this is not true, because any reasonable domestic space would become very hot indeed in that half hour, and their thermostats or a person would turn them off. It could be unsatisfactory if the circuit was feeding a lot of computers 24/7, although that would strongly depend on what they were actually doing, they take increased power with increased activity too, just like motors.


    I know that I keep on about this subject, the reason is that it is often very badly taught in college, and overload seems to be the one subject everyone remembers as the suggested results may be "interesting"! This is rarely true unless the designer is very stupid, cables themselves are rarely subject to overload failure, even when buried in the street and operated at several times the aparent rating. Cable failure usually is due to mechanical damage or wear-out by water ingress or similar. It can be fairly spectacular with a big cable, and a really big fuse on grided distribution in a large city, particularly central London.
  • 0
    Well that is interesting Graham because there is no BS number (or even an EN one) on my box of Ideal connectors. Perhaps I am an early adopter and they hadn't printed it on yet. It seems that no one trusts screw terminals any more, which is curious. Just imagine if your car enginge had all the bolts loose and how long it would last. This is a common cause of failure, but then there are an awful lot of bad mechanics about. It is true that one sometimes finds loose terminal screws, but the question is always "was it ever tightened correctly"? I was also rather under the impression that loose tails cables and screws had two causes, poor terminal design and mechanical movement of the cables. Cables with few big strands are always going to be a problen unless the terminal is considerably longer than the cable is thick (Like the old Wylex boards). It is very difficult to tighten the tunnel terminal common in CUs sufficiently to sort the strands out sensibly, and many electricians do not seem to pre-do this before tightening. These cables twisted are a nightmare, again bad workmanship. Torques given by manufacturers are not enough to compensate for this error, and if one tries much more the screw will usually fail, either the head or the thread strip. Why they don't fit Grade 8 screws is a mystry to me as the cost saving for mild steel ones is tiny. In fact I would like Grade 8 at least Screws with hex socket or Torx heads if designing such a product.
  • 0
    davezawadi (David Stone):

    Well that is interesting Graham because there is no BS number (or even an EN one) on my box of Ideal connectors. Perhaps I am an early adopter and they hadn't printed it on yet. It seems that no one trusts screw terminals any more, which is curious.


    Not really curious, the products are too small for sufficient information to be printed on them. However, I can confirm Ideal state compliance with EN 60998 series also ... https://www.idealindustriesemea.com/content/dam/europe/resources/catalogues/IDEAL%20Wire%20Connectors%20DL%20Flyer.pdf


    I think even the Ideal screwits are also stated to be compliant and suitable for BS 7671 use!


  • 0
    perspicacious:

    So, with this in mind, would a 20 A JB attract any coding on an EICR if found on a 32 A RFC?


    OK, I have rummaged around and found an old BS 6220 JB. 2¼" diameter. It is marked, "NOMINAL CABLE 2.5 MM²" and "250 V 20A".


    BS 6220 says:

    5 Classification

    A junction box shall be classified according to:

    a) the number of terminals as declared by the manufacturer;

    b) the nominal conductor capacity of the terminals, as declared by the manufacturer, which shall be one of the capacities shown in column 1 of Table 2;

    c) the method of mounting (e.g. surface or flush).

    6 Marking

    6.1 All junction boxes shall be durably and legibly

    marked with the following:

    a) the number of this British Standard, i.e. BS 62201);

    b) the name or trade mark of the manufacturer or responsible vendor;

    c) the nominal conductor capacity of the terminals, i.e. “n mm2” or “n” ‘ where n is the conductor size in mm2;

    d) the rated voltage, e.g. “250 V”. Compliance shall be checked by inspection.


    So what we have is a (4-terminal) 2.5 mm² surface mounted JB. There is no current rating!


    The BS goes on to specify that at least 2 x the nominal conductor must be accommodated (else it wouldn't be a JB! ? ) and that the test current for the 2.5 mm² version is 30 A.


    So the answer, BAD, is "no".


  • 0
    I have the 17th edition and am a student in electrical installtion , and on domestic ring circuits I think we have been told that additional sockets MUST have a 13a switch fused spur , I will ask my electrical science tutor this one ,all my tutors are experienced electricians and know the 18th edition well .
  • 0
    Chris Pearson:
    perspicacious:

    So, with this in mind, would a 20 A JB attract any coding on an EICR if found on a 32 A RFC?


    OK, I have rummaged around and found an old BS 6220 JB. 2¼" diameter. It is marked, "NOMINAL CABLE 2.5 MM²" and "250 V 20A".


    BS 6220 says:

    5 Classification

    A junction box shall be classified according to:

    a) the number of terminals as declared by the manufacturer;

    b) the nominal conductor capacity of the terminals, as declared by the manufacturer, which shall be one of the capacities shown in column 1 of Table 2;

    c) the method of mounting (e.g. surface or flush).

    6 Marking

    6.1 All junction boxes shall be durably and legibly

    marked with the following:

    a) the number of this British Standard, i.e. BS 62201);

    b) the name or trade mark of the manufacturer or responsible vendor;

    c) the nominal conductor capacity of the terminals, i.e. “n mm2” or “n” ‘ where n is the conductor size in mm2;

    d) the rated voltage, e.g. “250 V”. Compliance shall be checked by inspection.


    So what we have is a (4-terminal) 2.5 mm² surface mounted JB. There is no current rating!


    The BS goes on to specify that at least 2 x the nominal conductor must be accommodated (else it wouldn't be a JB! ? ) and that the test current for the 2.5 mm² version is 30 A.


    So the answer, BAD, is "no".




    Why? The test current is greater than that for a BS 1363-2 socket-outlet? The voltage and current rating is that of conductors in a cable in a RFC?


    What makes it unsuitable for an RFC?


  • 0
    If a single terminal is so good at clamping and maintaining continuity, why the requirement for a secondary earth terminal for some circuit accessories required?

    Perhaps because one common failure mode is not so much the screw coming loose but the conductor breaking (especially with re-terminated solid cores).

       - Andy.
  • 0
    I think we have been told that additional sockets MUST have a 13a switch fused spur

    I hope that either someone has mis-remembered or the advise has been taken out of context...

       - Andy.
  • 0
    AJJewsbury:
    I think we have been told that additional sockets MUST have a 13a switch fused spur

    I hope that either someone has mis-remembered or the advise has been taken out of context...

       - Andy.


    The correct answer (which is that an additional socket-outlet (single or double) spurred of an accessory on a ring, provided it is the only additional socket connected to that accessory, does not require a fuse ... provided you don't expect someone to use more than 13 A from that socket-outlet.


    It's all there for everyone to see in Appendix 15 to BS 7671. (page 505). It also shows you where you need to use an FCU for the spur.


    I guess the mix-up is that FCUs are called "fused spurs", and SFCUs are called "switched fused spurs" by many in the industry.


  • 0
    That link was very useful Graham, I have lots of Twisters (wire nuts) which I see are compliant as well. Thanks.

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