Should we be encouraging poor design which goes against the recommendations of the IET on this site?
Current Oven Wire
- Thread starter dball
- Start date
This argument, the attempts to draw distinctions between guidance and regulation, the conflict between the fact that so many people are only provided with ring finals in the houses they buy and the fact that so many appliances are provided with plugs, the difficulties that circuit designers face in trying to ensure that a ring final socket circuit will not be significantly overloaded anywhere, they all go away if we use radials.
And yet still some apparently rational intelligent people think that ring finals are good idea, and genuinely believe that if we had never had them, a proposal made today to JPEL/64 for their introduction would be greeted with approval and immediate adoption.
And yet still some apparently rational intelligent people think that ring finals are good idea, and genuinely believe that if we had never had them, a proposal made today to JPEL/64 for their introduction would be greeted with approval and immediate adoption.
We keep on encouraging ones which do not go against them.
If new houses used radials, I wonder how many complaints there would be of "my MCB's tripped. This house is faulty/poorly designed" there would be ?
this added to the extra cost of larger CU's (and lack of room for them)
and thicker copper cable required = £
this added to the extra cost of larger CU's (and lack of room for them)
and thicker copper cable required = £
That's a pretty loaded question, to which the short answer (at least to the first part - "should we encourage poor design?") clearly has to be 'no'.
The longer answer is, in this case, more complicated. The 'recommendation of the IET' to which you are referring exists in a list of measures which Appendix 15 says "can usually achieve" the requirement that "the current in any part of the circuit is unlikely to exceed for long periods of time the CCC of the cable".
However, as I've said, a designer who actually knows what they are doing (rather than just blindly 'following recommendations") can (particularly with Method C) usually/often guarantee (not just render it 'unlikely') that none of the circuit's cable will be overloaded for long periods of time, by the appropriate positioning of sockets (or FCUs) on the ring in relation to the CCC of the cable. I certainly would not regard that as 'poor design', would you? [and, in response to BAS's comment, if one does that, it is not really any worse (in terms of cable loading) than a corresponding radial circuit]
Kind Regards, John
I'm pretty neutral on this issue, but if you have 4mm² 32A radials instead of 2.5mm² 32A rings, there is no reason for any more MCB tripping, there is no need for larger CUs and the difference in terms of amount of copper is likely to be insignificant (might even be less with some circuit topologies, since 4mm² < 2 x 2.5mm²).
Kind Regards, John
In general there are probably no sockets within the first 10% of a ring bearing in mind rings usually start under the stairs. Manufacturers of appliances don't seem to recognise this recommendation, and there appear to be no known cases of problems. Perhaps some jobsworth in the IET dreamt this up and somehow it has become a recommendation.
Some on here would recommend not using ring finals at all but this is not spouted out every time someone mentions rings.
Some on here would recommend not using ring finals at all but this is not spouted out every time someone mentions rings.
A ring final can be installed with 4mm² copper and a 32A MCB and be safe no matter how the loads are spread around the ring. Only when it is installed using 2.5 mm² copper and 32A breaker does a ring final become "questionable".
Installing a radial with 4mm² copper is apparently beyond question. what is the cost of the extra cable making it a ring final.
In my opinion the ring has the benefit of having two routes for the CPC from socket to CU meaning there has to be two faults before the CPC is disconnected from a socket.
It is true that a single fault on a ring would go un-noticed until a full ( routine ) inspection was carried out or a second fault occurred to break the other leg of the CPC. In the event of two faults the loss of CPC could still go un-noticed, everything works OK, until test or a shock is noticed.
So a ring requires two faults to become hazardous due to loss of CPC. With a radial only one fault is needed to lose the CPC
Installing a radial with 4mm² copper is apparently beyond question. what is the cost of the extra cable making it a ring final.
In my opinion the ring has the benefit of having two routes for the CPC from socket to CU meaning there has to be two faults before the CPC is disconnected from a socket.
It is true that a single fault on a ring would go un-noticed until a full ( routine ) inspection was carried out or a second fault occurred to break the other leg of the CPC. In the event of two faults the loss of CPC could still go un-noticed, everything works OK, until test or a shock is noticed.
So a ring requires two faults to become hazardous due to loss of CPC. With a radial only one fault is needed to lose the CPC
A 4mm radial when compared to a correctly designed ring, will use more copper, as the return leg will be fairly short when designed correctly.
I guess people don't like terminating and and installing 4mm at every socket unless they have to.
I guess people don't like terminating and and installing 4mm at every socket unless they have to.
The fashion today is to waste as much copper as possible.
While they are happy to do this when installing a cooker circuit, or tails they draw the line when putting in socket circuits !
As I sort-of implied, that will often be the case. A potential problem arises when the CU is in a kitchen or utility room, with sockets for dryers, WMs etc., very close. If, due to installation method etc., the CCC of the cable is at the lower limit of what is allowed (20A), then the potential problem gets much larger, since that 10% increases to something like 37% - i.e. avoidance of cable overload is not guaranteed unless the loads are within about ±13% of the mid-point of the ring.
I would say that it is a well-intentioned recommendation but, as I've said, it is just one of a list of recommendations/suggestions as to ways in which long-term overloading of any of the cable can be avoided, and there is no reason why a designer who knows what they are doing should not use other (maybe even 'better') methods of achieving that requirement.
Kind Regards, John
That is likely to be the case if the circuit is supplying sockets fairly close to the CU. However, if most/all the sockets are more remote from the CU (e.g. 'upstairs sockets circuits'), that is much less likely to be the case.
I'm sure that is often the case. I used to be 'nervous' about wiring accessories with 4mm² cable, but it really is not much of an issue, and I certainly don't any longer regard it as a problem (and I'm not even an electrician)..
Kind Regards, John
I still don't see it as an issue on an upstairs socket circuit.
It obviously depends upon the arrangement of house and positioning of upstairs sockets but there is the potential for a fairly long return run to the CU when one has a ring. If you want a more dramatic example, think of a second floor (or higher!) sockets circuit fed from a ground floor CU.
Whatever, I think that, in terms of the big picture, the amount/cost of copper is probably a fairly minor issue in terms of the ring/radial choice/debate.
Kind Regards, John
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