Understanding FCU: BS7671 Appendix 15

[levstar]

Hi, I am in the thick of studying the 2330 L2 and I am curious to understand fully the diagram within Appendix 15.

Generally I am quite happy with what is illustrated however just want to confirm my understanding of Fused Connection Units and associated rules on an A1 Ring Final Circuit.

Am I right in thinking I can use a 13 Amp FCU to supply a radial circuit of a reasonable amount of sockets, providing I take into account diversity, voltage drop, R1 + R2, Zs etc.

If this is the case is it permissible taking design factors into consideration to come out of the back of final socket on the Radial and Fuse down to a 6A FCU to provide a light (Basically the whole idea being that the radial is a circuit that will provide a small amount power when needed / switched on at the 13A FCU to a separate outbuilding)

 

[riveralt]

Hi, I am in the thick of studying the 2330 L2 and I am curious to understand fully the diagram within Appendix 15.

Generally I am quite happy with what is illustrated however just want to confirm my understanding of Fused Connection Units and associated rules on an A1 Ring Final Circuit.

Am I right in thinking I can use a 13 Amp FCU to supply a radial circuit of a reasonable amount of sockets, providing I take into account diversity, voltage drop, R1 + R2, Zs etc.

If this is the case is it permissible taking design factors into consideration to come out of the back of final socket on the Radial and Fuse down to a 6A FCU to provide a light (Basically the whole idea being that the radial is a circuit that will provide a small amount power when needed / switched on at the 13A FCU to a separate outbuilding)

Once you have used the 13amp FCU to create the spur - the circuit the becomes a radial. So yes you can put another switched FCU into the radial for a lighting circuit - though I would fused it at 3Amps.

However, you mention an outbuilding - this opens a whole new can of worms and will probably involve notification to the Local Authority building control and the payment of a fee. The cable you use will need to reflect the environmental conditions and meet BS7671 guidelines in terms of the type and size of cable used, current carrying capacity, possibly bonding issues if there are extraneous conductive parts in the oubuilding etc.

 

[PrenticeBoyofDerry]

Am I right in thinking I can use a 13 Amp FCU to supply a radial circuit of a reasonable amount of sockets, providing I take into account diversity, voltage drop, R1 + R2, Zs etc.

Yes that is a fair assumption and you would also need to consider the load being applied to this, to make sure it suitable for purpose.

If this is the case is it permissible taking design factors into consideration to come out of the back of final socket on the Radial and Fuse down to a 6A FCU to provide a light

Down fusing and electrical discrimantion of circuits is permissable.

 

[riveralt]

Levstar, just because you are studying 2330 Level 2 does not exclude you from complying with the law.
A search of your posts reveals that you have done this before - maybe more than once and certainly in a property you previously owned.
So did you comply with the Building Regulations in your previous house when you ran cable out to a conservatory edit sorry Summerhouse at the end of your garden and then a shed next door?

Did you comply with the Building Regulations when you fitted an extractor fan and later I think downlights in your bathroom?

Have you contacted your LA BC in relation to this work you plan to do?

By the way what is Zs and how do you measure it?
What test equipment are you using?

 

[levstar]

Thanks for the information. I fully appreciate compliance with the law and strive to do things correctly.

As for previous works re the summerhouse, no lighting was spured off and a qualified and registered electrician assisted and signed off accordingly.

The purpose of this latest post was to clarify my understanding re numbers feeds on the radial + the installation of a light at the end of the ciricuit.

Work on the bathroom fan has not started as yet.

 

[OwainDIYer]

Am I right in thinking I can use a 13 Amp FCU to supply a radial circuit of a reasonable amount of sockets, providing I take into account diversity, voltage drop, R1 + R2, Zs etc.

Up to a point. The problem is that you have to assume that 13A sockets are for 'general purpose' and will be loaded accordingly, unless the particular circumstances are otherwise.

So a group of 5 doubles behind the telly or computer desk on a 13A FCU would be acceptable. 2 singles in different rooms would not, because each could have a 2kW heater in, which would be a sustained overload, and circuits must be designed so that sustained overloads that are not cleared by the protective device do not occur.

If this is the case is it permissible taking design factors into consideration to come out of the back of final socket on the Radial and Fuse down to a 6A FCU to provide a light (Basically the whole idea being that the radial is a circuit that will provide a small amount power when needed / switched on at the 13A FCU to a separate outbuilding)

You can do that, but if you need a 6A lighting circuit as well as a 13A / 16A / 20A radial socket circuit you should really be running a 20A or 32A submain to a CU and have separate socket and light circuits from that CU. Then (apart from the submain) you can use standard circuits. It will also make it more obvious if the lights fail where the householder has to go to correct the fault.

 

[levstar]

Thanks OwainDIYer good info sounds like the best practice in these situations is to always run a sub-main from the CCU.

Although going back to the worst case scenario mentioned re 2x 2kW heaters, would this not be protected by the 13A fuse in FCU? Cheers[/u]

 

[OwainDIYer]

Although going back to the worst case scenario mentioned re 2x 2kW heaters, would this not be protected by the 13A fuse in FCU? Cheers[/u]

Time-current curves.

Yes the cable would (should) be protected. That doesn't mean that it's acceptable to design an installation where a sustained low level overload is likely to occur, where the cable will be running above its design current but the protective device won't open.

Otherwise you could put a 10kW shower on a 2.5mm spur off a ring. It'll work but it's very naughty.

 

[ericmark]

The ring final circuit was designed for houses to reduce the amount of copper required in the old radial system. It also reduced the size of the consumer unit. but relies on the ring being maintained and it is so easy for rings to become two radials.

The rules have been put in to try and ensure cable does not become overloaded and some like the ensuring there is no figure of 8 formed seem a little hard to understand. The problem with a figure of 8 is we can end up with a final ring being feed by a single length of 2.5mm cable without it being detected and there are a few conditions like this which are more to do with reducing connection error than with overload like only one spur from a socket on a ring. However if we want to follow a type tested system we must keep to type.

The BS7671 seems to loop around many times to prevent saying the same thing over and over and it does sometime note the loop. However the 433.2.2 is often missed and this clearly limits the length of a spur to 3 meters which in most cases is ample. Appendix 15 is a useful guide but it refers back to 433.1 and although some of the rules are historical (was rule of thumb) the general idea of restricting the area served by a ring main is as valid today as when first quoted.

So although one can for example that a radial from a ring using a fused connection unit at 13A and with that feed the garage and in the garage reduce it further and with a 3A fuse in a fused connection unit feed the lamps. To do so means you have exceeded the floor area of 100 sq meters. As a result one has to be very careful the system is not overloaded. The design current for a ring is not 32A even though we use a 32A MCB. The design current is 26A and this is considered as 20A at central point and the remaining 12A is spread even throughout the ring so is calculated as being 6A. Now if one adds FCU's with large loads at points in the circuit then this balance could be lost. The supply to shed to use lawn mower once a week is very different to supply to garage to weld up latest vintage car refurbish. Even though both may be feed from a 13A fuse.

There are times were one should be using common sense and not just relying on blind following of regulations as although 314.1 is so early in the regulations it seems it is often missed and using common sense means likely even if you have missed a regulation you will still have likely complied.

 

[levstar]

This is all good information it looks like in summary that any circuit modification has to be based on worst case scenario of future users not just current occupants.

I am now curious re the comment concerning the figure of 8 scenario, Am I right in thinking this is not the same as an Interconnect and therefore would not appear during testing. If so how would could this be detected on an installation.

 

[dingbat]

...433.2.2 is often missed and this clearly limits the length of a spur to 3 meters...

Rubbish.

Firstly, this regulations has nothing to do with rings.

Secondly, it places no such restriction on the length of a spur from any kind of circuit, except where the overcurrent device is not placed at the point at which a reduction in current-carrying capacity occurs.

A fused spur is compliant with Reg 433.2.1 and so 433.2.2 has no relevance.

 

[bernardgreen]

Secondly, it places no such restriction on the length of a spur from any kind of circuit, except where the overcurrent device is not placed at the point at which a reduction in current-carrying capacity occurs.

Does this mean that a short ( restricted ) length of cable is allowed to be overloaded if the overcurrent protections is remote but not if the overcurrent protection is local ?.

I hope that the remote protection is defined as such that the cable is still only allowed to carry a current that makes it quite warm but not dangerously hot before the remote device operates.

 

[ban-all-sheds]

No, it means that the overcurrent protection can be at the far end.

An obvious example is a spurred socket. The fuse in the plug prevents the cable supplying the socket from being overloaded.

 

[bernardgreen]

An obvious example is a spurred socket. The fuse in the plug prevents the cable supplying the socket from being overloaded.

Obvious to us with common sense. We know that if the lower capacity cable has a short circuit live to neutral fault then the remote protective device will definately trip ( or melt ) before the cable has a chance to melt.

Not obvious to a member of a registered scheme who did not understand the backward protection of cables feeding a restricted number of 13 amp maxium sockets.

He could not ( I think ) differentiate between [1] the maximum safe continuous load current being limited by sensibly fused plugs in a limited number of sockets and [2] the very short duration fault current limited only by the impedence of the cable itself.