Fused Spurs

[dunny]

I have read on here lots of times about extending socket circuits by the use of a fused spur. Indeed, I have done this myself in a previous house based on similar advice.

Just curious what the reason for doing so is?

I can see that it may be convenient and also that it might help with knowing that a particular socket or sockets have been added to a circuit but presumably there is some other safety related reason?

D

 

[JohnW2]

I have read on here lots of times about extending socket circuits by the use of a fused spur. Indeed, I have done this myself in a previous house based on similar advice. Just curious what the reason for doing so is? I can see that it may be convenient and also that it might help with knowing that a particular socket or sockets have been added to a circuit but presumably there is some other safety related reason?

On the assumption that you are talking about a ring circuit (what electricians call a 'ring final circuit') the best way of adding sockets is to 'break the ring' and then insert the new sockets into the 'break' so that they become part of the (now larger) ring.

If one can't be bothered to do that (usually because of practical issues), a second best is to take a 'spur' from the ring. If it's not fused, then it is only allowed to supply one socket (single or double) via the spur, since the cable would be under-rated to supply the potential current load of more sockets. However, if one has a 'fused spur', with a 13A fuse, then one can supply as many sockets as one likes via that, since the total current drain by all the additional sockets is limited by the fuse (theoretically to 13A).

I don't think there is any reason, other than 'convenience', for favouring spurs (fused or unfused) as a means of adding sockets to a ring circuit - 'extending the ring' (making it into a larger ring) is the preferred way of adding sockets to such a circuit.

Hope that helps.

Kind REgards, John

 

[dunny]

Thanks John.

Its protected still by the fuse at the CU but I presume its more convenient to have a fuse in the spur to go so the whole socket circuit isn't affected.

I will be having a complete rewire done professionally in the next few months or so (providing I can afford it!!) and so this was purely out of interest.

Regards

D

 

[ericmark]

One reason for fitting a FCU today is where the original circuit is not RCD protected. Although the original can be left as it was any additions must comply with 2008 regulations requiring RCD protection both for sockets and some types of cable. Rather than use Ali-tube cable and a RCD socket using a RCD FCU allows twin and earth cable to be used and buried in the wall.

To extend the ring would mean the whole ring would need RCD protecting or Ali-tube cable and RCD sockets although the Ali-tube is now sold in short lengths on Ebay when the regulations first changed one could only buy ali-tube cable by the role with is rather expensive for a short extension. Either that or surface cables which is not really what we want in a modern home.

With a already RCD protected ring then yes I would consider extending the ring but before I could extend I need to work out the volt drop which it has already. To do this I guess at what will be centre sockets and measure the loop impedance from this I can calculate the volt drop. However the DIY guy will rarely have a loop impedance tester so he can't really test the volt drop so since he does not know if there is room to extend the ring (max of 106 meters of cable) then easy way is to use a FCU which will increase the permitted ELI from 1.44 ohms for a B32 MCB to 2.42 ohms for a 13A fuse. It is unlikely that any spur will be over 1 ohms so unlikely there will be a problem when the next electrical installation condition report (EICR was called PIR) is completed which should be done every 10 years or change of occupant which ever is earlier.

You may already have a EICR but often these are not done people just take a chance. Although for volt drop we should measure the line - neutral impedance or prospective fault current in practice only the earth loop impedance is recorded as I have said for a B32 MCB up to 1.44 ohms is permitted this may be recorded on your EICR.

For us electricians we have the meter we plug in press button and we know if we can extend in seconds but for the DIY guy not so easy. Hence why electricians will try to extend a ring but DIY often use spurs. Clearly even the DIY guy should measure or calculate the impedance but in the real world we know this does not happen.

 

[rjm2k]

Thanks John.

Its protected still by the fuse at the CU but I presume its more convenient to have a fuse in the spur to go so the whole socket circuit isn't affected.

It's not that simple, the fuse at the CU is based on spreading the load over 2 cables in a ring, so is normally a 32Amp fuse. When you spur, you normally use the same size cable as for the rest of the ring, but as a single leg that cable is rated at lower than 32Amps so in theory that spur leg could get overloaded and damaged if there is too much demand from devices on the spur because the fuse is rated higher than the cable. By adding a FCU with a 13amp fuse you create a weak point which will fail before the single cable does and before the main fuse trips.

I'm guessing it's considered bad practice to up the cable size to say 4mm (which even in an ideal situation is only rated upto 32amp) when spurring off a 2.5mm ring to avoid the need for a FCU but there would probably be little benefit in doing so anyway.

 

[JohnW2]

Thanks John. Its protected still by the fuse at the CU but I presume its more convenient to have a fuse in the spur to go so the whole socket circuit isn't affected.

That is rarely, if ever, the reason for fusing the spur. As I said, it's usually done to allow multiple sockets (with a maximum total load of 13A) to be spurred off a ring circuit using cable of a size which would be inadequate to safely supply more than one socket.

Kind Regards, John

 

[JohnW2]

...I'm guessing it's considered bad practice to up the cable size to say 4mm (which even in an ideal situation is only rated upto 32amp) when spurring off a 2.5mm ring to avoid the need for a FCU but there would probably be little benefit in doing so anyway.

We've discussed this before. From the point-of-view of cable CCCs it would obviously be acceptable to have a 4mm² cable protected by a 30/32A fuse/MCB to supply an unlimited number of sockets without any additional fusing (i.e. an FCU) - just as with a 4mm² radial. However, the reason why it's 'bad practice' (and probably non-compliant) to do as you suggest is that this arrangement could result in a full 32A load (from sockets on the spur) being attached to one point on the ring. Particularly if that one point was close to one end of the ring, this could result in the CCC of the ring cable (assuming it is 2.5mm²) to be exceeded for one leg of the ring.

Kind Regards, John

 

[dunny]

Thanks, I can see some good reasons to use the FCU especially with regard to creating a wek point to save overloading the cable.

A couple of observations:-

1. You guys really know your stuff!!
2. It's even more obvious that us DIYers have an awful lot to learn and the fact that many of you on here give your time and expertise is commendable!!
3. So glad I am getting my whole rewire done professionally - soup to nuts!!

Thanks for the replies.

D