converting DSSO spur in loft to single socket plus unswitched FCU

26 Oct 2013
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United Kingdom
looking for some advice. I have a DSSO spur in the loft coming from the upstairs ring. This was installed for a future burglar alarm control panel.

Got the alarm control panel now and looking to install, but it says an unswitched fused spur must be used...why this is I don't really understand. The DSSO is coming from the ring main, not the lighting circuit, so don't really get why I can't just stick a plug with a 3a fuse on it.

Ok I get that there's potential for someone to unplug it, which means that the alarm could potentially go off after a while, but nobody goes up in the loft (we use it for storage) so the only person that would ever unplug it is me.

Anyways....I want to do it properly, so was thinking I could convert the DSSO spur to a single socket plus an unswitched FCU for the alarm, so that I still have a spare socket if I ever did need to plug in anything else up there....can't think what right now, but you never know!

Was thinking of replacing the DSSO with a single, then running the FCU from the single for the panel, but that's running a spur from a spur, so appreciate I can't do that. Any suggestions so that I can have the FCU AND a single socket, or do I just need to ditch the idea of having a single socket, and just have the FCU?

Or just be away with it and stick a plug with a 3a fuse on the control panel?

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The 'correct' way to do it would be change the DSSO (?) for a 13 amp unswitched fused connection unit, then from the load side of that whatever amount of sockets you wish and a 3 amp unswitched fused connection unit for the alarm.

But as the other bloke said, you may as well just fit a plug on the alarm and plug into the DSSO (?) and leave it at that.

All depends on how anal you are, I suppose.
lol is DSSO not a double-socket switched outlet? :)

So if I replace the double socket with a 13amp FCU, then it would be:

upstairs ring -> "supply" 13a FCU -> "load" 13a FCU -> Single socket -> "supply" 3a FCU -> "load" alarm control panel.

Like this?

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It is a silly rule which results in silly arrangements.

Indeed it is...

The purpose of the rule is to limit the load on a spur to that which is acceptable for a 2.5mm cable. Around 26 amps, depending on the cable installation method. Many years and several editions of the regs ago, the rule allowed a double socket OR two single sockets OR a single socket plus a fused spur. At that time your original plan would have been OK.

However, it was felt that the original rule was too complicated and left room for confusion and error. In particular the possibility that someone might mistake the first socket on the spur as part of the ring because it had 2 cables present, and another spur might then be taken from that point. Of course, proper testing would soon reveal this was not the case, but those in charge of the regs decided to go for the idiot proof option. Thus the rule was changed to allow for a single POINT, regardless of whether a single or double socket.

In other words, your original plan follows the INTENTION of the rule in limiting the load on the spur cable to a safe level, a single socket plus a fused spur being electrically equivalent to a double socket, but it goes AGAINST the present wording of the rule. Make up your own mind....

Some modular accessory systems, e.g. Scolmore, allow you to build a single socket plus a fused spur into the same unit, and thus make it a single point. The jury is out on whether that gets around the rule or not.
I have a tendency to overthink things. Think I’ll just stick a 3a plug on it with a “do not unplug or switch off” reminder sticker and get on with it.
If you like, the single socket can be a double (or many sockets) if you follow the most recent diagram.
Some BA panel manufacturers ask for a 2A fuse in the FCU.
Only 3 and 13 amp are official sizes though. Yet another case of manufacturers instructions being wrong.

The fuse is to protect the cable not the panel. The minimum cable size allowed does not require a 2 amp fuse.

If the manufacturers require 2 amp it should be built into the panel.
There are some appliances where the fuse in the plug protects the appliance (for example the circuit board) in addition to the cable.
There are some appliances where the fuse in the plug protects the appliance (for example the circuit board) in addition to the cable.
That is not the 'fuse in the plugs' purpose. After all those same appliances sold in Europe, Australia, etc would only have the MCB in the CU for PCB protection if the manufacturer did not fit internal protection.
Some appliances do not have the internal fuse protection you speak of. Unfortunately things aren't always black and white.

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