Evening, Wondering if anyone can shed some light on this one for me please? I've seen an installation recently which I was initially concerned about but have since been told is safe??? The installation was a fcu spurred off of a socket on a radial circuit. The radial is in 2.5mm2 2 core and earth. The spur both supply and load side have been done in 0.75mm2 flex, and the fcu has a 13A fuse. The load is a couple of rack mounted servers, so low current draw on the fcu. However, I was under the impression that supplies to fcus must be in the same diameter cable as the main circuit i.e. 2.5 in this instance. I raised my concerns with a more senior electrician who told me not to worry, stating that because of the way current flows in a radial the supply cable will not be subject to the full current of the circuit, only what is being drawn by the fcu (unlike in a ring main). He agreed that the fuse should be lowered to a 5A, but told me that whilst not ideal, it was electrically safe to have a smaller diameter cable on the supply side of the fcu because this was a radial circuit. Whilst I am not sure, something about this doesn't ring true to me. Can anyone clarify the cable issue for me? Thanks
It's certainly not good practice, although safe enough if a 3a or 5a fuse in the fcu but the problem is that someone could easily put a 13a fuse in there!
Not good practice? Sen I've been doing things like this for years. There is NOTHING wrong with it. If there is what? Even a 13a fuse what pop under a fault on .75mm.
What I mean to say before everyone gets a cob on, no he should not put a 13a fuse in the fcu unit bet technically it's up to who ever replaces the fuse to use the right one. Same with re-wireables. I stick a label on the fcu saying 3a only in these cases. I don't mean use a 13a fuse and then wantonly wire away in .75mm.
I think he is concerned that the spur is fed from a bit of flex out of the back of the socket on a presumed 16 or 20A MCB. Although it would probably be safe as the spur cant draw any more than 13A and if the flex was to short out the MCB would go anyway, but at the end of the day its rough as a badgers **** and usually done by alarm fitters etc. I have seen a 2.5 ring/32A in a kitchen connected to a DP (unfused) switch and a single 1.5 T&E feeding a single socket below. I couldn't find anything particularly wrong with that install but the problems would begin is someone else fed additional stuff off the spur.
Ah right I see. Didn't read it proper. In that case as the flex has a higher with stand temperature than twin and it's length is kept short on a 20a breaker it would prob. be ok in fault but very lazy not to do it proper in the first place.
Thanks for your responses. Peter - you hit the nail on the head, the guy who did the install was from an alarms background. Good to know its safe, just not how I thought it should have been done. Appreciate all your time.
Nah, they should of used 2.5mm T/E from the spur from the socket to the incoming side of the FCU, after that use what U like and fuse accordingly.
1.5mm would be compliant Sine if less than 3m long and not liable to physical damage. (so it says in the big book!)
This is what I thought myself. However it seems less black and white than I first thought. There appears to be a grey area between 'compliant' and 'dangerous' where you're not going to burn down the building, but you're still not doing it right.
The idea is you can, if you really want to, take a very small conductor off of a large conductor BUT it must be protected by some form of type tested covering. The most common example would be a bus bar chamber that say, feeds a riser, with seperate, smaller bus bars taken off for each floor. Or a bus bar chamber feeding sockets on an office floor layout for each room. The other example would be for instrumentation purposes, ie you would not want to use the same conductor size to feed something like an amp meter if your monitoring huge currents. In any case, when doing this, the smaller conductor must be max three meters. For some reason the regs refering to this are often misinterpeted as being the max length of the meters tails. They are not and never have been to do with meter tails.
Yup ya changes the CSA of the cable ya reduce the size of the protective device as me old cable joint states Kin ell eh mate!
Are you aware that an MCB has 2 tripping mechanisms inside it. One is a simple overload type similar to a re-wireable fuse and just works on the heating up principle. The other trip is a coil and this detects the surge if a dead short occurs, even a piece of .5 flex shorted out would be enough to trip a 50A MCB with no damage to the cable.