My CH MCB is rated at 15A, however the entire system is fed through a fused switch with a 3A fuse. Shouldn't the MCB be rated 3A as well?
The existing cable from the MCB runs to a junction box then to the old fused switch. It looks like 1.5mm^2 cable. Is it OK to use 1.0mm^2 cable from the junction box to the new fused switch? Does it matter that the new cable is in the new colours and the old cable isn't?
Thanks Pete
No. The MCB's job is to protect te cable up to and including the 3A switched fused spur. The 3A fuse is to protect the wiring downstream of that.
There is no reason why you cannot extend the ring or spur on that MCB and add other sockets.
On Wed, 20 Sep 2006 10:17:14 +0100 someone who may be "PM" wrote this:-
Is this an "old fashioned" fused switch, complete with bakelite or metal enclosure and rewirable fuse inside, or is it a "modern" switched fused connection unit that fits into a single gang box?
Not unless you want any fault on the system to trip the MCB, as well as rupture the fuse. The way it is provides discrimination, which is desirable.
It would be a good idea to find out, rather then assuming.
Too little information to answer the question.
No, provided that you place the appropriate notice in the appropriate place.
It's unusual for a CH system to be fed from a separate circuit. The most common arrangement is for the boiler and heating controls, pump, etc. to be fed from a 30 or 32 A ring circuit, via a switched fused connection unit (FCU) fitted with a 3 A fuse. The immersion heater, if any, will be fed from a separate 15 or 16 A circuit. Sometimes the feed for the boiler etc, is taken from the immersion heater circuit, again via an FCU fused at 3 A.
So perhaps your 15 A circuit also feeds a water (immersion) heater as well as the CH, or perhaps it used to, and the heater has been removed, or a combi boiler has been fitted and there's no longer a HW cylinder, etc., etc.
If the circuit does only feed the CH then, yes, the MCB could be reduced to 3 A or 6 A. (3 A MCBs are available, but aren't very widely used.) In principle the FCU isn't needed either, but since you do need an isolator switch to allow work on the boiler etc. you may as well leave it alone. That said, the present 15 A MCB is likely to be adequately protecting the cable to the FCU, so there is probably no danger and no urgent need to change it.
New and old fused connection units? Unless the circuit is crazily long or there's an earthing problem in the installation, the 15 A MCB will adequately protect 1.0 mm^2 cable up to the FCU.
No, this is now normal, but you should fix a label like this
I have one MCB just for the CH and one just for the immersion heater.
I've switched it off and everything else is still working...it is labelled "Central Heating", also there are no other connections in the junction box except for the one going to the old boiler (via an unfused switch I have just noticed.)
Since the CH is the only load on that circuit I would rather a fault trips the MCB than blows the fuse. If a fault caused >6A to earth would a 6A MCB trip before a normal household 3A fuse blows? Or does it depend on the type of fuse?
Thanks for the info
It's a new switched FCU.
There is only the CH on this circuit. I would rather the MCB trips than the fuse blow.
Well..it's bigger than 1.0mm^2 :-) The general question is, is it OK to extend a thicker cable with a thinner one, bearing in mind that the whole is protected by a fuse & MCB.
What I mean is, the existing circuit goes from the consumer unit > junction box > existing CH switch. I want to remove the cable from the junction box to the existing switch and replace with new cable to a new switch. I want to use 1.0mm^2 for the new bit of cable.
Thanks David
Thanks NP.
The message from Andy Wade contains these words:
Really? I've seen quite a lot recently - circuit marked "Boiler". Seems handy.
The message from The Natural Philosopher contains these words:
Ah, I used to have this same argument with people who thought that whatever the Highway Code says they had a better way. I used to ask them to write down their idea, in accurate detail then ensure that everyone around them on the road had a copy and time to read it before they moved so that we were all singing from the same song sheet.
There are indeed a few bits of the Highway Code which don't make perfect sense, but at least they're codified and you can reasonably [1] expect other road users to have read them and apply them.
[1] If not practically.
It is quite common if the house was/is owned by and rewired by any South Yorkshire council or subcontractors working on their behalf.
Adam
The message from "ARWadsworth" contains these words:
Lot of it about in Telford, too.
I've seen quite a few recently and also wired a few that way myself. This is usually to get the CH on the non-RCD side of a split load box.
Where the boiler is in a kitchen, I tend to include it on the same (non-RCD) circuit as the fridge and/or freezer.
I would still fit a FCU (with a 3A fuse) beside the boiler though.
John
I don't have a split load consumer unit so it's all RCD'd.
Oh OK, it's obviously becoming a common practice. The OSG's advice that the RCD side of the board should only feed sockets is clearly getting home, at last.
In that case you would have to as your fridge/freezer circuit is presumably 16 or 20 A. For a CH-only circuit though, with a 3 A or 6 A MCB, ISTM that you really only need an isolator (20 DP switch) and not an FCU.
On Wed, 20 Sep 2006 15:11:24 +0100 someone who may be "PM" wrote this:-
Why?
It is done. One would need a lot more information to give an opinion on whether it is compliant to do so in a particular installation. Circuit lengths, loads and the like.
What is your objection to buying a bit of 1.5mm2 for this length?
Easier to re-set. No big deal if the fuse blows, just a bit more awkward - need to find another fuse, and I would turn off the MCB anyway before putting it in.
The load is about 1A, circuit length maybe 25ft.
None at all - 'cept I've already got some 1.0mm^2
MCB's dont provide protection from electrocution if thats what you are on about, thats the job of an RCCD.
With the table lamp eg the light fitting will be rated at a couple of amps so that is what the fuse should be at or below.
No, its the FLEX going TO the lamp that is only rated at 5A or less, and possibly the switch.
Which is why all my lights are on 6A breakers and 5A rated plugs, specially there for the purpose
On Thu, 21 Sep 2006 10:17:37 +0100 someone who may be "PM" wrote this:-
Do you expect the fuse to blow often?
It may be alright. You will need to do the calculations for it though.
On Thu, 21 Sep 2006 10:13:47 +0100 someone who may be "PM" wrote this:-
Your impression is wrong, very wrong. In general MCBs operate more slowly than cartridge fuses. For the MCB to trip the coil has to attract a bit of iron, which has to operate some linkages (with all the resistance to movement linkages entail), which move the contacts far enough apart to quench the arc. That all takes time. In contrast all a fuse element has to do is melt enough to quench the arc.
The exception to this is a small but prolonged overload, when the NCB will usually operate a bit more quickly, on the thermal part of its tripping mechanism.
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