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To RCD protect or not to RCD protect (a hopefully simple question)? |
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| message from Seri on 19 May 2004 |
My new boiler arrived today (feelings of happiness mixed with feelings of
emptiness in the wallet department).
As I was studying the wall that is to be its new home and making sure my
original designs would be accurate I realised I'd missed an area when
hatching my master plan.
I'm currently rewiring this house one room at a time.
In the kitchen I plan on having the only non RCD protected socket in the
property (for the freezer so if the RCD trips whilst on holiday the contents
won't spoil).
Questions:
Should the boiler be RCD protected or not (any pro's and con's?)
If the boiler shouldn't be RCD protected then is it okay to have it on the
same spur as the Freezer?
If the boiler can be on the same spur as the freezer then will I need to
convert that spur into a ring?
If the boiler should be on its own circuit, what rating MCB should generally
be fitted?
Thanks for any answers, as ever, it's all appreciated.
Seri
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| usenet replied to Seri on 19 May 2004 |
There's no regulations requirement for it to be RCD protected and I
don't really see any good reason for doing so.
Yes, why not.
No, why on earth would you need/want to do that?
Probably needs only a very small current so a 6 amp would do, size
the wire to match.
However my feeling would be to use the same circuit as for the
freezer, wire it with 2.5sq mm and have a 20amp MCB. It's then a
'conventional' radial circuit as in the IEE On-Site Guide. The boiler
should have some means of isolating it, a double pole switch or use a
socket.
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| Lurch replied to usenet on 19 May 2004 |
16A is better as 20A needs calculating correctly and quite often needs
a smaller MCB or bigger cable.
And a fuse, 3A usually. So a switched fused spur then.
Only if it's a switched socket, you shouldn't be able to remove the
socket while it's 'on-load'.
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| Martin Angove replied to Lurch on 20 May 2004 |
Just to confirm that - note table 6F (OSG) or 4D5A. Even under
installation method 6 (in conduit in insulation) 2.5mm cable is rated at
20A. Of course, if you're going to stuff it into the conduit with half
a dozen other circuits, or run it next to the heating pipes then you
might need to think again :-)
Oh yes, and don't forget the derating for rewireable fuses... in this
case according to my calculations even a 15A fuse is pushing it, but
we're talking about MCBs here presumably.
Hwyl!
M.
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| usenet replied to Lurch on 20 May 2004 |
I'm unconvinced by this, a 2.5sq mm radial on a 20 amp MCB is no nearer to
'needing calculation' than a 2.5sq mm ring on a 32 amp MCB is it?
Why the fuse? If the boiler installation instructions demand it then
yes but it's surely not a 'generic' requirement of any permanently
wired equipment is it?
That's a new one on me, is there an IEE regulations requirement that
states this?
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| Lurch replied to usenet on 20 May 2004 |
Well, I don't generally fit any 2.5mm radials with 20A MCB's unless
they're relatively short\exposed runs. I'll look into that though,
it's just the way I've always done it and I seem to think I did have
some theory behind it once!
So you'd fit a boiler to a DP switch with only a 16\20A MCB as
protection would you? I haven't come across a domestic boiler yet that
doesn't state "use a 3A fuse". However, I was only referring to this
particular appliance, it wasn't a generic requirement. If the CPD is
sized correctly to be able to offer overload protection to the
appliance then a DP switch is all that is required.
I seem to think there is, buggered if I can find it though! It's
probably written in such a way that it's doesn't mean that until
you've read it half a dozen times.
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| usenet replied to Lurch on 20 May 2004 |
Who are 'they'? A radial wired in 2.5sq mm protected by a 20 amp MCB
is one of the 'conventional circuits' for BS1363 sockets listed in the
appendices of the IEE "On-site guide". It's only the wierd insistence
on ring circuits in this country that means radials are not so widely
used, in some situations a radial makes much more sense than a ring.
Which is basically what I was saying, I think.
Anyone else any ideas?
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| Wheelbarrowbob replied to usenet on 20 May 2004 |
I agree with you, as far as i am aware the UK ( possibly Australia ) is the
only country to have fused 13A plug tops on a 32A ring. If i did a self build
etc i would have each room on its own 20A radial ( kitchem 32A ring). in the
event of a fault makes tracing it easier, easy life !
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| Lurch replied to usenet on 20 May 2004 |
You know, them, they're everywhere and know our every move!
I'll concede that one then as I never said you can't do it, put as
many 20A radials in as you want, I'm not going to stop anyone.
Sort of, depending on which way you read the question.
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| Christian McArdle replied to Lurch on 19 May 2004 |
And a DP switched socket at that!
Christian.
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| Lurch replied to Christian McArdle on 19 May 2004 |
No, a SP will do. The functional switching of the supply will be done
by the switch on the socket and the isolation is done when the plug is
removed from the socket.
Although a DP switched socket would be better.
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| Christian McArdle replied to Lurch on 19 May 2004 |
I was under the impression that SP switches weren't allowed for fixed
appliances in case someone mistakenly used the SP switch for isolation
rather than pulling the plug out. Possibly more of a best practice thing
than a specific regulation, though, I suppose.
Christian.
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| Lurch replied to Christian McArdle on 19 May 2004 |
That's why you usually use a DP switch for fixed appliances, and plugs
and sockets for portable appliances. Although you are right in a way.
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| ARWadsworth replied to Lurch on 19 May 2004 |
I thought it was a choice of double pole switch (or DP FCU), double pole
switched socket or unswitched socket for fixed appliances. Many fixed
appliances such as cooker extractor hoods come with a moulded plug but they
do state double pole switched sockets or unswitched sockets must be used.
Lurch if I may ask a quick question, you said
Only if it's a switched socket, you shouldn't be able to remove the
socket while it's 'on-load'.
What do I do about my iron? I know it is a portable appliance but there is
no on off switch on it and if it was used on an unswitched socket I must
always insert and remove it under load. Are some appliances banned from
unswitched sockets.
Adam
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| usenet replied to ARWadsworth on 20 May 2004 |
Yes, surely this is to make sure that the person disconnecting the
device really does disconnect it and doesn't (by mistake) rely on a
single pole switch.
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| G&M replied to usenet on 20 May 2004 |
What exactly is wrong with a single pole switch ? The earth is hard
connected to the neutral back at the consumer unit anyway and so the casing
is still connected. Either the regs should demand a 3P switch/removable
plug or allow single pole switches for fixed appliances.
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| Lurch replied to G&M on 20 May 2004 |
The regs say DP.
You should never ever under any circumstances even contemplate placing
a switch in the earth\cpc cable.
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| G&M replied to Lurch on 21 May 2004 |
Agreed - but I do wonder if the regs have lost the plot here.
Then why can you buy 3 pole switches with one pole marked "E" ?
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| G&M replied to G&M on 21 May 2004 |
switch/removable
Decided to check myself and these are used for fan isolation. Quite why
these are OK to switch the earth is another mystery of course.
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| ARWadsworth replied to G&M on 21 May 2004 |
Usually they switch L1, L2 and N.
Adam
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| Owain replied to G&M on 21 May 2004 |
"G&M wrote
| > Then why can you buy 3 pole switches with one pole marked "E" ?
| Decided to check myself and these are used for fan isolation.
| Quite why these are OK to switch the earth is another mystery
| of course.
Fan isolators are 3 pole because they switch
- permanent live
- switched live / trigger
- neutral
on fans that have a timer over-run trigged from the lightswitch.
They should *not* switch the earth, and I don't know why one pole is
labelled E. It's extremely misleading and irresponsible IMHO.
Owain
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| G&M replied to Owain on 21 May 2004 |
Well that's identified that the switch I have *isn't* a fan switch.
WTF it is though I'm not sure but having just checked with a meter it
definitely switches the connection from E to Eload as well as L to Lload and
N to Nload. It's a standard looking switch with no neon, front lead
connection or fuse. Any suggestions ? (other than bin it :-)
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| Lurch replied to G&M on 21 May 2004 |
Well. you've baffled me! Is it definitely for the UK market?
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| G&M replied to Lurch on 22 May 2004 |
Looks like a standard MKS 'clone' and I don't tend to buy electrical
fittings while abroad. But it might have come from some strip-out or other.
Unless there's a gallery of horrors somewhere I think this goes in the bin
in case somebody uses it.
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| Lurch replied to G&M on 21 May 2004 |
You can't, you must be looking at it wrong. Fan switches are for L,
Switched L, and N. The E is meant to be continuous throughout, with no
switches. Have another good look at your switch.
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| Lurch replied to ARWadsworth on 19 May 2004 |
Can't say as I've ever read the instructions TBH, but see the response
below, I think that answers this question as well.
That's why you're meant to have switched sockets. Unswitched sockets
are for where there are other means of switching, e.g. a switch fuse
above the worktop with an unswitched socket below.
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| ARWadsworth replied to Lurch on 19 May 2004 |
But there is no requirement AFAIK for switched sockets (but I always fit
them), only a requirement for DP switched sockets if an appliance is fixed
(or an unswitched socket or DP switch feed to an unswitched socket) as it is
the double dole isolation that is required.
It was the point you made about a loaded circuit not being allowed to be
unplugged that I was curious about as it would be impossible with some
appliances.
Adam
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| Lurch replied to ARWadsworth on 19 May 2004 |
I seemed to think there was, maybe I'm mistaken but it sounds
sensible.
I was referring to correctly wired installations, if there isn't a
switch to functionally turn of the supply to the socket then you're
right, you can't unplug an appliance 'off-load'. This is why you need
to have things done properly.
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| Christian McArdle replied to Seri on 19 May 2004 |
Put it off a 3A FCU off an existing circuit, or off a 3A MCB on its own
circuit. RCD protection is optional, but preferably not. The disadvantage is
that you lose heating in the event of a trip, which may lead to pipes
freezing if you're off skiing in January.
Yes, that's fine. The freezer and heating won't use anywhere near the 16A
MCB that would be expected on such a circuit.
No. Just take a 2.5mm cable from the freezer socket to the FCU. After the
FCU (with 3A fuse), you can run in 1mm lighting cable. In fact, you can
probably use less than 2.5mm to the FCU, but this would require calculation
and there really isn't much point.
3A if you choose to run on a dedicated circuit. Personally, mine runs off a
3A FCU on a 16A MCB circuit shared with the immersion heater. This enables
combined controls and the safety of knowing that all parts of the central
heating system are on the same circuit.
Christian.
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| Z replied to Seri on 19 May 2004 |
If you can put the boiler on it's own circuit to minimise the disruption
caused by an interruption elsewhere. 16A breaker 2.5mm cable or 1.5
Pirelli terminating in a switched fused spur with /without neon as per
your preference.
No reason to RCD protect the central heating in fact one might get
nuisance tripping if RCD protected. I get pretty sick in winter of
calling out the the central heating contractors to look at GCH systems
which are tripping the RCD protected side of split load consumer units.
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| Martin Angove replied to Z on 21 May 2004 |
Umm... can you enlighten me why Pirelli cable should be so much better
rated than ordinary? The only thing I can think of is that you are
referring to MICS cable...?
Hwyl!
M.
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| Martin Angove replied to Martin Angove on 24 May 2004 |
Now you've confused me >:-/
My problem was with "Z" who was apparently saying that whereas you'd
normally use 2.5mm cable on a 16A circuit, if it were Pirelli cable then
1.5mm is sufficient. TBH 1.5mm T&E should be ok for 20A under method 1
whatever the manufacturer.
I threw MICS in because I am aware (Table 4J1A) that it has a somewhat
higher current-carrying capacity than 70C PVC T&E and wondered if he
had been confused. I might go so far as to allow 90C cables a higher
capacity (more tables)... but there are plenty of manufacturers other
than Pirelli producing those :-)
Isn't LSF cable rated exactly the same as PVC of similar construction?
Never used your Flame Shielded stuff, though I have seen it, but I can't find
it in the regs either. Does it come under MICS?
Hwyl!
M.
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| BigWallop replied to Martin Angove on 21 May 2004 |
Or LSF (low smoke and fumes) or Flame Shielded cable (commonly used in the installation of
fire alarms).
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| BigWallop replied to BigWallop on 25 May 2004 |
You're actually better treating the cables as being the same as PVC T&E Martin.
For safety sake on the circuits you're designing anyway, it's best to use the
tables for the plain T&E cables.
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| Seri replied to Z on 20 May 2004 |
| >My new boiler arrived today
| >Should the boiler be RCD protected or not (any pro's and con's?)
Just wanted to say thanks to everyone for responding and I will be running
the boiler from a dedicated MCB on the non RCD protected side of the CU.
Really appreciated.
Seri
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| Z replied to Seri on 23 May 2004 |
Absolutely. Minimum requirements are exactly what they are- minimum
requirements.
Zero? you'd be surprised what I have seen being lowered out of upper
levels of multis.
RCD protected consumer units are a waste of time IMO. RCBOs are getting
a lot cheaper.
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| Tony Bryer replied to Seri on 19 May 2004 |
Having the CH on its own circuit is the best arrangement IMO, so
that if the main ring is off (either deliberately or as a result
of a fault elsewhere) the heating is not affected. You need to
ensure that you have an isolating switch that isolates all the
heating. 5A and 1.0/1.5 cable should be all you need.
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| Lurch replied to Seri on 19 May 2004 |
No, the only sockets\fittings that need RCD protection are ones that
are likely to supply portable equipment used outdoors. It has the same
pros and cons as the freezer, you don't lose heating or hat water if
the RCD trips.
Yes.
No, you can run two radials from one MCB. A 16A MCB with 2.5mm cables
is ample.
If it was on it's own circuit, then I would fit anything from a 3A MCB
upwards, and appropriate cabling to suit.
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| Lobster replied to Lurch on 21 May 2004 |
Really, is that right? I was under the impression that having RCD
protection was for added general safety in the home, is that not
right? Are you saying that, for example, in a non-ground-floor flat
(where there is a zero likelihood of using portable equipment
outdoors) it is pointless spending the extra on installing an
RCD-protected consumer unit?
David
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| Lurch replied to Lobster on 21 May 2004 |
Yep, certainly is.
No, and yes. An RCD does offer additional safety but isn't required
unless you're likely to be running power outdoors from the sockets in
question. Obviously in the case of a shower it is highly unlikely that
it will somehow become 'portable' and 'outdoors'!
Well, not pointless as such, it does offer additional protection, a
common misconception, held by quite a few electricians too!
An RCD isn't 'required' in such circumstances. At my house I have some
of the downstairs sockets on an RCD but as most of the outside has a
socket conveniently located somewhere nearby it's not really
neccesary.
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