Shame whoever wired this place did not think of that... they did one 5A
(Type 1 MCB) and then worked out it was not going to hack the 40 or so
lamp fittings on the ground floor, then split of two rooms on a second
one. Even then the first circuit would not tolerate all the lights on
for long!
I was just thinking the 'Grand Designs' usual of thousands of low voltage
downlighters must need some thinking about - certainly before LEDs. When I
wired my place the usual was a single bulb pendant per room. ;-)
--
*What happens when none of your bees wax? *
Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
People here before were fond of the multi armed dangly thing with three
or five spokes and a lamp on each... they soon ramp up the lamp count if
not used sparingly!
Judicious use of low energy lamps, and a few fitting changes, and bulb
wattage reductions has tamed the excess a little. Lots of high ceilings
though, so they take a reasonable amount of lighting to be comfortable.
Its not a departure at all - its one of the standard circuits listed in
the On Site Guide:
http://wiki.diyfaq.org.uk/images/2/2b/Standard-Lighting-Circuits.jpg
(you will note there is also a standard 16A lighting circuit - although
that is very unlikely to be used in a domestic situation)
Sounds like a very useful learning experience. Keep in mind though that
for subtle details that was several issues of BS7671 ago, and stuff has
(and will continue to) evolve.
Alas in those days many professional electricians had very little clue
about equipotential bonding (even the term "earth bonding" is a misnomer
when you think about it - earthing and bonding are two different systems
designed to add protection in two different ways). Not aided much by the
15th edition regs that also over egged the requirements somewhat.
You may find this useful:
http://wiki.diyfaq.org.uk/index.php?title êrthing_and_Bonding
The key requirement in situations where EQ bonding is used is to
establish if the metalwork in question is capable of bringing a
potential into the equipotential zone. If its not, then there is no
point in bonding it. Hence why things like sinks and bath do not require
bonding in spite of so many electricians believing they do.
Many of these things are chosen for backward compatibility as much as
fundamental theory.
Depends on how you define "manage". It an installation that does not
provide adequate discrimination[1] in the event of a fault
[1] i.e. the requirement that the effect of a fault be localised to it.
Plunging an entire house into darkness due to a lamp failing and
tripping a MCB is not acceptable today. (it never really was, but the
implications of it had not really been considered at the time)
I sense an anti 10A fuse agenda here ;-)
I think you will find that modern lighting accessories are fine on 10A
circuits (note that even the limitation on the "small" version of the
normal lamp holders was removed).
(and TBH a 6 way CU is woefully inadequate for many places these days)
If doing it now, it would be quite common to do that, and also add an
extra ring circuit for the kitchen / utility room - since the modern
compliment of kitchen appliances can rapidly use all 7.2kW capacity of a
ring circuit.
Would it not have been simpler to provide a dedicated circuit for this
purpose? Then you would *know* its not going to be tripped regardless of
what happens elsewhere.
Its not uncommon to find all sorts of strange stuff in existing
installations - especially if they are ageing and have been hacked about
a bit in the following years.
(I recall digging a 5A circular junction box out of the plaster above
the sink in the kitchen of a neighbour's property. (yes it was the screw
terminal type that is intended to remain accessible for maintenance))
I would expect that as long as you fill in the test results correctly
they will be happy (they probably don't even look at the name of the
contractor on top of the page!)
Your suspicion is this case is unfounded - or at least not for this
reason. Needless to say Part P has complicated the "certification"
process somewhat in subsequent years.
Quite possibly, although you would need to consider what you are
attempting to achieve, and also the unintended consequences of the change.
Keep in mind that when providing over current protection for any
circuit, there are two discrete issues that need to be addressed:
overload, and fault handling. (i.e. long term over current resulting
from applying more load than the circuit design current, and the very
short term effects from massive over current caused by someone nailing
through the cable etc). Traditionally these two functions are provided
by the same protective device at the origin of the circuit[2]. If you
are using one of the standard circuits, then both of these are allowed
for in the design; The protective device nominal rating is lower than
that of the "as installed" cable current carrying capacity[3], and
someone has already done the sums to make sure the cable won't melt
while carrying 1000A for the few ms necessary to open the MCB on its
magnetic trip).
So for example changing a "whole house" 10A lighting circuit to a
"normal" type B 6A MCB on the grounds that modern lamps no longer
require the current may be fine. But you are starting with a circuit
that was already adequately protected. As consequence of changing the
protective device, you have also reduced the fault current tolerance of
the MCB from 50A to 30A (type B MCBs need 5x nominal current to trip on
the "instant" part of their response curve)[4]. A result you may now
find is that when one of the few remaining incandescent lamps does fail,
its far more likely to trip the whole circuit. (and trips and falls in
the house kill orders of magnitude more people each year than are
electrocuted!) (note that this is easy enough to mitigate in this
particular example since you could use a type C MCBs instead)
[2] this does not always have to be the case - fault current protection
must be provided at the origin, while overload protection may be
delegated elsewhere or in some cases may not be required at all.
[3] Remember that the "clipped direct" current carrying capacity of even
1.0mm T&E is 16A
http://wiki.diyfaq.org.uk/index.php?title Êbles#Cable_Sizes
[4]
While that all sounds nice and warm and fuzzy, can you actually justify
it on technical merit?
Agreed in most cases... although keep in mind the increasing use of
ancillary loads on lighting circuits - e.g. extractor fans for forced
ventilation in kitchens, loos, bath/shower rooms etc, localised space
heating with "heater" lamps etc.
Well thanks for that revealing information. It's additional knowledge
I can retain for future reference. Obviously, I'm not going to uprate
the fusing on my owm lighting circuits since the efficacy of the 5A
fusing has amply proved itself in my case over the past 3 decades.
Still and all, the whole point of a single fuse at the CU to protect
a lighting circuit is essentially to minimise the risk of a house
fire. Being 'plunged into darkness' is very rarely as fatal as smoke
inhalation or worse.
Of course there is always the counter argument in the automotive case
where it is (or at least, was) deemed preferable to eliminate a sudden
blackout of the headlamps due to a fuse fault by foregoing any
protective fusing whatsoever, making the risk of an electrical vehicle
fire higher as a result. The idea being that it is better to still be
able to safely navigate along a narrow country road on a dark night
for as long as it takes to bring the vehicle safely to rest and permit
a rapid bailout.
I believe modern practice with vehicular wiring is to fuse each
headlight independantly now that the fusebox is housed within the less
harsher environment of the passenger compartment, usually just under
the dashboard over the driver's footwell.
Well yes but you _know_ why. Even now, the idea of using a 10A fuse
over a 5A fuse for lighting circuit protection still leaves me with a
feeling of uneasiness.
I'm familiar with the 'moving goalposts' syndrome. Obviously a fine
tuning of the safety versus utlity compromise. At least you still have
the option 'to make it safer'.
Funny you should mention that. :-)
We needed to have a downstairs toilet and shower room extension just
after moving in for the benefit of the XYL's elderly parents whom we
had moved in with us (the whole reason for buying such a capacious
property in the first place).
What we landed up doing was to convert the woefully inadequate
kitchen (probably an earlier conversion from a cloakroom) into the
downstairs shower room and use the extension to create a new kitchen.
I remember recovering the 2.5mm FT&E cable and sockets (along with a
redundent porcelain DC fuse holder and fuses that had simply been left
in situ) after the old kitchen space had been gutted prior to its
conversion.
In the meantime once the kitchen extension had been completed, the
electrical contractor added a 2 way wylex fuse box to extend a
seperate kitchen ring main and supply a cooker point feed. The
lighting was served by the addition of one extra fluorescent tube
luminary to the existing ground floor lighting circuit.
Overloading the kitchen ring main has never been a problem. I suppose
it is just possible to overload it if we were to switch the 3KW kettle
on whilst using the "George Formby", toasting bread , microwaving some
food and 'Brevelling' some tasty sandwiches whilst we (the XYL & I)
each run a Dyson over the respective halves of the kitchen dining room
floor areas when the noise of the dishwasher will be nicely drowned
out. We may even have enough 13A sockets without having to disconnect
the fridge and the freezer nor cheat by plugging into the 13A cooker
point socket.
I'm sure that would be a good 8 or 9KW's worth right there (maybe
even 10). The strange thing is, we've never felt impelled to "Work so
hard at cooking and cleaning as to make our eyes bleed" Not even at
Christmas time when it was 'our turn' to host the 'christmas family
do'.
Perhaps we simply don't posses enough of the latest energy consuming
'Must Have' kitchen gadgets to bless us with such overloading
problems.
Think it through. What's important? having a dedicated circuit that
can still go faulty despite its splendid isolation from the other ring
mains or a choice of three alternative sockets less than an extension
lead's length away. Remember, this sort of eventuality ideally should
never happen or at least no more than once in a lifetime so there's no
good reason to over engineer a solution when a perfectly good one is
already to hand.
Just the fact that lower fuse ratings result in lower fire hazard
risks. I wasn't concerned over the electric shock hazard which remains
unchanged when simple fuses provide the protection.
If my floor standing cooling fan is anything to go by, I'd imagine a
small vent axia type of fan wouldn't draw much more than 50 watts. As
for 'heater lamps' you're probably looking at 500W max (in a suitable
lamp fitting) or else 1KW for a radiant 'bathroom heater' that's
mounted high up and operated by the obligatory pull cord.
The more powerful radiant heater would be better served by a 5A fused
junction box spur tapped off the ring main. Quite frankly, I wouldn't
feed such loads from a lighting circuit other than maybe a vent axia
fan or two. You seem to be trying to justify the higher 10A fuse
rating with extra appliances that would be better served from a ring
main supply (via a suitably fused connection box).
My own personal view is that the the primary function of a CU is
safety and, to this end, selecting a lower fuse rating option over a
higher one when conditions allow maximises its effectiveness at
minimising the risk of a house fire.
The risk can never be totally eliminated whatever size fuse is chosen
but the risk does become more improbable as you lower the fuse rating
used on any existing circuit. Take my 15A ring main as an example of
this. I've never ever had to replace the 15A fuse link with a 30A one
because of 'overload' issues (nor for any other reason).
The 15A fuse link has sufficed quite nicely for the loading this
particular ring main is subjected to. IOW, its utility hasn't suffered
as a result of the lower fuse rating so I've gained an extra margin of
safety 'for free' by this simple expedient.
I suspect that most domestic 10A lighting circuits could be just as
well served by a 5A fuse link or 6A MCB these days simply by virtue of
the extensive use of CFL lamps in place of the tungsten filament GLS
lamps they'd originally been sized for. It's certainly worthy of some
consideration if you have 10A lighting circuits in your home.
On Thursday, February 13, 2014 1:25:03 AM UTC, Johny B Good wrote:
The oldest vehicle I had had no fusing whatever. The original fusebox with space for 2 fuses was present, but not connected.
Exceeding 30A on a ring circuit isnt a problem, theyre designed to deliver much more for limited periods. 30A is the continuous rating. We regularly used to heavily exceed 30A at the last house, but nothing got hot or blew.
NT
On Wed, 12 Feb 2014 18:13:03 -0800 (PST), snipped-for-privacy@care2.com wrote:
The manufacturer probably decided the risk with 6 or 12 volt wasn't
worth the expense and trouble of 'fuse protection' (plenty of other
hazards to consider) in a box on wheels that could be rather more
swiftly evacuated than a typical house (plus, one might hope, at least
one of the occupants would be wide awake and alert to such a danger).
ISTR, that 2.5mm FT&E has a continuous rating of 27A (depends on how
the cables are routed (trunking with other cables versus single run
clipped onto beams and joists with adequate air space).
The 27A rating is only 3A shy of the full 30A fuse rating but on a
ring main it would be difficult to persuade more than 27A to flow over
the shortest section between the CU and the nearest socket (but not
impossible under the right (wrong?) circumstances).
The remaining 3A is modest enough that in most installations this
would be supplied over the 'scenic route' via the rest of the ring
main wiring.
What helps is the 13A fusing of the plugtops limiting the current
draw from a twin gang outlet to 26A (the regs might have something to
say in regard of the use of 3 and 4 gang outlets - I just don't know).
In most domestic properties, the risk of overloading the shortest run
to the socket nearest the CU is vanishingly small so the use of 27A
rated cabling in a 30A fused ring circuit is deemed acceptable in the
regs (it's only a 10% overload at its worst after all).
On Thursday, February 13, 2014 3:03:21 PM UTC, Johny B Good wrote:
th space for 2 fuses was present, but not connected.
Maybe. But I doubt it following a convo I had with them :) Trying to find o
ut what the engine was, the oldest guy that had been there forever said 'we
used to put whatever was lying around in those, could be anything.' I dont
think vehicle hazards were considered much in postwar designs.
er much more for limited periods. 30A is the continuous rating. We regularl
y used to heavily exceed 30A at the last house, but nothing got hot or blew
.
Its routine, but not a problem. Think about it, washing machine dishwasher
kettle microwave... we often exceed 30A.
NT
I don't think anyone was suggesting that you do. Just highlighting that
it is an allowable and properly designed option.
Missing the point I think... fusing good; single lighting circuit bad!
The chances of any lighting circuit causing a fire in the first place
are vanishingly small, but having a pair of them (and in consequence a
lower average current load on each) can only improve the odds further.
So if a house is wired with a single 10A circuit, you would get a much
greater improvement in safety by splitting it into two circuits (of
whatever rating), rather than just keeping it as it is and lowering the
fuse size.
There are a few permitted cases where fusing may be omitted on similar
grounds - i.e. where failure of the supply is more dangerous than the
circuit being damaged by over heating etc. (unlikely to crop up
domestically though!)
Whilst I agree with the general principle of using protective thresholds
of no bigger than they need to be, I also recognise that the absolute
size of the fuse is not the sole guide to how safe a circuit is.
Possibly - but that does not always equate to a lower current protective
device. Nuisance trips from "close to tripping" protective devices can
in themselves harbour dangers.
[snip]
With kitchens you can generally ignore the kettle and microwave etc -
they are not usually on long enough to pose a problem. The big white
things with heater elements like washing machine, dishwasher, tumble
drier etc can swallow more "long term" capacity. Some single ovens are
also often plugged into a kitchen ring circuit these days.
Keep in mind the circuit is *designed* to be able to cope with short
term overloads of several kW without any damage. A normal 32A MCB will
happily supply 45A for some time, and even 50A for 10 mins or so. This
reflects the case that the wiring itself also requires some time to
reach a temperature where it will start to suffer damage.
Most people don't. Although many have enough that it would become an
issue if the kitchen was sharing a circuit with large parts of the rest
of the house as well.
Whichever ring you power it from, its sharing with other appliances,
which may cause a nuisance trip. Without sharing, there is no possible
cause for a trip other than catastrophic failure of the freezer itself!
(it would also allow the provision of a freezer supply that does not
have RCD protection, which eliminates another set of trip scenarios from
an application where enhanced shock protection is not an issue)
[big snip]
Yup fans are typically no more than tens of watts at most. Most heater
lamps are usually under 750W. Although there are a few over 1kW. E.g:
http://www.tlc-direct.co.uk/Products/SHHL1.html
A 6A circuit gives you 1380W to play with. So that would only allow
another two lamps (allowing the required 100W per lighting position) on
the circuit if using a 6A one.
Other common reasons for larger capacity circuits would be a largish
bank of strip lights - since they can have a fairly heavy switch on surge.
I find no need to "justify" the circuit - they exist, and are
appropriate for some situations. I was just pointing out some of the
reasons that the "normal" 1380W may not offer the required capacity.
Nothing wrong with the logic... although you need to focus attention on
the right things. Adding RCD protection would give a much more
significant reduction in the likelihood of of house a fire cause by an
electrical source than say worrying about (properly installed and
configured) 10A lighting circuits. As would Fitting mains powered
interlinked smoke alarms.
Any circuit is capable of supplying ample current to heat something
enough to cause a fire... Quality of terminations and cable insulation
are more relevant.
Your 15A rewireable will supply 28A pretty much indefinitely anyway - in
these days of central heating, most ring circuits outside a kitchen are
less heavily loaded in the past (lots of small appliances rather than
heavy load space heating etc)
Personally I don't have any 10A ones, although as I alluded to
elsewhere, one would have actually been a quite a good choice here due
to the huge number of lamps employed on the ground floor! (when we moved
in, turning on half the downstairs lights all at once would trip the 5A
MCB after about 10 mins) (I have replaced the CU since)
Now I have three 6A Type C MCB circuits for the house, another for the
outside lights, another in the garage / workshop. (I also have battery
backed emergency lighting fed from the adjacent normal lighting
circuits, since there are no street lights here, and it can be *very* dark)
I appreciate that. I was pointing out that just because I could
uprate to 10A fuses without violating the regs, I wasn't about to do
so (you're not obliged to fuse up to the allowable limits if you don't
need to).
If I'd been aware that use of 10A fusing was permitted on lighting
circuits when I was rewiring the house, I'd probably have chosen that
option (but I've no doubt, after replacing all bar one incandescent
lamp with CFLs, I'd have long since downrated to 5A fusing by now
anyway).
I agree that there's more to having two lighting circuits simply to
limit the maximum current draw. It's just that there are a lot of
quite modern 3 bedroom properties (less than 40 years old) relying on
only a single lighting circuit (along with a single ring main) and I
thought this was standard practice. I suppose it was by the cost
concious "Barret Homes" of the building industry at the time.
You'll get the same safety benefit regardless of whether you simply
replace the 10A fuse with a 5A one or split the lighting into two
seprately fused circuits, each fused at 5A. In the latter case you'll
have improved utility over the single circuit and retain the same
safety level against faults in the fittings and lamp cordage.
If we ignore the bare faced lie that CFLs have 5 times the luminous
efficiency and assume a more reasonable 4 times improvement, a total
relamping of the original filament lamps with equivilent light power
CFLs will reduce the load current to only 25% of the original
requirements used to define the size of fuse.
For anyone who has replaced most of their incandescent filament lamps
with CFLs or LEDs, they can probably simply replace the 10A fuse with
a 5A one without detriment to the utility of the lighting circuit.
There _just_ might be a reference to life support system power in the
home but other than that I wouldn't have thought the regs would allow
any such 'dispensation' whatsoever.
I'm aware of this.
"House Owner Rage"?
Well, it helps that the kitchen's ring main is not cursed with the
washing machine and tumble drier loads, they live in the utility room
right next to the downstairs shower/toilet and are plugged into the
ground floor ring main circuit.
I see your point but my setup is protected by simple fuses with no
ELCBs in sight so I don't have the problem of 'nuisance trips' to
contend with. In any case, a freezer can go unattended for 24 hours
before it just has to be powered back up.
I think we'd notice a ring main outage well within that time limit
and pay the basement a visit if only to check the fuse box, allowing
us to swap the freezer to an adjacent working socket if need be which
is not even one step away from the fuse board.
That chest freezer has long since been sent to the council tip. I
don't think we even bothered to relocate it to the newly built kitchen
(BICBW on this point) electing instead to simply buy a new more energy
efficient unit more in keeping with our 'brand new' kitchen.
As I said, not an issue in my case.
Well, you're offering a justification regardless. I'm not saying
you're wrong to provide such justification, just pointing out that
such justification doesn't apply in my case since, like most homes
connected to the gas main we enjoy the benefit of central heating
which makes such additional loads redundent.
For those with electric only homes, I can see the justification to
use 10A fusing on the lighting circuits to increase the utility over
and above the bare minimum of lighting only.
I might even consider uprating the fuse to allow the use of such a
heating lamp if I decided to improve the comfort of taking a bath
whilst a CH fault had forced us back to using the immersion element
for heating the water. TBH, I think I'd forego such luxuries on the
grounds that it'll 'help concentrate the mind' on getting the CH
repaired in a timely fashion.
I made sure to include such redundency when the CH was installed but
it did rather help that the previous owner had installed a hot water
tank designed for both immersion and central heating which reduced the
parts cost significantly of the CH installation - it was only a matter
of plumbing the heat exchanger coils into the CH system.
We've got plenty of the stand alone battery powered smoke alarms
scattered throughout the house. That's a 'last line of defence'
measure to protect life and limb (but not goods and chattels - any
saving of those would simply be a side effect of the primary
protection).
You make a very good argument for my case. :-)
You've built a system that meets your requirements, seemingly taking
the more cautious approach with the lighting circuits. I guess you've
been playing "Devil's Advocate" in offering the counter argument to my
assertions which were initially prompted by my ignorance of the regs
in regard of permissable lighting circuit fuse ratings.
I've learnt something new so the whole discussion hasn't been a
pointless waste of time and I'm sure it's given others something to
ponder upon which might prove to be of some use in the future.
433.3.3 does indeed include life support systems, but it also includes
supplies to the exciter of a rotating machine, to a lifting magnet, the
secondary circuit of a current transformer, a supply to a fire
extinguisher system, and ones to safety systems like smoke or gas alarms.
(these are in addition to the overload exceptions permitted for other
reasons)
that and falling down the stairs in the dark etc.
A lamp tripping a MCB when it fails, is still a "nuisance trip", and not
RCD related... (one area where BS3036 rewireable fuses have an advantage
over MCBs)
(Not having *any* ECD protection kind of renders any worries about
fusing somewhat moot, since you are forgoing the single biggest
contribution to shock and fire protection to have come about as a result
of electrical regulation changes in quite a number of years).
+1 And, if possible a half amp fuse (the actual draw for a typical
aerial amplifier will be way less than 0.1A). The lower the better in
this instance.
On one occasion we had a small domestic amp (of reputable make) develop
a fault which caused it to make the nearby carpet smoulder (we'd
supplied it and left it inside the TV cabinet, but the customer had
re-arranged things and screwed it to the skirting board.) The amp had
obviously got very hot indeed. It was of a type with a thin walled steel
case inside a black plastic case, and the latter had melted quite a bit.
The amp had not blown the plug fuse, which was 3A. The smell had alerted
the customer when he came home and he had disconnected the power.
Incidentally I've told this before on here, but there was the young mum
who, offended by the smell coming from the masthead amplifier power
supply, and concerned that it might burn the baby's fingers, wrapped it
in towels.
Bill
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