On Thu, 22 Nov 2012 13:53:44 +0000 (GMT), "Dave Plowman (News)"
Plantpot Fuses R Us ?
Once used a Farnell 60v 50A PSU that was wired to a normal 13A plugtop. On
startup after being in cold storage you needed a small handful of 13A fuses that
lasted about a few mains cycles so you could get enough charge in the smoothing
caps to reduce the inrush. Once you'd done that it worked ok. It wasn't a case
of reforming the caps as the were replaced, ideally it needed two plugs to the
Let me help you with a translation for that. What dribble was trying to
The fuse should never have a greater rating than the fault withstand
current of the cable. If you have a high inrush surge to deal with, then
fit a 13A fuse since there is no such thing as a time delay BS1362 fuse,
and a 13A will give adequate fault protection anyway while withstanding
On Wednesday, November 7, 2012 at 6:40:34 PM UTC, David Robinson wrote:
To "the fuse is there to protect the cable" ? Hmmm, that's rather to vague,
and technically incorrect. I think you need to ask what are we protecting
really, and I beg to differ that the fuse is there to protect the cable, he
re are some reasons why:
Really what we are protecting is ourselves and other property from electroc
ution and fire, The simple fact is that if the cable is overloaded then it
will melt and cause a fire, it will then also short out the supply causing
further risk. To illustrate lets consider the service fuse in your electric
ity supply, without it any short or overload would probably take out at lea
st part of your street.
It's easy to forget that the fuse is there to protect against overload beca
use in most cases it's blown due to a direct short, but if you are running
a 3KW heater on a 5A lighting circuit with a 13A fuse, that wire is going t
o melt sooner or later, but even assuming it doesn't, over time the excess
heat will make the plastic brittle and one day it will crack, perhaps when
you pick up the cable! A 13A fuse will certainly protect a direct short an
a 3A cable as it won't really have time to melt before the fuse blows, but
if you want a demonstration of what can happen try dropping a some 13A stra
nded wire across a 12V car battery, make sure you aren't holding it and the
re is nothing combustable around though as the whole length will instantly
burn all it's length before it vapourises- It's quite spectacular, yo might
want to wear suitable eye protection as it could be quite bright.
On Friday, 18 November 2016 22:49:09 UTC, firstname.lastname@example.org wrote:
You're replying to a 4 year old discussion for some reason.
e, and technically incorrect. I think you need to ask what are we protectin
g really, and I beg to differ that the fuse is there to protect the cable,
here are some reasons why:
ocution and fire, The simple fact is that if the cable is overloaded then i
t will melt and cause a fire, it will then also short out the supply causin
g further risk.
That certainly isn't fact.
without it any short or overload would probably take out at least part of y
maybe for strangely forgetful folk
nning a 3KW heater on a 5A lighting circuit with a 13A fuse, that wire is g
oing to melt sooner or later,
stic brittle and one day it will crack, perhaps when you pick up the cable!
A 13A fuse will certainly protect a direct short an a 3A cable as it won't
really have time to melt before the fuse blows, but if you want a demonstr
ation of what can happen try dropping a some 13A stranded wire across a 12V
car battery, make sure you aren't holding it and there is nothing combusta
ble around though as the whole length will instantly burn all it's length b
efore it vapourises- It's quite spectacular, yo might want to wear suitable
eye protection as it could be quite bright.
It's all quite simple. 3A fuses aren't required for new goods that conform
to the law, if extension leads aren't used. IOW in most cases 3A fuses are
unnecessary. Now add the real life facts of old appliances, nonconforming g
oods, extension leads and daisy chained extension leads and it's easy to se
e why 3A fuses are still a smart idea.
On 19/11/2016 02:44, email@example.com wrote:
conform to the law, if extension leads aren't used. IOW in most cases
3A fuses are unnecessary. Now add the real life facts of old appliances,
nonconforming goods, extension leads and daisy chained extension leads
and it's easy to see why 3A fuses are still a smart idea.
Just a thought but do you know if they have ever been "required". I ask
because, when I were a lad (late 60s early 70s) our council house was
rewired from 15A Round pin to 13A square pin. The contractors who did
the rewiring put new plugs on all of the appliances that were about and
in use and every one of these plugs had a 13A fuse in it.
As a 14 year old who was just starting to get into engineering I thought
this was wrong and changed some of the fuses appropriately. However I
thought the contractors might have been doing a dodgy job and I wrote
off to someone (Cant honestly remember who but BSI comes to mind - their
kite-mark was everywhere at the time).
I definitely got a reply, the gist of which was "Any fuse is better than
none and yes there is nothing really wrong with putting a 13A fuse in a
I have been pretty relaxed about fuse ratings ever since.
On Saturday, 19 November 2016 08:37:48 UTC, Chris B wrote:
It sounds like you were wrongly advised. At that time a lot of table lamps
& some other goods used what is now regarded as speaker flex for a mains le
ad. This stuff is not protected by a 13A fuse, it requires a 3A to be safer
. Also other appliances in use at that time could not be counted on to have
inbuilt overload protection.
OTOH the safety standards of the day were a lot laxer. There was plenty of
class 0 stuff in use, wires stuffed under plugs, heating run on 1930s light
ing circuits, taped twisted flex joints and so on.
On 18/11/2016 22:49, firstname.lastname@example.org wrote:
Yup plenty ;-)
One needs to be specific about which fuse, and in what circumstance.
There are some circumstances where the primary of job of a fuse is to
provide fault protection to a cable. However there are other times it
may be required to provide overload protection (to a cable or an
In this particular case it seems to be talking about the plug fuse on an
appliance lead. If one ignores some very old appliances, then its true
the only purpose of the fuse is for *fault* protection of the lead. If
the appliance needs overload protection, then it must include its own
(note this is a different situation from hard wired appliances that
don't have fitted flex and plug - there a manufacturer may require a
particular fuse rating in the supply)
Bit of a red herring there. We are talking about a DVR with its own
internal overload protection. So overload is non issue.
The only thing we need demonstrate is that the plug fuse will clear a
fault at the far end of the lead.
(and that is just a matter of establishing the Prospective Fault
Current, and the applying the adiabatic equation to establish if the
flex has enough cross section to withstand the fault - we don't even
care if it ends up with a conductor temperature north of 150 deg C in
the process, so long as the thing does not vaporise or burst into flames)
More twaddle really... Even with lighting circuits wired in 1mm^2 T&E
the current handling capacity of the cable will take a 3kW load
indefinitely in the majority of cases (unless the cable is buried in
again, probably incorrect.
The current carrying capacity of a cable is dictated by its maximum long
duration conductor temperature. For PVC insulated cables that is
typically 70 deg C. So long as the cable can reach thermal equilibrium
at or below that, then you can expect a long service life from it.
and that is really the end of the discussion - its purpose in life has
been achieved ;-)
Not sure how the suggestion that one sticks a 0.5mm^2 CSA flex across a
source capable of supplying several hundred amps for an extended period
demonstrates anything other than how do dangerous stuff and abuse a
ISTM the poster in question (and I have a feeling this ancient thread
has been discussed here before) does not really have a clear
understanding of the difference types of over current situation that one
may need to protect against.
Overload protection (i.e. stopping something drawing more than its
design current for a prolonged period) is different from clearing a
fault where the only limit on current draw is the resistance of the
wires in question (i.e. a short between live conductors, or between an
line and earth conductor).
A fuse may do either or both in different circumstances.
(plug fuses are not usually called on to provide overload protection,
apart from the situation where they are protecting a spur from a high
capacity circuit. (you could also include multiway extension leads in
Having said that, there is a slight advantage to using lower rated plug
fuses when running appliances from extension leads or in other
circumstances when cascaded from another BS1362 13A fuse, since they may
discriminate better in the event of a fault.
i.e. put the metal leg of your ladder through your drill's battery
charger flex, and a 3A fuse in its plug may blow without taking out the
13A fuse in the multiway extension lead feeding it.
On 18/11/16 22:49, email@example.com wrote:
Now try putting your 13A rated bare wire in series with a 13A fuse
across a car battery. I guarantee that the wire will be fine. Google
"Let through energy" for more detail.
The fuse is designed to heat up and melt before the cable heats up to
beyond its operating temperature (70C for PVC insulated).
Yes, a 13A fuse will not help a 3A cable, but you should have had a 3A
fuse there in the first place.
As for your 3kW heater on a 5A lighting circuit - yes, the breaker at
the source WILL protect the cable and will trip before the cable gets to
70C for an otherwise correctly designed and installed circuit.
Blimey 2012, almost up to date.
On an allied subject, Remember some time back an incident where a person
died due to a fuse not blowing when a mains cable was taken up by an old
style beater hover, mostly made of metal. The cord was not the one on the
hoover but one on a table light, which only had a 13 amp fuse not a lower
one so it never blew.
However I'd not suggest taking up live cables in any case to be honest!
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