I was wondering if someone can help me with a problem I'm having.
With the days getting colder I have started using an electric heater
in my garage. The only problem is that it has a very short power lead
so I have plugged it into a 2way extension which is plugged into a
double socket in my garage which in turn runs to a fuse box in the
garage and then into the house (into an old cooker outlet).
Anyway, the other day, after being in the garage with the heater on
for a couple of hours, I unlugged it and noticed that the plug was
warm, rather foolishly I stuck my finger on the prongs and the live
one was bloody hot.
Can anyone tell me whats causing this and what I can do to fix it as
I'm a bit scared of melting the socket or burning the garage down.
Not at all foolish: the plug wasn't live, after all, and you've found
something which needs attention.
The heater is a fairly significant load (2kW typically, maybe 3kW).
You may well have a loose connection in the extension socket or in
the heater plug. Open both up (if you need telling to unplug the
extension lead first, don't read on or do any of this stuff!!) and
see which it is; you may need to cut back to fresh cable if the
insulation's got charred.
It could also be that the 2-way extension socket is simply cheapo
junk which makes poor contact with the plug pins - many trailing
sockets are fine for light loads like, well, lights, drills, computers
and so on, but don't make the best of contact and will heat up when
used on a heavy load. A better form of trailing socket from that
point of view is a metalclad socket, with a cable gland used to
provide proper strain relief. Being finicky, they're not really meant
to be bashed about on their own - it's possible one of the round
blanking discs could fall out if subjected to rough handling, exposing
the innards to a careless finger. Mounting it on a bit of scrap wood a
little larger than the socket would improve its robustness.
HTH - Stefek
Stefek Zaba wrote
| A better form of trailing socket from that point of view is a metalclad
| socket, with a cable gland used to provide proper strain relief. Being
| finicky, they're not really meant to be bashed about on their own -
| it's possible one of the round blanking discs could fall out if
| subjected to rough handling, exposing the innards to a careless finger.
It is possible to obtain metalclad back boxes without knockouts, for use in
schools etc where vandalism is a problem. Obviously, it would need to be
20mm drilled for the cable gland.
Could be a loose connection in the socket you were plugged into...
If the pin was that hot, it might be worth checking the connections in
the plug itself in case of heat damage there, and it might be worth
considering throwing away the old extension lead as well (heat can weaken
the contacts in the socket, which will only make the problem
If the 2 way extension lead was on a reel, it may be that the load and
inductive resistance (the magnetic field caused by the load can
affect the way the cable copes with the load*) caused the heat - if
you`re going to use a heavy load on an extension reel it`s wise to fully
unwind it first.
For distribution purposes, electricity supply companies used (some may
still use) "reactors" on the 33Kv network - these are essentially big
coils of wire, much like a cable reel, but in fault conditions the
magnetic field caused by the attempt to draw excess current would limit
the total amount of current that could feed into the fault.
Please add "[newsgroup]" in the subject of any personal replies via email
* old email address "btiruseless" abandoned due to worm-generated spam *
It could also simply be that the 13 amp plug fuse is running hot, with
a 3kW load it will certainly run warm and even with a 2kW one it may
be a little warmish. Depending on how well the plug and socket
conduct the heat away this may be the only problem. There are not
many appliances that take two or three kilowatts *continuously* so the
problem is often only evident with an electric fire.
The heating in a coiled extension cable has nothing to do with
induction. Nothing. The live and neutral cores run right next to
each other, and carry the identical current (barring L-to-E leaks!)
in opposite directions; thus the magnetic fields cancel (almost)
completely. The heating of coiled extension cables is *all* about
the boring *resistive* losses warming up the copper, and having a
hard time escaping through radiation, conduction, or convection,
because the cable's all bunched up together. You'd get just as much
heating if you folded the cable there, back, there, back, side-by-side,
tightly bunched, as you get by having it coiled. Unwinding an extension
reel before passing a heftier load through it *is* essential, but it's
done to allow air to circulate between the loosely-looping spillage of
cable you leave behind (where it won't create a tripping hazard, of
course; can you tell we've got an EHS audit in the Labs this week? All
our sharp blades locked away in the tool cupboard, and I might even bother
to put the side panel back on my PC before the day is out, *and* try
to remember to keep my shoes and socks on...)
The shape of a coiled extension lead powerfully suggests "transformer",
"solenoid", "speaker coil", "motor winding", and similar constructions
to all of us: but you'll notice all of those - and the 'reactors' Colin
mentions on the 33kV network - are wound with a *single* conductor, not
with a flow and return right next to each other!
Avoiding inductive effects is the main reason for good wiring practice
requiring phase and neutral to be carried either in the same multicore
cable, or on single conductors running in the same conduit. Separating
them can cause increased radio-frequency interference (some of the
schemes for wiring two-way light switching play hell with induction-loop
hearing aid pickup, f'r example!). And those who separate phase
conductors as they enter machinery - especially if there's a nice thick
iron panel between the entry points - lose power and may encounter
embarassing heating from the eddy currents created; you're supposed to
cut slots between the entry points to reduce this effect if you can't
feed the conductors in through the one hole.
Stefek 'slayer of urban myths' Z
On 7 Oct 2003 15:01:14 -0700, firstname.lastname@example.org (Skirrow)
Dodgy fuse clip. The fuse is on the live pin, so a difference in
temperature on the two pins usually points to this clip, not the pins
Multi-kW appliances should be fitted with a good brand of plug. The
"market stall" grade usually do the job for most things, but you
notice the difference with vibration (the screws work loose) or high
Die Gotterspammerung - Junkmail of the Gods
Presuming that the plug etc. are of the correct size and that the
heater is not oversized for a normal plug-in application (as
opposed to permanently wired in electric heaters etc.);
Sounds like a poor connection of some sort, they can build up
some resistance and get warm/hot.
Could be loose wire connection screw inside the plug? Poor
contact between plug pins and the socket?
The plug pin is maybe either dirty/corroded or damaged. Or the
tension of the socket that the pin goes into is weak or worn or
If it's getting hot and nothing is done it may well get much
worse. Heat will tend to corrode more or weaken spring tension on
contacts even more.
It may be a simple case of carefully sand papering corrosion off
the pin of the plug right through to replacing the socket AND the
plug to gt rid of a damged part and make sure.
Suggest don't leave it unattended in it's present state.
What ampage rating is on the heater ?
How many watts (kW) is the heater ?
What ampage rating is on the extension lead ?
Was the extension lead fully unwound off its reel ?
Is the power socket in the garage wired with the correct sized cable for the
type of loading you're applying to it ?
What else was plugged in along with the heater ?
Is the garage lighting taken from the same supply as the socket ?
Does the garage light go dimmer when the heater is switched on ?
Does the heater have a fan ?
Did you notice if the extension cable itself was also getting warm / hot ?
Although the garage is wired into an old cooker outlet, it may not be heavy
enough to run a lot of small appliances like heaters, lighting, fridges,
freezers, power tools etc. etc. I take it that by saying "an old cooker
outlet" you mean that the old cable is diverted into the garage and it is
now supplying a separate fuse box.
If this is the case, then the earth connecting centre (the bare copper
conductor in the cable) may not be heavy enough to be used as a proper
connection for such an installation, and you may need to take a separate
bonding cable back to the main earth point in the house and create a lower
resistance path for fault signals to trip/blow fuses as quickly as possible.
If it is just this one extension that is causing its plug to heat, then it
is more than likely not up to the job of supplying the heater properly. A
simple test is to try plugging the heater directly into the socket and see
if it causes the same symptoms.
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