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SAFETY WATCH
Safety Watch is an occasional feature in SILICON CHIP drawing attention to
issues of electrical safety in the workshop and home.
Making your electric radiator safe
Does the cord or plug of your electric
radiator get hot? If so, you should think
seriously about installing a cord and plug
more suited to the job.
These days many electric radiators have plugs and cords which
are just not suitable for the task.
We're talking particularly about
appliances rated at between 2000
and 2400 watts and which draw
between B and 10 amps from the
mains.
If they're used on the lower
power settings, they're OK but
when you switch up to full power on
those cold winter nights, the cord
and plug starts to go a bit limp literally.
Two of the photos accompanying
this article show a particular type
of plastic 3-pin plug which is commonly fitted to the cords of electric
radiators. It is nominally rated at
10 amps [as indicated by the lOA
marking between the pins) but
when the appliance is drawing this
current, the plug gets very hot - so
much so, that the plastic starts to
melt. This shows up where the cord
enters the rear of the plug [Fig.1)
and around the brass Active and
Neutral pins (Fig.2).
Part of the problem is that the
pins are fabricated out of thin
brass and thus have a higher
resistance to the current flow than
plugs with solid brass pins.
When you are drawing 10 amps,
even quite a small resistance will
result in a lot of power dissipation.
Say, for example, that the total
resistance associated with the pin
of a power plug is only 50 milliohms
or 0.050 [this figure includes the
contact resistance plus the resis16
SILICON CHIP
tance of the pin itself). With 10
amps flowing, there will be 5 watts
dissipated. With two pins having
the same resistance, the total
power dissipated in the plug will be
10 watts.
Now 10 watts of heat dissipated
in a plug is enough to make it stinking hot. It certainly is enough to
melt it. So you can see that
resistance must be avoided as much
as possible.
Often combined with these puny
plugs is a puny power cord which
also has a nominal rating of 10
amps. The operative word here is
"nominal". As far as we're concerned, it doesn't mean 10 fairdinkum, full-on continuous amps; it
means pony sized amps which probably come out of the same labs
that used to do the exaggerated
horsepower tests for cars.
Such cords are OK if used for a
short period but are not adequate
for radiators that are used at maximum setting for hours on end.
These puny 10-amp power cords
have a plastic sheath with an
overall diameter of about 8mm.
When they are running at 10 amps
they get quite warm and limp. If
they are running over a shag pile
carpet or are in any situation
where air circulation around the
cord is poor, the cord will get surprisingly hot.
Some blower type radiators are
of all plastic construction and so
are double insulated. This means
that they have a 2-core flex which
is even thinner in overall diameter
at only 6mm.
In some situations, we believe
these cords and plugs get so hot
that they could eventually lead to a
fire. In our opinion, they should not
be approved by the electrical
authorities for 10 amp applications.
We reckon they're OK for any appliance drawing up to 5 amps on a
continuous basis but for anything
more than that they are not
suitable.
Replace the cord and plug
Fig.1: carefully inspect the rear of
the plug. If the insulation is starting
to melt, the plug should he replaced.
There is only one type of 3-pin
plug which is suitable for an appliance which draws 10 amps and
this is the HPM Cat. 99. This is not
made of soft plastic but hard white
phenolic. It has solid brass pins and
a large cord entry at the rear.
Heat will not distort these plugs.
So even if you have poor connections inside the plug, the plug will
not melt.
And there is only one type of
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Fig.2: the pins on plastic plugs used at high current can loosen as the plastic
melts. Here, the active pin (at the end of the screwdriver blade) has worked
quite loose, creating a hazardous situation.
3-core flex which we would recommend. It is rated at 15 amps continuous and has an overall diameter
of 10.5mm. It is labelled 301.25mm
and 1.5 sq mm. What this means is
that the three cores are each made
up of 30 strands of 0.25mm copper
wire which gives an overall conductor cross section of 1. 5 sq mm.
Why go for a 3-core flex which is
rated at 15 amps'? There are two
reasons but they add up to the same
result - the cord won't get hot. The
first reason is that this heavy duty
cord has thicker insulation and is
therefore less likely to kink and
strain the internal conductors. Second, the voltage drop along the
cable for a given current is a lot
less than in the so-called 10-amp
cables. This means more voltage in
the radiator and less heat in the
cable.
cable. That way, if you decide that
it looks too hard to mess about with,
you can let an electrician do the
job.
By the way, many radiators don't
have a long enough power cord - if
you decide to replace the cord you
can make it longer.
Let's assume now that you have
. purchased two or three metres of
the recommended cable and the
HPM plug. Remove the existing
power cord from the radiator and
note the length of the Active,
Neutral and Earth wires. Depending on how the cord is installed,
these will probably have different
lengths. You must duplicate these.
The longest wire determines how
much of the outer insulation must
be removed.
Stripping the cable
When you buy your HPM Cat. 99
plug it will come with instructions
on how to wire it and how much to
strip the cable. That's OK for the
plug end but what about where it
terminates inside your radiator'?
What you have to do is to duplicate
the connections inside your radiator.
In fact, it would be a good idea to
have a look at these connections
before you go to purchase any
Fig.3: the HPM Cat. 99 plug is the one
that we recommend for 2400W
radiators. It has solid brass pins and
will not distort due to heat build-up.
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17
Fig.4: a Stanley utility knife with a
sharp blade is the recommended tool
for stripping mains cable.
Fig.6: use the Stanley knife to strip about 12mm from the ends of the inner
cores but be careful not to nick the individual strands of wire. The inner cores
should duplicate the lengths of those on the original cable.
Fig.5: be careful not to nick the
insulation of the inner wires. If you
do, cut off the damaged section and
start again.
Naturally, you have to strip both
ends of the cable, one end to suit
the radiator and the other to suit
the plug. But many people are not
sure how to go about stripping
cable. The recommended tool is a
Stanley utility knife with a sharp
blade.
The idea when stripping the
outer sheath from the flex is to
make sure that you don't nick or cut
the insulation of the three individual insulated wires. If you cut
the insulation of these, you can't
safely wire the plug. A cut in the insulation can eventually lead to a
flashover in the cord and perhaps
even a fire.
18
SILICON CHIP
So if you do nick one of the three
cores, you must cut off the damaged
portion and start again.
Fig.4 shows how to start stripping the cord. You bend it over and
carefully cut around half the circumference but don't go deeper
than 1mm. As you bend it, the cable
covering will then split open as
shown in Fig.5. Having done that,
bend the cable over in the other
direction and cut it in the same
way. The outer covering will then
slip off.
Note that if you have nicked one
of the inner cores it will be immediately visible as you bend it
over.
Now strip about 12mm from the
inner cores as shown in Fig.6.
Again, go easy with the knife otherwise you will nick the individual
strands of wire. As you can see in
Fig.6, you just roll the wire along
the blade edge so that it penetrates
but doesn't cut through. Then by
bending the core back and forth,
the insulation will break and can be
slid off.
Terminate the cable inside the
radiator first. Make sure that all
connections are secure and that the
cable is anchored exactly as before.
Incidentally, if the radiator is a
double-insulated appliance, do not
connect the Earth wire - just clip
it off.
You can now terminate each of
the three wires on the mains plug.
Don't forget to slip the plug back
over the cable before you terminate
the wires. There is nothing more annoying than wiring up a mains plug
and finding you have left the plug
cover on the table, or on the floor!
Now before you apply power, get
out your multimeter and switch it to
a low Ohms range. Check that you
have continuity between the Earth
pin of the mains plug and exposed
metalwork on the radiator.
Check that you have continuity
between the Active and Neutral
pins. If the radiator has a switch
you will have to move this to the on
position to obtain a reading. If the
radiator has a rating of 2400W, its
resistance in the highest heat setting will be about 240.
Finally, switch your multimeter
to the highest available Ohms setting and check that there is an open
circuit (ie, no reading or no pointer
deflection) between the Earth pin
on the plug and either the Active or
Neutral pins.
That done, your radiator can be
put back into use.
One further point: if the power
plug still gets hot when the radiator
is at the maximum heat setting,
then the mains power point itself
may be a little tired and may need
replacing.
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