Yep - no disagreement with all that but it does little to support your
You know - I have seen a lot of your advice and it has always been
good but on this occasion I am struggling with your logic!
news@xxxxxxxxxxxxxxxxxx declared for all the world to hear...
Overbudget to start with, and maybe he lives in a place than only
has one mains socket on each wall with pile of extensions plugging
Honeplugs work perfectly well over extensions. I'm using two in my
house right now, one of which is in a 6-way which itself is plugged
into an 8-
way surge protector.
Just an aside:
Plugging an extension cord into an extension cord is not a good
idea. You can easily end up with too high an earth impedance - which
greatly increases the risk of fatal shock and fire.
If you simply must do it, replace the fuse in the second extension
with a 5A one and limit the load to under that. It quite possibly is
well under that, already (ie <1.2kW). A fuse is only a few pence and
only takes a moment to change. Compare that to how much time and
cost would be involved if there was a fire...
Better, of course, is to get the ring main right, with sockets where
you want them - and run a single extension from each.
Sorry to be an old maid but yes - I do know of fires that have
resulted from daisy chaining extension leads..
As you say, the ring main is the best option
Greater risk of shock from Earth impedance increase over an extension
lead or two - give me a break! Nonsense! The risk of a shock is there
regardless of earth. No earth will ever prevent the risk of shock! If
someone diverts the current into themselves then the earth is
irrelevant! In fact, touching the live with one hand and the earth
with the other is an example of when an earth can be more lethal!
Just get an ELCB and this is the best thing for safety in existence.
The fuse in the second lead to 5A - why? Each lead can be 13A and the
combined load of any number of piggy backed leads will only be
governed by the first fuse anyway. If the first lead is fused at 13A
and each of the leads is rated at 13A then there is no fire risk from
overloading at all (with the exception of long leads left coiled up
in which case it can certainly get hot and the effective load rating
is dramatically reduced).
The earth connection to exposed metalwork is there so that any
breakdown in insulation will cause a very large fault current, through
the live conductor, the earth conductor and the protective device,
such as to cause the protective device to operate quickly enough to
prevent the voltage on the exposed metalwork rising to a high enough
voltage, or sustaining that voltage, for long enough to pose a lethal
This relies on the earth impedance being low enough to cause a very
high, but very short duration, fault current.
Fault clearance is dependent on the i2t characteristic of the
protective device. Too low a fault current will result in too long a
fault clearance time, to be safe. A 5A fuse has a lower i2t and will
thus tend to clear a particular fault current much faster than a 13A
This is nothing to do with overloading. This is all about earth
impedance and how fast faults can clear, when they happen.
A typical thermal fuse characteristic is shown here:
Note that (for these typical fuses), a 15A fuse will take 10 seconds
to blow with 40A current. If the per-wire extension lead resistance is
just 3 ohms, the fault current will not exceed 40A, even with a
dead-short between live wire and equipment case. The case will rise to
120v and remain at that for 10 seconds, until the fuse finally blows.
If a 5A fuse had been fitted in that extension lead, it would clear
that fault in less than a tenth of a second.. The case would still
rise to 120v, but only for less than a tenth of a second.
The other factor with extension leads plugged into extension leads is
that this introduces another set of, potentially resistive, brushing
contacts, reliant on spring pressure to ensure a good contact. These
can very easily have enough contact surface to provide a low impedance
at low currents, but not enough surface area to provide low impedance
at high currents. It only needs an extra few ohms to go into circuit
to limit the fault current so that the protective device will not trip
(quickly - or even at all). That high resistance point will, of
course, rise to a very high temperature, very quickly, under those
conditions. Hence the fire risk.
An ELCB is an /additional/ safety device. It should never be a
substitute for correct fault-clearance design.
Never plug extension leads into extension leads. But, if you really
must, at least fit a lower rating fuse.
My original suggestion:
"Plugging an extension cord into an extension cord is not a good idea.
You can easily end up with too high an earth impedance - which greatly
increases the risk of fatal shock and fire. "
"If you simply must do it, replace the fuse in the second extension with
a 5A one and limit the load to under that. It quite possibly is well
under that, already (ie <1.2kW). A fuse is only a few pence and only
takes a moment to change. Compare that to how much time and cost would
be involved if there was a fire..."
The above totally supports my original syggestion...
Your examples and web link argument relates to distribution box fuses
that protect ring mains or radial circuits - NOT fuses in 13A plugs or
No, it relates to *thermal fuses* - exactly the same type of fuse fitted
to 13A plugs and extension blocks. It clearly shows the time v current
characteristics of thermal fuses of different current ratings.
Anyone relying on these to prevent a spontaneous short circuit (what you
were essentially saying) would be daft anyway - they are only there to
protect the ring or radial circuit.
Fuses are not there to "prevent a spontaneous short circuit" - they are
there to clear such faults.
Getting back to the extension leads - they use far more efficient fuses
and in any case they protect the specific extension lead - not the device.
Define "efficient fuse". The cartridge fuses in plugs and extension
leads obey exactly the same laws of physics as thermal fuses in
distrbution panels. They have the same problem in clearing faults
quickly where the fault current is very constrained, eg by too high an
Now - each item plugged into the extension lead will of course be
properly fused to protect that specific item - and will likely be 3A or
perhaps 5A for typical PC items.
The only remotely sensible point you have made. However it is quite
possible to plug a 13A unit in. Indeed, probable in many situations.
The office cold in the morning? How about plugging in a fan heater,
where you work? Happens all the time..
On this basis, I still fail to see your point.
If you're essentially on about doubling up protection then the original
point was - well pointless made really!
But that wasn't your original point!
My point was and remains that plugging extension leads into extension
leads is inherently dangerous. A real possibility exists that a fire
will result. A real possibility exists that a fatal electric shock will
My secondary point remains that, if you must do so, then derating the
fuse in the extension lead is a good idea. It would prevent anyone using
a high current unit individually fused at 13A. As I have shown such an
item, if it develops a fault, can present dangerously high voltages on
exposed metalwork for long enough to kill. It can cause a fire.