An extension lead is a simple and useful piece of electrical equipment. For a phone charger, a lamp or another low-power device its use is generally no particular problem.
The situation changes when high-power appliances are supplied through an extension lead: electric heaters, fan heaters, hotplates, machines, larger kitchen appliances or other equipment that draws a large current for a long time.
An extension lead is then no longer just „a few more metres of wire“.
It becomes part of a circuit through which a current close to its permitted load can flow for hours.
The power of an appliance determines the current
For a single-phase load that behaves approximately as a resistive load the basic relation is:
P = U × I
where:
- P, the power of the appliance;
- U, the voltage;
- I, the current.
At a voltage of approximately 230 V, a 2,000 W heater draws about:
2.000 / 230 ≈ 8,7 A
A 3,000 W appliance draws about:
3.000 / 230 ≈ 13 A
Those are currents that can flow through the cable, the plug, the socket and every contact point continuously for several hours.
That is why there is an enormous difference between a 10 W lamp and a 2,000 W heater, even though both are plugged into the same socket.
Why does a conductor heat up?
Every real conductor has electrical resistance.
When current flows through it, part of the electrical energy is turned into heat.
The amount of heat depends on the relation:
Ploss = I² × R
This is a very important relation.
Heating does not grow in proportion to the current, it grows with the square of the current.
If we double the current, the heat losses on the same resistance become approximately four times greater.
That is why problems that go unnoticed at a small load can become very pronounced once a powerful appliance is connected.
The cross-section of the conductor matters
The thinner and the longer a conductor is, the greater its electrical resistance.
A greater resistance means:
- greater losses;
- a greater voltage drop;
- more heating of the cable.
It is therefore not enough for an extension lead simply to have a plug that physically fits the wall socket.
It has to be intended for the current, the power, the length and the conditions it will be used in.
The declared maximum load of an extension lead should not be exceeded.
The most critical place is often not the cable itself
In practice overheating very often appears first at the contact points:
- between the plug and the socket;
- inside the socket of the extension lead;
- on screws and contacts;
- at the joints of conductors;
- on damaged or worn contact surfaces.
The reason is again electrical resistance.
A good contact has a very low transition resistance.
A poor, loose, oxidised or mechanically worn contact has a greater resistance.
At a small current that may not be noticeable.
But if 10 or 13 amperes pass through the same contact for hours, a significant amount of heat starts to be released at that small place.
The contact heats up.
With heating the material can soften and deform.
The contact pressure becomes even weaker.
The resistance grows further.
The temperature rises even more.
A process that makes itself worse can arise in this way.
Why does a socket sometimes melt while the fuse does not trip?
This is one of the most important things to understand.
A circuit breaker primarily protects the electrical line from excess current and from a short circuit.
But a poor contact can overheat even at a current lower than the rated current of the breaker.
An appliance can, for example, normally draw 10 A.
The breaker of that circuit sees nothing abnormal, a permitted current is passing through it.
At the same time one small contact point in a worn socket can have an abnormally high transition resistance and locally reach a very high temperature.
That is why it is possible to see:
a melted socket or plug, while the breaker has not tripped once.
A breaker and a poor contact are not the same kind of problem.
Why is a coiled extension lead a particular problem?
For extension leads on a reel, manufacturers usually state a different permitted load depending on whether the cable is fully unwound or stays coiled.
The reason is not an electromagnetic „coil“ in the sense in which that is sometimes explained, it is above all the removal of heat.
Every metre of conductor creates a certain amount of heat.
When the cable is stretched out, its surface is exposed to the air and the heat is given off to the surroundings relatively easily.
When several dozen metres of cable are packed on a reel, the inner layers are surrounded by other heated parts of the same cable.
The heat is removed far less easily.
The temperature of the insulation can therefore rise significantly.
When a powerful appliance is supplied through a cable on a reel, the permitted load stated by the manufacturer for the coiled and the unwound state has to be respected.
More sockets does not mean more available power
An extension lead with four or six sockets does not thereby gain four or six times the electrical capacity.
All those sockets are supplied through the same cable and the same plug.
If we connect:
- a 2,000 W heater;
- a 2,000 W kettle;
- and one more powerful appliance,
their currents add up.
That is why the number of free sockets on an extension lead is not an indication of how many powerful appliances may be used at the same time.
The limit is the weakest part of the whole current path.
An extension lead plugged into an extension lead
Connecting several extension leads gives:
- a greater total length of conductor;
- more contact points;
- a greater voltage drop;
- more potential places for a poor contact;
- a greater possibility of an overload that the user no longer keeps track of.
That does not mean that joining two extension leads will automatically cause a fault, but the number of weak points increases.
With large and long-lasting loads such improvised installations should be avoided.
Heaters are a particularly unfavourable example
An electric heater is almost an ideal example of a large continuous load.
While it is heating, it can draw a current close to its full rated value for a long time.
That means the cable, the sockets and the contacts do not have a short load of a few seconds, they can be loaded for hours.
That is exactly why with heaters the consequences of:
- poor contacts;
- weak extension leads;
- old sockets;
- distribution blocks of doubtful quality
are especially visible.
And what about a water heater?
A storage water heater also has a high-power heating element and can work for a relatively long time during one heating cycle.
The way a particular heater is connected has to follow the manufacturer's instructions and the designed electrical installation.
An extension lead should not be used as a permanent substitute for a properly made connection for a high-power appliance.
If there is no suitable electrical connection at the place where a water heater, a heater or another powerful appliance is needed, the correct solution is to check and if necessary adapt the installation, not to run an extension lead permanently across the room.
How to recognise a problem?
When using powerful appliances, pay attention to:
- heating of the plug;
- heating of the socket;
- a smell of heated or melted plastic;
- a change in the colour of the housing;
- crackling or sparking;
- a loose fit of the plug;
- deformed contacts;
- damaged cable insulation.
The plug of a powerful appliance should not be so hot that it is unpleasant to hold in the hand.
If strong heating, a smell of burning, melting or sparking appears, use should be stopped at once and the cause removed.
Simply replacing a melted extension lead is not always enough, the wall socket, the connections and the state of the installation itself have to be checked as well.
The most important thing is to look at the whole current path
When we connect a high-power appliance, the current does not pass only through its own cable.
It passes along the whole path:
consumer unit → installation conductors → wall socket → plug of the extension lead → contacts of the extension lead → its conductor → its socket → plug of the appliance → the appliance itself.
Each of those elements has to carry the current passing through it safely.
The whole system is as reliable as its weakest place.
The essence
An extension lead is not dangerous because it is an extension lead.
The problem arises when a large current is passed for a long time through an unsuitable or faulty extension lead.
A few simple rules of physics then apply:
a greater current → considerably more heating
a greater contact resistance → more heat released locally
a thinner and longer conductor → a greater resistance and a greater voltage drop
poorer removal of heat → a higher cable temperature
That is why with powerful appliances the question is not enough:
„Does the plug fit into the extension lead?“
The real question is:
„Can the whole current path carry the current of that appliance safely and for a long time?“