UK 13 Amp Plug and Mains Extension Cable Safety Issues

Mains Extension Cable Safety


The initial purpose of this post is to raise awareness of a UK 13 Amp Mains Extension Cable Safety Issue. I came across it by chance when I landed on the PlugSafe site. I’ve taken the liberty of reproducing their information below and enhancing their picture. Full credit to PlugSafe for making us all aware.

As a consequence of a comment on the initial post I have also included an issue with the safety of 13 Amp Mains Plugs too.

Mains Extension Cable Safety Loophole in BS 1363 – Credit to PlugSafe

There is a problem in BS (British Standard) 1363. It has a mains extension cable safety loophole which allows manufacturing of extension sockets with the earth pin socket too close to the edge. When it is too close to the edge the plug can be plugged into the socket in an inverted fashion exposing the LIVE and NEUTRAL pin sockets.

Most multi-way portable socket outlets allow this insertion of the EARTH pin into the EARTH pin socket. Doing so opens the shutters of the sockets for the other two pins – LIVE and NEUTRAL.  People can then poke metallic objects such as pins, paper clips and small keys and screwdrivers into the open holes and get a severe electric shock. This is a clear danger visible in these photos below:

Mains Extension Cable Safety
Live & Neutral Connections Shown Exposed by Insertion of an Inverted Plug’s Earth Pin

Note: It is important to ensure that children are not allowed access to this type of extension.

It is to be hoped that this oversight will be corrected at the next revision of BS 1363. 

Further Issues With Mains Extension Cable Safety

After publishing this post I received a couple of comments from Alastair Wilson. He reminded me of two other issues regarding 13 Amp Mains Extension Cable Safety and 13 Amp Plugs. They are:

  1. The fire risk when using extension cables with a heavy current passing through them if they’re still coiled up.
  2. The danger to persons (particularly children) if they use old 13 Amp Plugs without the insulation on the Live and Neutral pins that later plugs have.

Read on for further explanations.

1. Using A Coiled-up Mains Extension Cable Can Be Dangerous

The wire in a mains extension lead has some resistance even though it is very low. Electricity flowing through any resistance generates heat, so heat is dissipated in extension cables when in use.

The equation used to calculate the power dissipated in an extension cable is:

PC = (IC)2 RC


  • PC = Power dissipated in the cable in Watts,
  • IC = Current in Amps drawn by the appliance connected to the cable,
  • RC = The Loop Resistance1 in Ohms of the Live & Neutral wires in the cable.

An Example of A Mains Extension Cable

In this example I will work out the power dissipated in an 18m long extension cable capable of carrying a current of 13A. This mains extension cable can be used to supply a 3kW electric heater if used to its maximum capacity. See the picture below:

Mains Extension Cable Safety
18m Cable Reel with Live and Neutral looped by shorting the terminals of a mains plug together.

Measuring An Extension Cable’s Loop Resistance

The following measurements were made using the schematic diagram in Fig. 1 below:

Mains Extension Cable Safety
Fig. 1. Schematic Diagram of the Loop Resistance Test Circuit for an Extension Cable.

Checking The Test Lead Resistance

First I measured the resistance of the test leads of my ohmmeter because it can be significant when compared to the low value of resistance found in a good electrical conductor such as wire in a cable. To do this I had to:

  1. turn the ohmmeter on and set it to the lowest range,
  2. connect the test lead probes together with their crocodile clips.

See the picture below:

Mains Extension Cable Safety
Taking a measurement of the Ohmmeter Test Lead resistance.

As you can see, from the ohmmeter screen above and Fig. 1, the sum of the resistance of the two test leads was:

0.05Ω + 0.05Ω = 0.1Ω.

Measuring The Total Loop Resistance

Secondly I shorted the Live and Neutral wires together at the socket end of the cable. I did this by inserting an open mains plug into the socket with its Live and Neutral terminals connected by a thick piece of wire.

I then connected the crocodile clips on the ohmmeter test leads to the live and neutral pins of the mains plug at the other end of the cable. As you can see, from the ohmmeter screen below and Fig. 1, the loop resistance of the test leads plus the cable was 1Ω.

Mains Extension Cable Safety
Measuring the loop resistance of the cable reel.

Calculating The Extension Cable Loop Resistance

Using the formulae:

RC = R2 – R1


  • R1 = Resistance of the test leads,
  • R2 = Combined resistance of test leads and cable,
  • RC = Loop resistance of the cable on the reel.

I subtracted the resistance of the test leads from the loop resistance of the cable and test lead combination thus:

RC = 1 – 0.1

This gave a cable loop resistance of:

RC = 0.9Ω

Calculating The Power Dissipated In The Cable

Putting values in the equation for the power dissipated in an extension cable gives:

PC = (13)2 × 0.9 = 152.1W


  • PC = Power dissipated in the cable in Watts,
  • IC = 13 Amps,
  • RC = 0.9Ω.

Using The Mains Extension Cable At Full Capacity

The power dissipated by this example extension cable at full capacity is:

152.1 Watts.

It’s 18m long so the power per metre is:

152.1W ÷ 18m = 8.45W/m (Watts per metre).

I will give this some sort of meaning:

A person’s hand is about 10cm wide. So grabbing a handful of cable (a tenth of a metre) will let a person feel the heat. They will feel 0.845W of heat which, I can assure you, is nothing. They won’t notice that it’s above ambient temperature.

I conclude that uncoiling a cable in the open air alleviates heat dissipation problems. So that’s how we should use them – uncoiled.

Take this same 18m cable and coil it up tightly onto a reel and that 152.1W will be confined to a relatively small volume. The surface of the coil does have the air around it but deeper into the coil there will be no circulating air and plenty of plastic insulation. The insulation is both an electrical insulator and a heat insulator. So the inner core just gets hotter and hotter.

Unlike metals PVC insulation melts at relatively low temperatures. Soft PVC allows tensions on the conductors to make them cut through the insulation and short out or start sparking. This can result in fire. The circuit breaker or fuse may cut the power off but the damage to the cable will be permanent.

Using The Mains Extension Cable At Less Than Full Capacity

Mostly extension cables are used at less than full capacity so they don’t get anywhere near as hot as I have indicated they might at full capacity.

Note: There is a square law involved so if the current is halved the power dissipated will only be a quarter of the maximum at 38.025W.

Consequently it is possible to use an extension cable coiled up if a low power is required by the connected appliance and if it is only required for short periods. So using it to supply an electric heater might be dangerous but to extend an electric light may be O.K.

NOTE: The only advice I can safely give is to:

  1. uncoil an extension cable under all circumstances before use to prevent it overheating,
  2. ensure coils of wire on the ground don’t become trip hazards by using the shortest extension cable that will do the job.

2. Using Old Mains Plugs Without Insulated Pins Is Dangerous

The Danger Involved

When inserting or removing a 13 Amp mains plug fingers can wrap themselves around a plug and touch the pins. This can happen to anyone who finds them tough to extract from the socket. Children and the aged with their weak grip are particularly susceptible. These people can potentially get a lethal electric shock by doing this.

It doesn’t matter if fingers touch the earth pin since it makes contact with the earth connection in the socket before any other pin. (That’s why it’s longer.) Consequently the earth pin is always safe.

If someone’s fingers touch the live or neutral pins after the live pin makes contact in the socket they will get a serious electric shock.

The Danger In Detail:

  1. If the neutral pin connects before the live pin it should be safe since neutral is at earth potential.
  2. If the live pin connects before the neutral then the live pin will be dangerous and, if the appliance is switched on or it has no switch, the live potential will appear at the neutral pin, until it is connected, making it dangerous too.

New & Old Style BS1363 Mains 13 Amp Plugs

New Style Plugs

The danger has been removed for some years now since the introduction of BS1363 – 13 Amp plugs with plastic insulation on their live and neutral pins. Since 1984 insulated pins have been a requirement.

See this article in Wikipedia.

safe mains plugs
BS1363 plugs with insulated pins.

Old Style Plugs

The danger comes from continued use of old style plugs made before 1984 which have uninsulated pins. If anyone is using them please get them changed. I have to confess I have found some on items I have inherited or assembled before that date. They look like this:

unsafe mains plug
Pre 1984 13amp plug with uninsulated pins.


  1. Loop Resistance is measured by joining two wires of a cable together at one end of the cable (to make a loop) and measuring the resistance with an ohmmeter (or Wheatstone bridge) connected to the same two wire at the other end of the cable. This method measures the resistance of the two wires added together because they are in series.


Author: Helpful Colin

I have a background in telecommunications and a fascination with all things scientific and technical - from physics to electronics, and computing to DIY.

5 thoughts on “UK 13 Amp Plug and Mains Extension Cable Safety Issues”

  1. Even more dangerous are old plugs whose brass conductors extend right up to the body of the plug. If a British Standard child pulls such a plug halfway out of its socket, its fingers can get underneath the body of the plug and touch the live pins, at the same time as the pins are connected to the live source in the socket. (All recent plugs have a plastic sheeth along half the length of the pins to prevent this problem.)

    1. Hi Alastair,

      You make a very good point. Thank you.
      I come from the days of the old plugs and I’m familiar with the issue. I think about it each time I have some difficulty pulling a plug out of the socket. Even with big mans fingers I think they might still touch the brass of the insulated pins if I grab hold of a plug with anything other than a delicate finger and thumb grip.
      I will edit my post and include this subject.

      Regards, Colin.

  2. Another even more serious problem with large coiled extension leads is that people do not realise they need to be completely uncoiled before large currents are passed through them (such as the power needed by an electric fire). If they are not fully uncoiled, some sort of heating occurs in the coil, which given time melts the insulation and can cause a fire. I often wonder how many fires occur this way, having seen a coil with melted insulation. (If you look very closely at some extension coils you will see a warning in small print – but who reads such warning?)

    What I would also like to know is whether the effect is some sort of exotic inductive effect, or is it just that high currents generate a little heat in the wires, which cannot escape if the wires are still coiled tightly on top of each other instead of snaking over the floor.

    1. Hi again Alastair,

      Thanks for this comment too.

      This is an issue I’m even more familiar with since my father did just that – get his coiled extension overheated. He had a really long extension with cable which went back to the days of 5amp & 15amp round pin plugs. This cable was geared up for 5amps and was used for his B&D hedge-cutter. I didn’t see what he did but I visited afterwards to find him having the greatest difficulty unwinding what was left of the cable. I don’t remember it being burnt. It had just melted together, more as he got to the core of the reel. The final few turns had to be cut through to remove them from the reel.

      As to your second point, the source of the heat: I am sure the heat comes only from the resistance of the wire and the inability of it to escape. The current in the live and neutral are always traveling in opposite directions so their magnetic flux cancels out. If the live wire was wound on one steel reel and the neutral on another then I think eddy currents would be induced into the steel and there would be extra heat from that.
      I will also include these points you’ve made when I edit my post.

      Regards, Colin.

    2. Hi Alastair,

      Sorry for the delay but I have updated my post considerably since your comments. You may wish to take a look at it.

      Thanks again for pointing things out to me.

      Kind regards, Colin.

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