Tag Archives: safety

Plug wiring colour scheme

UK plugs use brown insulation for the live wire, blue insulation for the neutral wire and green with yellow stripes insulation for the earth wire.

But why this particular combination of colours? The answer is deceptively simple: there is no type of colour blindness that will result in these wires becoming confused.

Above: how a UK plug looks to someone who is red-green colourblind.

Above: how a UK plug looks to someone who is blue-yellow colourblind.

One of the lesser-known safety features of a UK plug is the extra distance that the neutral wire has to travel when compared to the live wire. If someone pulls on the mains cable the live wire will disconnect first, making the plug safer.

Under the IEC 60446 standard only black, brown, red, orange, yellow, green, blue, violet, grey, white, pink and turquoise are acceptable colours for labelling wires. Countries must choose an appropriate selection of colours that eliminates the possibility of confusion.

IEC 60446 colours. From top to bottom: normal vision, deuteranopic vision, tritanopic vision.

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How does the damage caused by exposure to radiation vary as the dose of radiation increases?

Most people assume that if you double the amount of radiation you double the damage caused, and that there is no threshold below which no damage is done. This is called the Linear No Threshold (LNT) model and is represented by the graph below:

The LNT model has been the subject of some disagreement in recent years. The American Nuclear Society said in a 2001 Position Statement* that:

“There is substantial and convincing scientific evidence for health risks at high dose. Below 10 rem (which includes occupational and environmental exposures) risks of health effects are either too small to be observed or are non-existent.”

This linear threshold model holds that there is a limit below which no damage is caused, but that damage then increases linearly beyond that limit.

There are other possible models. Damage may increase in an exponential way, with very low damage at low doses but increasing amounts of damage at higher doses.

In a logarithmic model the damage would be very large at first, but taper off as the dose increases.

So which model is correct?

The LNT model remains the most commonly used by regulatory bodies, but there is growing interest in threshold models and in the idea of radiation hormesis, the idea that a small dose of radiation is actually good for the body by somehow stimulating the body’s repair systems. I think the most likely candidate is a J-shaped curve with a significant threshold but then a fairly rapid linear increase in damage caused.

* American Nuclear Society, Health Effects of Low-Level Radiation, Position Statement 2001.

Long half-life ≠ dangerous

Nuclear waste is often quoted as having a “half-life of millions of years” as if this is a bad thing in and of itself.* But there’s another way of looking at it.

Radioactive decay occurs when an unstable atom emits either a helium nucleus, a high-speed electron, an electromagnetic wave called a gamma ray or more rarely one of a number of other possibilities. Being in the way of these emitted particles and waves is generally considered to be a Very Bad Idea.

Radioactive decay occurs at random, with each atom having a chance of decaying at any given moment. The more likely it is that atoms decay, the quicker they decay, and the shorter their half-life.

Imagine the radioactive atoms are ammunition cartridges; when they decay the cartridge “goes off” and a bullet is released. Now imagine you’re standing next to two piles of cartridges representing some nuclear waste: one pile with a short half-life and one pile with a long half-life

The bullets in the short half-life pile will go off over a short period of time, and the bullets in the long half-life pile will go off over a longer period of time. Which pile would be safer to stand next to?

Caesium-135 and caesium-137 are both common isotopes found in nuclear waste: Cs-135 is formed when xenon-135 produced as a fission fragment decays by beta emission; and Cs-137 is formed as a fission fragment itself (a uranium nucleus splits to form one caesium-137 and one rubidium-98 nucleus).

Cs-135 has a half-life of 2.3 million years and emits beta particles with an energy of 267 keV. Cs-137 has a half-life of 30 years and emits beta particles with an energy of 605000 keV. On a graph of 100 years the change in caesium-135 is invisible; only at a scale of a million years does the change become visible:

If you stood next to a million atoms of Cs-137 for a year 22840 atoms would decay, for a total energy release of 2.2 nanojoules. Standing next to a million atoms of Cs-135 for a year less than one atom (0.301) would decay and the total energy released would be 13 femtojoules, less than 150 thousandth of the energy released by the caesium-137.

So you have a tradeoff: caesium-135 is less dangerous than caesium-137 but becomes less dangerous more quickly. Both Cs-135 and Cs-137 decay to form stable (non-radioactive) barium so if you can turn a profit selling barium then you’re better off buying a truckload of Cs-137; you’ll be able to sell it as barium sooner.

* It’s worth bearing in mind that nuclear waste eventually becomes safe. Chemical waste from the production of solar cells like silicon tetrafluoride and cadmium telluride remain toxic forever.

A BP anecdote

You might have noticed that BP is in the news at the moment. Some people have accused it of taking a rather lax approach to safety.

Every year I support a team taking part in the Engineering Education Scheme run by the Engineering Development Trust. Our team’s Engineer has always been from BP’s headquarters at Sunbury and last year the “Celebration and Assessment Day” (!) took place there too.

I’ve never taken pupils anywhere that places more emphasis on safety than BP’s offices in Sunbury.

I made the mistake of crossing the road, with the pupils, at somewhere other than one of the designated crossing places and was immediately reprimanded by a member of BP’s staff. Not an officious security guard, just a regular member of staff. Before the kids were allowed to go anywhere at BP they had to have a safety briefing that included the “stair code” – BP’s rules for using the stairs. Everywhere we went in the labs there were boxes with disposable safety goggles.

Anyway, it’s just an anecdote. It doesn’t look like they’ve done as well in the Gulf of Mexico.

A cautionary tale

If you’ve ever been involved with weather monitoring you’ll know that a spherical lens can focus sunlight to a point. The Campbell-Stokes sunlight recorder counts the hours of sunlight per day by burning a trail across a calibrated sheet of paper.



When teaching vision I use a large spherical flask full of dyed water and a number of different lenses to simulate the eye, including short- and long-sightedness.

Until very recently this flask was stored on a windowsill in direct sunlight. Can you guess why we moved it?