What is a ‘dead-man switch’?

A so-called dead-man switch is (usually) a switch where the circuit breaks automatically when the operator of the machinery ‘dies’. Don’t worry: most dead-man switches don’t actually involve people dying. But they’re crucial for keeping people safe, and—importantly for us—these switches often use keys.

Why do we have them?

Dead-man switches are all over the place. My lawnmower has one, for instance. In order to start it, I need to grip the handle in a certain way. The moment I let go of that part of the handle, the lawnmower stops in its tracks.

Why? Because suppose something terrible had happened? Suppose I fainted. I would fall over and could possibly get a limb caught in the path of the rotary blade. However, if I fainted, I’d have let go of the handle, and therefore the circuit would be instantly broken, and the blade would be still.

It’s not massively likely that I’d faint while using a lawnmower, but imagine I’m driving a train. I’m dragging hundreds of people behind me at high speeds, and their lives are essentially in my hands. If I faint or suffer a heart attack or am in any other way incapacitated, the dead-man switch ensures that the train stops moving, keeping the passengers as safe as is possible under the circumstances.

How do they work?

These switches work in various ways. They’re not switches, usually, in the traditional sense, where somebody actively flicks the switch. Instead, they engage automatically.

Dead-man switches are always ‘fail-safe’, i.e. when they fail the machinery stops. This means that, like the aforementioned lawnmower, many systems require you to be actively holding something in place for the circuit to remain intact.

The second common type of dead-man switch involves a key. This is common on motorboats, for example, but also snowmobiles and fairground rides. A rudimentary key sticks inside a hole, which completes a circuit. The key is attached to some string which is attached to the operator’s wrist or waist. If the operator faints or moves away from the controls, the key will be pulled out via the string, and thus the circuit is broken.

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Dead-man switches are extremely important in matters of personal safety, and the simple technology has, I’m sure, saved countless lives over the years. However, not all switches are designed to save lives…

Fail-deadly

Sometimes switches are made that will specifically still work when the operator is dead. This is usually only the case with suicide bombings and similar attacks: if the police kill the bomber, the fail-deadly system (if one is being used) will ensure that the bomb still goes off. Scary stuff!