Residual-current devices (RCDs)

Last updated: December 4, 2009.

Electricity is amazingly useful—but it can be amazingly dangerous too! Did you know that it takes only a fifth of a second for a strong electric current flowing through your heart to kill you? That should give you pause for thought if you regularly use electric power tools. Think about if for a moment: if you're using an electric hedge cutter and you accidentally chop through the cable, the electricity has to go somewhere. If the tool has a metal case, you're holding on to it, and you're standing on the ground, there's a very high risk that your body will form a "short circuit"—the path of least resistance for the current to flow through. It takes just the blink of an eye for a current that's doing you a favor to change its mind, zap through your body, and kill you. One way to reduce the risk is to use a clever protective gadget called an RCD (residual current device), which automatically shuts off stray currents before they can electrocute you, cause fires, or do other kinds of damage. Let's take a closer look at how these brilliant bits of equipment can save your life!

Photo: A typical RCD adapter with a three-pin outlet for the UK electricity system. Plug the RCD into your electricity outlet, plug your appliance into the RCD, and you're all ready to go. The blue button tests the device: press it in and you cause a temporary short circuit that should cut the power. The green button is a reset switch that restores the RCD to normal operation after a test or a real cut-out.

Electricity + magnetism = electromagnetism

If you've read our articles on electricity and magnetism, you'll know these two phenomena are like two sides of a coin—a single phenomenon called electromagnetism. Electric currents can produce magnetic fields, while magnetic fields can cause electric currents to flow. RCDs use the connection between electricity and magnetism in a particularly ingenious way.

Suppose you're running a power tool such as a lawnmower. There are two wires coming from the electricity supply to the electric motor that spins the cutting blades. One wire is called the phase or live and the other is called the neutral. If you're unlucky enough to cut through one of these wires, the current has to flow somewhere. If you cut through a live lawnmower cable with a pair of stainless steel garden shears, for example, the shears, your hands, arms, and legs would form part of a circuit through which the electricity would flow. You could be dead within a second! But if the cable is plugged into an RCD, the RCD detects the sudden change in the current and breaks the circuit in around 30-50 milliseconds. That should be plenty quick enough to save your life in most circumstances.

Photo: Electricity—never forget how powerful it really is. Photo by Dave Parsons courtesy of US DOE/NREL (Department of Energy/National Renewable Energy Laboratory).

RCDs are a bit like transformers

RCDs work in a similar way to electricity transformers; if you're not familiar with those, you might find it helpful to review our detailed article on transformers so you understand what's coming up next.

In a transformer, two coils of copper wire (called the primary and the secondary) are wrapped around a circular iron core (sometimes called a toroid). By using different amounts of wire in the two coils, a transformer can change a high-voltage electric current into a low-voltage one (or vice versa):

Artwork: A simplified illustration of a transformer. The copper primary and secondary cables are connected magnetically, via the iron core, rather than electrically.

Inside an RCD, the live and neutral cables from the electric supply wrap around an iron core much like the one in a transformer. The live cable wraps around one side of the core and the neutral cable goes around the other, so:

1. Alternating current flows back and forth through the live wire (yellow).
2. As it does so, it induces (creates) a magnetic field in the iron core, just like in a transformer (blue arrow).
3. Meanwhile, an opposite alternating current is also flowing back and forth through the neutral wire (orange).
4. The neutral current induces an equal and opposite magnetic field in the core (red arrow).
5. Under normal conditions, the magnetic fields induced by the live and neutral wires cancel out: there is no overall magnetic field in the core and there's nothing to stop current flowing to the appliance you're using.

What happens when there's trouble?

Now suppose you cut through or damage the cable leading to your appliance. If you cut through one of the wires, there's effectively a current "leak" from the circuit, so there will be unequal currents flowing in the live and neutral wires. One of the wires will carry more current than the other, so the magnetic fields they produce will no longer cancel out and there will be a net magnetic field in the core.

How does that helps us? The iron core has a third, smaller coil of wire wrapped around it. This is called the search or detector coil and it's wired up to a very fast electromagnetic switch called a relay. When a current imbalance occurs, the magnetic field induced in the core causes an electric current to flow in the search coil. That current triggers the relay, and the relay then cuts off the power. See how it happens in the animation in the box below.