Safety switch or circuit breaker? They do completely different jobs

The question that matters

Go and look at your switchboard. What is on it decides what happens to somebody in your house who touches something they should not.

There are four generations of device out there and plenty of Douglas Shire houses have more than one of them on the same board. They are not interchangeable and they do not do the same job.

Ceramic fuses

The original. A ceramic carrier with fuse wire in it, in a few different sizes, still sitting in a great many older boards here.

A fuse protects the circuit. It cuts the power when the current through it is too high, which covers both a slow overload and a dead short, and that is what stops a cable overheating inside a wall. What it will not do is protect a person from an electric shock, and it never could.

Plug-in circuit breakers

The next development, and the one to be most wary of. A plug-in breaker does the same job as a fuse without the fuse wire, which is convenient.

The trouble is how easily one gets replaced with the wrong rating. It takes seconds, it looks entirely correct afterwards, and the result is a circuit protected above the capacity of the cable feeding it. That is how a circuit gets overloaded without anything tripping, and it is a genuine fire risk. Like a fuse, it offers a person no protection from electric shock. To be plain about it: changing a protective device is licensed electrical work, whatever it looks like from the front of the board, and it is not a job to have a go at.

The safety switch, or RCD

The residual current device was the first device designed to protect people rather than wiring, and it is the important one.

It works by comparing the current flowing out to the current coming back. If some of it is going somewhere else, which is what is happening when it is going through a person, it cuts the power to that circuit in a fraction of a second. Quickly enough to matter, whether the fault is a damaged power point or a hairdryer that has finally given up. It does not make electricity safe to touch. It very substantially reduces the chance that touching something faulty kills you, which is not the same promise and is still the most valuable device on the board.

Two things it does not do. It does not protect the circuit: an RCD watching for leakage will not notice an overloaded cable, so a board with safety switches and nothing else still carries a fire risk. And it cannot help someone who has managed to get across both wires at once, active and neutral together, because the current still balances and there is nothing for it to detect. That is rare, and it is the reason nobody should treat a safety switch as permission to poke about.

Combination devices, which do both

The current answer combines the two functions in one device, per circuit. The circuit gets overload protection and the people on it get earth leakage protection.

Doing it per circuit rather than across the whole board has a practical benefit that owners notice immediately: nuisance tripping largely goes away. A single faulty appliance trips its own circuit instead of taking out half the house, which also makes finding the fault trivial rather than a process of elimination in the dark.

How to tell what you have

A safety switch has a test button on it. That is the quickest check anyone can do without opening anything.

Press it. The switch should trip immediately. If it does, reset it and you know that circuit is protected and the device works. If nothing happens, ring an electrician. If there is no test button anywhere on the board, you do not have a safety switch at all, whatever else is on there.

Do that test every three months, which is what Queensland's Electrical Safety Office recommends. It takes about a minute, and the easiest way to remember is to do it when the quarterly power bill arrives.

What to do about it

If the board has fuses or plug-in breakers and no test button, the upgrade is worth doing and it is not a large job. A switchboard upgrade replaces the protection, generally improves the labelling to something a future electrician can read, and brings the board up to current standard.

If you are not sure what is on your board, send us a photograph of it with the cover on. We can usually tell you from that, and it costs you a text message.