Safety Switch Tester

Working on cars very often involves the use of power tools. Electric drills, electric jigsaws, electric sanders, soldering irons – all are mains-powered. When you consider that each of these tools has the ability to cut, sand or burn its way through the power cord, you can see why it’s very important that a ‘safety switch’ – a residual current device – is in use. You can buy safety switches that are installed on the main meter board or you can buy portable ones that are integrated into a multi-outlet power-board.

But how do you know that they work?

I was recently working on a mains-powered car battery charger. In my stupidity I forgot to switch it off at the outlet - instead I switched off at the charger itself...which isn’t of much use when you then pull the (live) switch out of the box! So I got a real kick from the switch – the power travelled down my fingers, down my arm and then through my body to the ground I was sitting on. That’s exactly the situation in which a safety switch is designed to trip, turning off the power in 30 – 40 milliseconds, before the electricity has a chance to do you damage.

And before starting work on the charger, I’d been careful to plug it into a portable safety switch. But the switch didn’t trip!

So how do you know that your life is being protected? In houses and workshops where the safety switch is installed on the main power board, how do you know that each power point is protected? How do you know that a portable safety switch actually works? Despite all safety switches having a ‘test’ button, self-testing can be flawed – really, you need an external checker to be absolutely confident.

The answer is to build a safety switch checker.

Warning! This story is based around Australian 240V AC mains power systems. The instructions for building the Safety Switch Tester are specific to Australia, Australian regulations and Australian power outlet configurations. Unless you have a high level of expertise with mains power systems, you should not apply this information to non-Australian mains power wiring. In some Australian states, working on mains powered devices may be illegal. Building the Safety Switch Tester involves dealing with mains power wiring. Never work on the circuit while it is plugged in to a power socket. Mains-rated wiring must be used and a high level of workmanship employed.

Building a Safety Switch Tester

It sounds like it’d be really complicated to build a device for testing safety switches but actually it’s pretty easy. That’s because Silicon Chip magazine has detailed how to build one and basically it’s just a few neon indicators, a pushbutton and some resistors. It’s covered at Silicon Chip's "Build A Safety Switch Checker" and I recommend that in addition to this article, you also read that text.

The parts needed are:

1 x 3-pin plugpack case (Jaycar HB-5900 or equivalent)

1 x 250VAC plastic momentary pushbutton switch (Jaycar SP-0716 or equivalent)

2 x plastic neon bezels with in-built resistors (Jaycar SL-2630 or equivalent)

2 x 2.2kΩ 5W wirewound resistors

1 x 1.5kΩ 5W wirewound resistor

You’ll also need some AC rated hook-up wire, heatshrink, silicone and some other bits and pieces like cable ties.

This diagram, reproduced from the Silicon Chip article with permission, shows you all the wiring. When doing the wiring, take great care to get the layout correct, and insulate exposed 240V connections.

The one I built differs a fraction from the Silicon Chip parts layout in that I placed the string of three resistors down the bottom and covered them completely with heatshrink. They’re held in place with a few dobs of Silastic.

Once the wiring is done, the two halves of the enclosure are held together with the supplied screws.

Using the Safety Switch Tester

The left-hand neon indicates if there’s a valid earth connection. If there is (and on every power outlet there should be!), the neon will glow when power is switched on. The right-hand neon indicator shows that power is available from the power outlet.

When the test button is pressed and the safety switch is working correctly, both neons will immediately go off.

If the safety switch is not working correctly, or none is connected, the right-hand neon will go out but not the left-hand one.

So, when you turn on power you want to see two neons; when you press the button on a circuit equipped with a safety switch, you want to see both neons go out.

Simple, huh? And once you’ve built a safety switch tester, it’s also a good device to lend to your friends so they can test their own systems...

How a Safety Switch Works Mains power systems comprise three conductors: Active, Neutral and Earth. When a power tool (or anything else drawing power) is working normally, the current flow to the tool through the Active is the same as the current flow back in the Neutral wire. But if there is a fault, power might be returning not through the Neutral but instead through Earth. That could be the case because (a) there is an insulation fault inside the appliance, or (b) the power is flowing to Earth through your body! In either case, you want the power switched off ASAP. The Safety Switch monitors the current flows in the Active and Neutral conductors and switches off the power if there’s a difference in these current flows. Note: a safety switch cannot protect you if you get across the Active and Neutral connections – it won’t be able to tell that the power is going through you and not the appliance! However, it will protect you if you are between Active and Earth – that’s if the Safety Switch works, of course!