I have to test prototype PCBs that convert household AC into several DC voltage levels.

I concern about my safety when working with AC and would like to know how to properly setup a testbench that, for example, will trip a circuit breaker or break a fuse if something goes wrong.

I plan to build a testbox that is located between main AC line, and the test object. The box will probably have the following stuff built into it.

  • Cable for the connection to the main household AC
  • Variac (variable transformer)
  • Emergency stop button (big red button)
  • Circuit breaker that has a lower rating than the house circuit breaker
  • Output socket for connecting to the test object
  • Fuse between the main line and the output socket

What else should I include in this testbox? Or is this testbox thing really helping?

  • 3
    \$\begingroup\$ isolation transformer maybe? I also have a high voltage testbox at home that is completly transparent acrylic and has lots of interlocks to prevent you from opening while anything is live. \$\endgroup\$
    – PlasmaHH
    Commented Oct 12, 2017 at 10:00
  • 1
    \$\begingroup\$ How will you test this box... \$\endgroup\$
    – Trevor_G
    Commented Oct 12, 2017 at 12:30
  • \$\begingroup\$ If there is ANY chance that someone will ask "Hey.. what does this switch do?" while messing with it, you should add a key-lock switch. \$\endgroup\$
    – Trevor_G
    Commented Oct 12, 2017 at 12:33
  • 1
    \$\begingroup\$ I hope not Trevor! But that will be redundant for now. I have a friend who always come by and learn the stuff i'm doing, so I might consider adding it in the future. \$\endgroup\$ Commented Oct 12, 2017 at 12:59
  • 1
    \$\begingroup\$ Make sure that there are people around to assist if required, and that those people are clear on the safest procedure if something does go wrong. \$\endgroup\$
    – gamesmad
    Commented Oct 12, 2017 at 16:29

3 Answers 3


An isolation transformer will make it safe to touch live, but definitely not live and neutral simultaneously.

Another option is a RCD (residual current device). I purchased a sensitive one which trips at 10mA leakage current to ground, which is safer than the 30mA one which protects the whole house. You will still get a large jolt if you touch the wires, but the RCD will protect your life.

It is also quite useful when using power tools outside: it did trip several times, and it turned out the extension cords had water inside the sockets, that kind of thing. It's a nice safety feature.

None of this protects against the big SPMS input cap, which is the most dangerous component, not only for electrocution, but also if it is shorted accidentally by something, there will be fireworks and bits of molten metal flying around, which is hazardous for eyes.

In the end, working on mains is all about state of mind, common sense and prevention: for example, if you only want to probe the low voltage side of the PCB, tape some insulating plastic sheet on the back of the PCB over the high voltage bits. Mylar photocopier transparent works well. Put some heatshrink over Wires when they're soldered to pins and other connectors, and basically make sure the amount of live exposed metal is kept to a minimum.


Some RCD/GFCIs will also act as breakers and detect overcurrent, some won't and must be protected with a circuit breaker. You must check and use the proper combination. Those we use here for residential electric installations require properly dimensioned circuit breakers.

(In France it is called a "differential breaker" because it measures the difference in currents flowing in Live/Neutral wires. The difference is leakage to Earth, so when it exceeds the threshold, it trips.)

Please note that some will detect DC leakage, some will only detect AC leakage, so if your AC is rectified by a SMPS for example, and you want to protect against a fault to ground after the rectifier, make sure you read the docs to pick the right protection device!

You can also get adjustable circuit breakers which are designed to protect motors. Anyway, for most electronic work, a 2A breaker should do the job just fine unless you're doing high power stuff.

About the order:

If you use an Isolation Transformer then the RCD will be useless. Suppose you touch one of the wires on the secondary: since the transformer provides isolation, no current will flow in your finger, thus the RCD will not trip. However if you touch both live and neutral on the secondary, the RCD will still not trip.

So the isolation transformer is a bit of a compromise. It makes it safer in a way, also you can use your scope to probe mains voltage without blowing it up... but you can't use a RCD.

Using a RCD without transformer means the RCD will trip if enough current runs through your finger, no matter if you touch one wire or live and neutral. So it's safer in a way (but you still get electrocuted a bit).

Also none protect against big capacitors charged to high voltages, which is why I put an emphasis on common sense and prevention...

For example, soldering test points (ie, bits of resistor legs) and then using the scope or multimeter grip tips on that means you don't have to hold the probes with one hand while looking at the scope and fiddling with it. It prevents the risk of having your probe slide, rip across the board and make a short...

  • 4
    \$\begingroup\$ In the US and Canada, the RCD (residual current device) is more commonly known as ground fault circuit interrupter (GFCI), ground fault interrupter (GFI) or an appliance leakage current interrupter (ALCI). \$\endgroup\$
    – Tut
    Commented Oct 12, 2017 at 12:06
  • \$\begingroup\$ Are circuit breaker on the input side still necessary? because if the RCD trip early then the circuit breaker will be redundant. \$\endgroup\$ Commented Oct 12, 2017 at 12:50
  • 1
    \$\begingroup\$ @NatthapolVanasrivilai Only if you trust the RCD/GFI/whatever to never be faulty. \$\endgroup\$
    – JAB
    Commented Oct 12, 2017 at 13:41
  • \$\begingroup\$ @peufeu How should the order of the connection be? AC --> RCD --> Isolation Transformer --> Variac --> Switch --> V/A-Meter --> Output Fuse --> Output \$\endgroup\$ Commented Oct 12, 2017 at 14:02
  • 1
    \$\begingroup\$ @NatthapolVanasrivilai Order should be AC-Fuse-Switch-Variac-Transformer-RCD-Fuse-Meters-Output \$\endgroup\$
    – Trevor_G
    Commented Oct 12, 2017 at 14:42

You are missing the very first thing I'd use, which is a isolation transformer. The next most useful device is the variac. Your redundant fuse and circuit breaker are just silly.

When using both a variac and a isolation transformer, put the variac in front of the isolation transformer. This is because many common isolation transformers are toroids that can have significant residual magnetism. If you happen to shut off the input when the core is magnetized one way, then happen to turn it on next at just the wrong part of the power cycle, the primary can look like almost a dead short for half a cycle. I once blew a 30 A breaker this way when first turning on a isolation transformer rated much less than that.

With the variac before the isolation transformer, you are (sometimes at least) slowly increasing and decreasing the voltage on the primary. That leaves little residual magnetism when turning off, and limits the current until the magnetic field follows the input again on when turning on.

  • \$\begingroup\$ I will now also consider having isolation transformer in the build. The fuse was meant to protect the output side of the testbox. In case of it go overload due to short. Circuit breaker is there because i didn't want part of the house to just shut off, because the PCB trips it. But and RCD recommendation from peufeu was anothoer good suggestion to replace circuit breaker. \$\endgroup\$ Commented Oct 12, 2017 at 12:52

I agree with earlier posters, but would go a bit further. Your life is valuable so it is worth spending some time and money to make a reasonably safe test rig.

0) Unless your application is very unusual you will probably find a [set of] suitable PSU[s] from a reputable manufacturer at a reasonable price. I avoid mains work where possible.

1) Keep all high voltage / high energy circuitry under a rigid cover while energised.

2) Never manually probe the dangerous section while it is energised.

3) Only make measurements on low voltage, ground-referenced test points.

4) Have another person present. This person must know both how to safely isolate the circuit and how to revive you.

The points above are not as difficult to achieve as it might seem.

5) Use an isolation transformer. This allows you to ground the reference point in the circuit, which is usually the negative side of the storage cap.

6) Rig a small mains contactor to connect a discharge resistor across the high voltage storage cap, via a normally-closed contact. Ensure that the resistor can safely absorb the energy in the cap within a second or so. You can start up the circuit by manually operating the contactor test button before powering the circuit on.

If you forget to press the test button your input circuit breaker will swiftly open.

7) Make potential divider test probes to the interesting points in the circuit. This will probably involve several >100K ohm, high-voltage, chip resistors soldered to the relevant nodes. Wires will lead the test points out from under the protective cover to your instrumentation.

8) Start with a low-voltage, resistively-limited mains supply. This will help protect things until you are sure the circuit is working as intended.


Stay safe, friend.


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