It is often a surprise to see how other people react to mains electricity when they encounter it in a piece of equipment. As engineers who have dealt with it both personally and professionally for many years it is easy to forget that not everyone has had that experience. On one hand we wince at those who dive in with no fear of the consequences, on the other we are constantly surprised at the number of people who treat any item with more than a few volts in it as though it was contaminated with radioactive anthrax and are scared to even think about opening it up.
We recently had a chat among the Hackaday writers about how we could approach this subject. The easy way out is to be all Elf-and-Safety and join the radioactive anthrax crowd. But the conclusion we came to was that this site is a resource for hackers and makers. Some of you are going to lift the lid on boxes containing significant voltages no matter what, so we thought we’d help you do it safely rather than just listen for the distant screams.
So here follows the first in a series on how to approach electronic devices containing high voltages, and live to tell the tale. By “high voltages” we mean anything up to mains voltages, and those directly derived from them such as the few hundred volts rectified DC you’ll find in a switch-mode PSU. For multi-kilovolt EHT you’ll have to wait for another article, because that is an entire subject in itself. We’ll mention these higher voltages in passing, but their detail is best left for a Hackaday colleague with more pertinent experience.
The very first point to make clear when writing an article like this one is that you should disregard any ideas of “safe” voltages or currents. While the story about some significant number of people who die every year from licking a 9-volt battery is probably an urban legend and you have little to worry about from low voltage equipment, it is best to look at any higher voltages as potentially dangerous and react accordingly.
If you had a mains-voltage shock once and got away with it, you were lucky, that doesn’t mean the voltage is safe, you were merely playing a game of Russian Roulette with Ohm’s Law and a low impedance high voltage supply. Your domestic mains can dump the hefty amounts of current your home heating, your cooker, or your electric kettle demands, so if it finds a low resistance path through you then it is going have no problems dumping whatever current Ohm’s Law allows it to through that path. The chances are if it happens to you the path will be a high enough resistance that you’ll only get a very nasty jolt and live to tell the tale, but if it’s not your lucky day the resistance will be low enough that you’re just going to sit on the end of it twitching until the power is turned off, whether you’re alive or not. That’s the gruesome truth. Mess with this stuff and you can die, end of story. You are responsible for keeping yourself safe, and this is not a joke. OK? Now to work!
The place to start when talking about mains-voltage equipment is your bench. We realise you’ll probably be working at your kitchen table or wherever you happen to be, but it’s best to start an article like this with a baseline, of what you really should have when doing this stuff. There are three things we’d consider essential in the mains power system on a mains-voltage bench. An isolation transformer to power the item you are working on if it needs a mains supply, a residual current circuit breaker for the rest of your mains supplies, and an emergency isolation switch for all bench power.
An isolation transformer isolates the mains supply from the earthing system. This does not change any of the risks inherent to the mains voltage or other high voltages in your device, but it does provide you with some level of protection should you accidentally provide a path to earth from a live component.
It’s probably worth at this point explaining for a minute how earthing works with respect to mains power. You will have a local earth connection at your house, and your utility company will connect the neutral line to an earth at the substation to ensure that line voltages are not induced at a higher voltage with respect to their surroundings than the voltage they are supposed to carry. There may be 110V or 230V between the live and the neutral depending where you live, but without that earth connection both of those conductors could end up at thousands of volts higher than their surroundings, for example in a thunderstorm. The earth connection provides a fixed relationship between line voltage and the surroundings, such as the utility poles, trees, your house, and you.
Earthing is thus a vital part of power distribution safety. The only snag with open equipment on your bench is that any earth connection becomes a valid part of the return circuit for the power, and since that earth connection can come through you, that’s dangerous. The isolating transformer breaks that earth circuit for your bench, thus removing that particular hazard. The upshot of this is that you can safely touch either one of the two wires and because there’s no path to ground, you won’t get zapped. (Touching two wires completes the circuit. You still have to be careful!) Isolation transformers are also used to lift the ground so that you can connect mains circuitry up to your oscilloscope, even though the black probe clip is connected to earth ground.
Residual Current Circuit Breaker
A residual current circuit breaker compares the current flowing in the live mains conductor with that flowing in the neutral. In a normal situation these currents will be identical as current flowing out one must flow back in the other. If they differ, it is likely that the missing current is due to a hazard or fault, and the circuit breaker is activated. So if the power shorts to earth through your body rather than the normal return path, it is detected and cut off. It is important to understand that a residual current circuit breaker will not protect you from circuits on the other side of an isolation transformer, however this is to provide some protection for the other circuits on your bench. You may well find that the wiring codes in your country mean you already have a residual current circuit breaker in place.
Emergency Isolation Switch
You will be familiar with emergency isolation switches if you have worked with large machine tools. A big red switch designed to be easy to hit to turn all the power off. You may want to hit it if something gets out of hand, or someone else may want to hit it for you to turn off the power that is electrocuting you. Either way it provides a very quick way to manually make any mains sources on your bench as safe as possible in the event of an incident. Remember the bit we mentioned earlier in about sitting on the end of the line twitching until the power is turned off? This is the big red switch that makes it very easy for someone to do that for you.
Having sorted out your bench, how about your tools? The chances are you’ll already have everything you need in this department, but it’s worth saying here. If you are using a tool or an instrument on a piece of equipment carrying electricity, it has to have the appropriate insulation properties for the voltage in hand. Perhaps you have to have experienced insulation breakdown in a cheap test probe at first hand to truly understand this point.
A decent quality electrician’s screwdriver will have its insulation voltage rating stamped on its handle, though the absence of a rating is not necessarily a reason not to use it. A lot of screwdrivers have first-rate insulated handles, but without a voltage rating because they are sold for general-purpose use rather than specifically electronic. When you choose a screwdriver or other tool, consider the amount of plastic it puts between you and the metal, and pick accordingly. Wooden handles may not provide adequate protection, and metal handles should be avoided at all costs.
Your instruments such as multimeters, oscilloscopes, and their probes will all carry an appropriate safety rating for maximum voltage, surge voltage, and current. In modern equipment this will be denoted by specified classes each with their associated voltage defined under IEC 61010, though you may find older equipment with IEC348 or other national standard ratings (Fluke publishes a handy PDF explaining these standards in detail if you would like to know more). These ratings are important in ensuring that the instrument keeps you safe as you use it.
However there is a snag. In recent years there has been a flood of cheaper instruments arriving on the market, and some of the manufacturers of these devices have been found to put IEC 61010 ratings fraudulently on items which do not in any way meet those standards. We have covered tales of dodgy cheap multimeters and test failures more than once here at Hackaday, unfortunately this is a widespread problem. So please bear this in mind when buying a multimeter or other instrument for use with mains voltages: the cheapest may not be good enough. We have all used £5/$10 bargain meters with our low-voltage analogue or microcontroller work because a flashing LED on a breadboard is hardly hazardous, however for higher voltages those IEC class markings have to come with a believable provenance. Buy the most expensive instrument you can afford from a decent and reputable manufacturer, it’ll keep you safer and you’ll get a top-quality meter that will last you a lifetime.
So far we have discussed your bench and associated equipment. We are not however quite done though, because there is one final component in keeping your bench safe: you. If you have a piece of equipment in front of you and you are about to open it up, the most significant part of your safety routine doesn’t lie with your equipment, but in how you approach it.
Think for a minute, what are you going to find inside that box? You need to consider what voltages it takes as a power supply, what it does with those voltages, where any hazardous voltages might lie inside it, and whether or not they are likely to be accessible once the cover is off. It’s important to realise that simply because an item isn’t mains-powered does not mean you no longer have to consider this, for example if you’ve ever opened a camera with a built-in flash you may have encountered a capacitor with several hundred volts remaining on it. Where is the high voltage going to be?
The likely hazards identified, what are you about to do with it? Are you just opening it for curiosity’s sake, are you going to work on the high voltage circuit to fix it, or is it surplus that you’re scavenging for parts? If you’re just curious or simply scavenging parts, you may not have to increase the risk by applying power in the first place, so what other risks might remain?
Once you’ve established these ground rules you can then tailor your approach based on a realistic expectation of the risk involved. This process might seem rather long-winded, but in practice it should only require a moment’s thought. It’s infinitely better to take that moment to think than get an unexpected and possibly lethal high-voltage electric shock.
In the next part of this article we will venture inside some of the typical devices you will be working with to examine the individual hazards and techniques you might encounter.
Filed under: hardware, Skills