Many threads in diyAudio Forum deal with mains connected equipment and/or equipment capable of producing dangerous voltages and powers. This page is here to tell you about the safety rules you need to know about before you embark on your “DIY adventures”.

Electronic equipment is dangerous. Every person is responsible for deciding if they are willing to risk their life, and for ensuring that whatever information they receive is correct. This forum and it's members accept no responsibility for any death, injury or property damage that result from any of these suggestions; your safety is your own responsibility

Before worrying about how safe your electronics are (this is just as critical, but it comes later on in the cycle), you need to consider your own personal safety while working with your electronics projects.

Several members of this community can tell you stories of people who have been killed whilst working with electricity. In some cases, these were people who knew the rules and, because of a mere moment's carelessness [they lost the fear] they left friends and families to mourn them.

This is serious stuff. Too much fear is better than not enough.

"It's mils that kills and volts that jolts"

It always is milliamps of current that will kill you. The general rule of thumb is that voltage will give you a jolt; it's what gives you the tingles or causes your muscles to contract.

Current is what kills you and it doesn't take much; a few milliamps (mA) running through your body, for a just a few seconds, can stop your heart.

You can actually withstand a considerable amount of voltage as long as the voltage doesn’t travel a long distance through your body such as from the front of your finger to the back of your finger (or between the probes on a tazer).

The safest way to learn electrical safety is by spending time around experienced people. Never work alone.

If you intend to work with high voltages (1000V+) all of the rules are different and you would be INSANE, no, SUICIDAL to tackle that kind of project without the direct supervision of someone who experienced in that field.

The following best practices are not applicable in these cases. You really do need to learn from people who know what they are doing as there are just so many ways that high voltage systems can electrocute you (like arcing through the air just because you are standing too close to them).

• Read a book like Electrical Safety Handbook
• Work slowly and carefully.
• Never work when you are tired or angry, this is when you are most likely to get careless.
• Take off all jewelry before starting (gold and silver are, coincidentally, the best conductors out there).
• Wear rubber soled shoes, not socks or bare feet.
• Look at what you're wearing, what you're sitting on and what you're working on top of. Lots of chairs and desks have metal components that will provide a pathway to ground, as do some items of clothing; like the metal buttons /rivets in jeans (Small contact area = very painful shock)
• Wear long legged trousers. If you're measuring current and the probe slips the trousers might protect you from the lead. Trousers also protect you from molten solder (there are situations where solder can boil off and splatter everywhere).
• Wear rubber gloves that are rated for the voltages that you will be working with.
• Wear safety glasses. Electronics components (resistors, capacitors, transistors) can explode for a number of reasons: accidental short, over voltage and even just plain old age. When they do blow, they are extremely loud and throw shrapnel everywhere.
• Wear ear protection. When stuff blows (see above), it is almost always accompanied by a very loud bang which can, under certain circumstances, permanently damage your hearing.
• Work with one hand, keep your other hand in your pocket. This prevents you from shorting your body across a chassis or other circuit. In this situation, the power will enter one arm, go through your chest (aka, your heart) and out your other arm to ground. This is very very likely to stop your heart.
• Discharge all charge storage devices (like capacitors) before handling them. There are many ways to do this. The best is to wire two or three 240v bulbs in series with insulated wire and then short the terminals of the capacitors with the insulated wires. You could use 100K ohm 2W resistors for this kind of task but resistors can fuse internally without smoking or exploding, leaving no evidence that they've failed, leaving you with a false sense of success. ALWAYS meter the caps after discharge to make sure you're safe.
• When metering with a multimeter (henceforth DMM), begin with the power plug out, the power switch off (and double check it), clip the DMM probes to the unit using alligator clips/clip probe, plug the unit it in, turn on the power, stand back while taking your reading, turn the unit off, unplug it and disconnect your probes (with one hand).
• Use parts/wire that are rated for the voltage/current that you'll put through them. Exceeding the ratings for components can create electrical shorts or cause small explosions. In the case of wiring, exceeding the current ratings can cause fires.
• Use an isolation transformer on your work bench with a ground fault automatic switch. This won't prevent you from getting electrocuted when (not if) you make a mistake but the lower amounts of power that these devices deliver can increase your chance of surviving your mistake.
• Assume that every piece of exposed metal is "hot" with voltage (because it only takes one short to make this true).
• Act as if every bit of insulation is not truly insulated
• Be aware of what's around your hand at all times! If you get a small jolt you will reflexively pull your hand away. In tight spots, you might brush up against another contact point.
• If possible, test your projects with low voltage/power supplies before scaling them up to full power supplies. This reduces the risk to you while you're debugging the unit.
• Try to make sure that your workbench is 100% wood. Wood is a great insulator. This is in contrast to electrical component handling where you need to be concerned about damage to the component from static discharge.

The European Standard “IEC 60950-1” defines that electrical safety relates to protection from:

• A hazardous voltage, that is, a voltage greater than 42.4 V peak or 60 V DC;
• A hazardous energy level, which is defined as a stored energy level of 20 Joules or more or an available continuous power level of 240 VA or more at a potential of 2 V or more;
• A single insulation fault which would cause a conductive part to become hazardous;
• The source of a hazardous voltage or energy level from primary power ;
• Secondary power (derived from internal circuitry which is supplied and isolated from any power source, including DC)

On top of that, there are many other normative reference that stipulate rules and regulations dedicated to electronic equipment safety, such as fire hazards, risk of injury, etc.

Whenever possible, it is recommended to use commercially available and approved (UL, VDE, KEMA) power supplies to power your equipment. These power supplies are tested for safety by independent institutions, they are readily available and usually quite cheap as well.

If it is necessary to build a mains-connected application, protection against electric shock can be provided in two classes:

Class I equipment uses basic insulation; its conductive parts, which may become hazardous if this insulation fails, must be connected to the supply protective earth. Examples are computer power supplies and kitchen appliances.
In summary, Class I isolation requires adequate insulation between mains and any user accessible part, capable of withstanding a test voltage of at least 2120V peak. An isolation distance of at least 3 mm must be maintained between any user accessible part and mains carrying parts. Furthermore, any conductive, user accessible part must be adequately connected to the safety ground.

Class II equipment uses double or reinforced insulation for use where there is no provision for supply protective earth. Examples are power tools, hair dryers and cell phone chargers.
The double insulation requirement means that the insulation between mains and any user accessible part is capable of withstanding a test voltage of at least 4240V peak. An isolation distance of at least 6 mm must be maintained between any user accessible part and mains carrying parts.

In practice, it is recommended to separate mains carrying parts and user accessible part as much as possible, but never less than required (see above).

When building a power supply, the use of a Class II insulated transformer is preferred, but note that when this is fitted in a Class I equipment, this does not, by itself, confer Class II status on the equipment.

It is always recommended to use an approved mains entry with an integrated fuse holder and switch. If this is not possible, you should do the following.

• In case you intend to route the mains cable directly into your equipment, use a proper strain relief at the point of entry.
• The first thing after the mains enter your equipment must be a properly sized fuse in an approved fuse holder. If you use a mains plug, keep the wiring to the fuse as short as possible. Use properly insulated wiring, and be sure to make firm and mechanically strong connections.
• A separate mains switch should be of the double pole type. In case of three-phase appliances, all phases and neutral must be disconnected simultaneously.
• It is not required, but strongly recommended to place filtering components after the mains switch. Placing them before the mains switch will expose them to continuous stress and reduces their lifetime.
• A secondary on/off switch is allowed if the equipment consumes less than 10W in the “off/standby” state. In such case a visible means (e.g. a stand-by LED) must be provided to indicate that mains are connected.

Anyone constructing a Class I equipment, must pay special attention to proper safety grounding:

• Use yellow/green insulated wire (if necessary, green with a narrow yellow stripe. Do not use yellow wire with a green stripe)
• If you route the mains cable directly into your equipment, the ground wire must be the longest, such that it is the last one to come free if someone accidentally pulls the wire.
• Any user accessible metallic part must be adequately grounded.
• Adequate grounding may require, for example, an additional ground wire to be connected between the front plate and the mains ground wire.
• If the entire enclosure is conductive, and all parts are firmly connected to each other using metal screws, one grounding point is considered adequate.
• Take care when using metallic switches or potentiometer shafts. They must not provide any hazard!

Source: Elektor Construction and Electrical Safety Guidelines (Dutch, English)

!!! DO NOT MESS WITH MAINS !!!

The following are other threads where safety in certain situations have been discussed.

• Safety Practices: General and Ultra-High Voltage
  
• Safety First
  
• 83 safety rules
  
• Safety issues and CE Certification
  
• Document on electric safety
  
• Auto Transformer - safety advice
  
• On soft start circuits and safety
  
• Power Supply Safety
  
• Safety Question about Capacitors
  
• Electrical safety with pcb mount transformer
  
• Question regarding 3rd wire grounding and safety
  
• Building a class II PSU (double insulated, no safety earth)
  
• Workbench safety practices?
  
• one safety question (Painting components)
  
• Safety question concerning ground loops
  
• Safety with high volts: 101
  
• Safety when testing headphone amp for muso's
  
• SMPS -- "safety and the 5 cent zener"