The safety card

Every rule here exists because somebody was hurt or a building burned. None of them is a matter of judgement, and every one of them is a rule you can be sent home for breaking.

Print it. Put it on the bench.

The four that are absolute#

1. Prove dead before you touch. Live, dead, live. Test your meter on a known live source, test the circuit, test your meter again. A meter with a flat battery, a broken lead or the wrong function selected reads zero volts on everything, and zero is exactly the answer you were hoping for.

2. Lock it off with your own lock, and keep the key on you. Not in a drawer, not with a colleague, not "I told him". People go home. Shifts change. A circuit that is off for no visible reason gets switched on. One lock per person working, and the isolation cannot be restored until every lock is removed.

3. Never defeat an interlock, and never work past one that has been defeated. A magnet taped to a reed switch, a cable tie on a limit switch, a jumper across a terminal. Finding one is a stop-work event, not a note for the production meeting. It was designed in by somebody who identified a hazard.

4. Never lift a protective earth to cure a noise problem. It works, and it is how people are killed. The correct fix for a ground loop is a balanced interconnect, a screen lifted at one end, or a transformer. Those are in Class 7, they take five minutes, and they do not remove the thing keeping somebody alive.

Before you work#

• Know what you are working on and what feeds it. If you cannot name the isolation point, you are not ready to start.
• Know where the isolation is and that it works.
• Know who else is affected. Anything shared, anything a performer touches, anything above somebody's head.
• Work with somebody else in the building for anything above extra-low voltage. Not in the room necessarily, but aware and reachable.
• Know where the first aid kit and the nearest defibrillator are, before you need to know.

While you work#

• One hand where you can. Keeping the other hand out of the circuit keeps the current path off your chest. It is a habit worth building on low voltage so that it is automatic on high.
• No jewellery, no watch, no loose lanyard. A metal bracelet across a battery terminal will deliver hundreds of amps and it cannot be removed while it is welding itself to your wrist.
• Eye protection when soldering, cutting or drilling. Solder spits, flux spits, cut ends fly, and an eye does not repair.
• Ventilation when soldering. Flux fumes are a respiratory sensitiser, and sensitisation is permanent.
• Wash your hands after handling solder, before eating. Especially leaded.
• Tidy as you go. A clipping across two pads is a short. A trailing lead is a trip. A hot iron off the stand is a burn or a fire.

Voltages, and where the thresholds are#

Below 50 V is not "safe", it is lower shock risk. A 12 V battery can deliver hundreds of amps into a short and start a fire in seconds. A 48 V DC supply can sustain an arc that mains AC would self-extinguish. Energy and shock are two separate hazards.

Damp changes everything. Body resistance falls from tens of kilohms to about a kilohm with sweat, water or broken skin, and the current at a given voltage rises by the same factor. This is why the rules tighten outdoors, near water, and on a stage under lights where people sweat.

What current does#

The let-go threshold at about 10 mA is the one to remember: above it, the muscles that close the hand overpower those that open it, so the grip tightens. The victim cannot release. This is why a bystander must switch off rather than pull.

What protects what#

No fuse or breaker anywhere protects a person. That is not their job. Only an RCD, isolation or double insulation does, and each of them has a specific gap.

If somebody is being shocked#

1. Do not touch them. You will become the second casualty and there will be nobody left to help.
2. Switch off. The isolator, the breaker, the plug. This is why you know where it is before you start.
3. If you cannot switch off, push or pull them clear with something dry and non-conductive: a wooden broom handle, a dry rope, a plastic chair. Not your hands, not anything damp, not anything metal.
4. Call emergency services. Immediately, and before you assess anything.
5. Start CPR if they are not breathing. Get the defibrillator.
6. Anybody who has taken a mains shock goes to hospital, even if they feel fine. Cardiac arrhythmias can appear hours later, and this is the rule people most often talk themselves out of.

Hong Kong and most of the region: 999. Europe: 112. North America: 911.

Fire#

• Electrical fire: isolate first if it is safe to do so. Removing the energy source is more effective than anything you can spray at it.
• CO₂ or dry powder on electrical fires. Never water.
• A lithium battery fire is not a normal fire. It supplies its own oxygen, it reignites, and it emits toxic gas. Evacuate, isolate if you can do it safely, and let the fire service deal with it.
• A burning smell with no visible fire is still a fire. Isolate and investigate. Almost always a connection running hot, which is Class 1's loose-terminal lesson arriving in earnest.

Soldering and the bench#

• Iron in its stand, every time, without exception. There is no "just for a second".
• Assume every iron is hot. Assume every joint you just made is hot.
• Never catch a falling iron. Let it fall and move your feet.
• Solder is hot enough to burn through skin quickly, and a solder burn to the eye is permanent.
• Cut ends fly, so cut with the end pointing at the bench.
• Ventilate. Wash your hands.

Capacitors#

• Any large capacitor in mains equipment is live until you have measured it, and it can stay live for minutes after the plug is out.
• Discharge through a resistor, not with a screwdriver. Shorting one welds the screwdriver, sprays metal, and can rupture the capacitor.
• Measure after discharging, then measure again a minute later: some capacitors recover a surprising amount of charge on their own.

When to stop#

Three situations where stopping is the professional response and continuing is not:

• Anything on mains you are not qualified for. This course does not qualify you for mains work. It qualifies you to specify it, check it, and refuse it.
• Any failed or defeated safety system. These are not repaired under time pressure and never bypassed to get a performance through. There is no production worth it, and the phrase "just for tonight" is how every one of these incidents begins.
• Any time you have been going more than thirty minutes with no boundary established. Another pair of eyes is faster than another twenty minutes of yours, and asking is cheaper than the mistake tiredness is about to produce.

The one sentence#

If you are not certain it is dead, it is live.

Everything else on this card is an elaboration of that.