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A beginner's safety guide to repairing electrical and electronic things.

So you're nervous about repairing something electrical...

Let's look at some basic truths.

Yes, electricity can kill, but only in the right circumstances.

Of course you shouldn't take risks: if you're not 100% certain about what you're doing then find someone knowledgeable to help you, but there's no reason to be scared. Electricity isn't a magical force, it's a well understood scientific phenomenon with some very simple rules.

Voltage v. Current

The first rule for us is:

• Voltage Shocks. Current Kills.

If you think of electricity as being like water:

• Voltage is the pressure, formally known as "potential difference" measured in volts, symbol "V"
• Current is the flow rate, measured in amps, symbol "A".

It's the flow of electricity (amps) that will kill you, but it's the pressure (volts) that will push the flow through you, so you need enough of both to kill you.

Some examples

• A very high voltage with a tiny current may hurt you but it won't kill you. This is what static electricity is: when you get a shock from, say, walking across a nylon carpet, it may be several thousand volts but it's only millionths of an amp, so it may hurt, but it won't kill.

• On the other hand, touching a car battery which is capable of supplying a huge amount of current (say 50 amps), but only a a little bit (12V) of "pressure") will give you at most a slight tingle:

there's not enough "pressure" to push all that current (flow) through you.

• Mains electricity is 230V in Europe, usually 110V or 220V in the USA: that is enough "pressure" to push the current through you and, if it goes through your heart or brain, it can kill you even if there's only a tiny bit of current. The amount of current it takes to kill someone is not much: about 0.2A (200 milliamps), which is about enough to light a 40Watt lightbulb.

What is safe for beginners

OK, enough talk: how much is safe? Simple answer: Anything below 20 volts won't kill you. Anything above, say 40 volts can give you a nasty shock or kill you.

In practice what this means is that these things are safe

• low-voltage lights
• most battery operated devices, especially mobile phones, tablets, torches and most other battery operated devices (**except digital cameras with flashes**)
• most car electrics except the spark-plugs and the circuits which supply them
• laptops but not the mains-operated power supply.

Unless you know what the voltage is, DON'T TOUCH IT!

When you look at a piece of electrical equipment, it will usually show the voltage, so check all labels and markings first. Let's look at some real examples...

Some concrete examples

• Work out the voltage of a torch by adding up the voltages of all its batteries: if it has 4 batteries each of which is 1.5V, the total voltage is at 6V, which is not enough to do any harm.
• A laptop power supply (picture) is connected to the mains (240V), so that part is dangerous, but the part that supplies the laptop is only 18V, so that's safe to do repairs on: you'll get at most a bit of a tingle if you touch it.
• A mobile phone charger is connected to the mains, so that part (240V) is dangerous, but the part that plugs into the phone is probably 5V (check the label on the part that plugs into the mains), so very safe.
• USB sockets provide a maximum of 5V, so they're very safe.

Now, be aware that by "safe" I mean that you can't hurt yourself by touching any of the wires, but you can damage the equipment by, for instance, accidentally joining 2 wires together and causing a "short". In other words, causing 2 wires which want to be at different voltages to try to get to the same voltage...

So, the next thing to think about is