Hmmm, where does it say how to switch this thing off?
Electricity is the presence and flow of electric charge.
Electricity in contact with the human body can cause electric shocks which can
- be painful
- cause strong involuntary muscle contraction
- cause heart contraction strong enough to cause haemorrhage or stroke
- kill heart muscle tissue
- disrupt the natural heart pacemaker action, causing heart attack
- burn flesh & organs
Pain and muscle contractions can cause anything from a mild shock to fatal consequences, e.g. a fall off a ladder or roof.
The magnitude of a shock depends on the current which flows through the body, the time it flows for, and which part of the body it flows through. Voltage is one of the factors determining shock current. The voltage is obviously higher for mains than a small battery. (Batteries can kill a person, but only under unusual conditions.)
The current which flows through the body depends not only on the voltage applied but the resistance of skin, clothes and any other objects in its path. The worst case is for wet skin with no clothes which is why mains electricity in bathrooms requires more safety precautions to stay safe. In contrast a person with dry skin wearing rubber-soled shoes standing on a dry wooden or vinyl-covered floor might hardly notice contact between a finger and a live mains terminal under ideal conditions.
The severity of a shock depends on the path it takes through the body, its duration and current flow. The worst case is current flowing through the head, but a more common risk with DIY is current from one hand (or arm) to the other, as current then passes through the heart, sometimes with grave consequences. Current from a hand to a foot is usually not fatal.
The current it takes to kill varies according to the current path through the body, shock time, and individual tolerance. A current of 0.1A is likely to kill. This is readily available from the mains.
Socket circuits are now increasingly often protected by an RCD (Residual Current Device) which cuts off power in a fraction of a second in most cases with the most common type of shock path (Live to earth). RCDs have no effect on Live to Neutral shocks, or shock currents below their tripping threshold.
The voltages associated with static electricity (e.g. from walking across a carpet) are kilovolts or 10s of kilovolts, but the current multiplied by time product is so small that the danger is more or less zero.
Lightning bolts are very short-lived but the voltages and currents associated with them are enormous, typically in the region of thousands of amps, and a million volts. The power of lightning is so high that it usually causes death through internal burning, and direct effects on heart, brain and nervous system.
Safe Working on Electrical & Electronic Equipment
Many mains-operated appliances sometimes store lethal electric charges for days after it is switched off, and this must be discharged before any work on the appliance. The worst offender in this respect is microwave ovens, which store enough charge, at thousands of volts, to kill a queue of people. A minority of appliances can store enough energy to create a small explosion if some parts are short-circuited.
Some mains appliances use a live chassis connected direct to the mains. This was standard practice in TVs until the 1990s, and in radios until the 1960s. However some goods produced more recently also use this system. Thus fitting an earphone socket can be a very bad idea with many TVs.
There are various other safety issues with mains appliances as well. The main risks are listed in Appliance repair hazards
Microwaves have the greatest risks during servicing than any other household appliance, with CRT TVs probably ranking second.
Electronics in some battery-operated equipment can create dangerous voltages and charges, usually produced via the action of transformers or inductors. Cameras with electronic flash (not the LED flash found on mobile phone cameras), invertors and UPSes are well known examples.
This article should not be misinterpreted as being a complete guide to the risks present in servicing appliances. It can not be.
A thorough guide to safety in working on electrical equipment (including some battery-operated appliances) is the
It's worth a read even if you're fairly familiar with and experienced in dealing with electrical & electronic appliances.
Fire is the biggest electrical danger. Britain sees 400-500 deaths in house fires a year, with a percentage caused by an electrical problem.
Shocks, even very mild ones, can cause falls from a ladder or rooftop.
Testing after work is done helps pick up on mistakes, and check the work meets requirements.
Electrical fittings deteriorate and get damaged. Screws come loose. People do unwise things with installations (occasionally due to a loose screw). Periodic inspection can pick up on these. Inspection after buying a house turns out to be wise many times.
Some recommend domestic wiring is periodically inspected once a decade. However the inspection reports can sometimes be used to scare owners into work that doesn't need doing. It's sensible to get a realistic and impartial perspective on the findings, otherwise who knows how to interpret the findings. This is one of the questions that gets asked on news:uk.d-i-y from time to time.