Mostly - increasingly switched mode PSU's require an earth (and noise filter) to meet the EMC directive. There is some discussion going on now as to whether house circuits should be classified as high protective conductor current circuits (which would effectively stop
30mA RCD's being fitted to these circuits).
No you don't, it's nothing like that simple. The chances of death from a brief shock caused by touching a conductor carrying below several thousand volts is quite low. The figures vary depending upon the conduction path - the most dangerous being through your left arm to the feet or right arm (hence the advice to keep your left hand in your pocket when working on live circuits). Whether you grasp the conductor is also critically important, unless you grasp the conductor 240V is rarely fatal (always use the back of your hand for first contact if you really have to touch something which may be live). Even small currents cause the hand muscles to contract preventing you from letting go.
There are widely varying figures depending upon where you look for the simple reason that not a lot of practical work has actually been carried out on the subject. For some reason they couldn't get many volunteers. Most of the levels are therefore extrapolations from the results seen at lower levels.
Critically important is the "Let go" current, the current above which you cannot release your grasp upon a live conductor. 99% of the female population have a ?let go? limit above 6 mA, with an average of 10.5 mA. 99% of the male population have a ?let go? limit above
9mA, with an average of 15.5mA.Prolonged exposure to 50 Hz currents greater than 20mA across the chest causes the diaphragm to contract which prevents breathing and causes the victim to suffocate so it is quite possible (albeit improbable) to die while your RCD stays firmly in place.
The more immediate cardiac effects depend upon the frequency of the electrical current as well as its magnitude. Humans are most susceptible to frequencies at 50 to 60Hz as the internal frequency of the nerve signals controlling the heart is approximately 60 hertz. Ventricular fibrillation occurs when 50/60Hz current from the electric shock interferes with the natural rhythm of the heart. At
100mA the current would need to be maintained for about 3 seconds to start fibrillation, at 900mA it would take only 3 milliseconds. The heart loses its ability to pump and death quickly follows.Ventricular fibrillation can occur at current levels as low as 30 mA for a two year old child and 60 mA for adults. Most adults will go into ventricular fibrillation at hand to hand currents of about 100 -
200 mA.Humans are able to withstand 10 times more current at DC and at 1kHz hertz than at 50 or 60 Hz. Interestingly the most dangerous current range is somewhere between 200mA and 4A which causes the heart to fibrillate which cannot be stopped by first aid methods. Above that current the heart is paralysed rather than going into fibrillation and simply stops - it can sometimes be restarted with a blow to the chest.
The following figures are taken from NASA, Stanford and the Royal Navy.
Current Physiological reaction 5 A Tissue burning
Sub lethal electrical shocks can often cause permanent nerve damage which only becomes apparent weeks or months later when the wastage of muscle tissue is noticed.
50-150 Milliamperes - Extreme pain, respiratory arrest, severe muscular contractions. Individual cannot let go. Death is possible.0.2-4A - Ventricular fibrillation. (The rhythmic pumping action of the heart ceases.) Muscular contraction and nerve damage occur. Death is most likely.
10A - Cardiac arrest, severe burns and probable death.Under 1 mA - Not perceptible
1 to 8 mA - Mild sensation 8 to 15 mA - Painful shock but person still has control of muscles and can let go of source of shock 15 to 20 mA - Painful shock. Unable to let go 20 to 50 mA - Severe shock. Severe muscular contractions. Breathing difficult 50 to 100 mA - Extreme shock. Extreme breathing difficulties 100 to 200 mA - Death by Ventricular Fibrillation. Not reversible 200 mA up - Severe burns. Breathing stops. Chest muscles clamp and stop the heart but no ventricular fibrillation. Victim may survive if given immediate resuscitation.Assuming the skin (which is quite a fair insulator)is intact typical resistance levels for an adult are:-
Dry skin 100k to 600k Ohms
Wet skin - 500 to 1k Ohms
Internal (broken skin) hand to foot - 400 to 600 Ohms, Ear to ear ~100 Ohms.
Unless you have wet hands it much more difficult than you might think to dangerously electrocute yourself with 240VAC. Certainly in your changing a light bulb scenario it would be most improbable.
A 30mA RCD will pass its test if it trips at 16mA.