Year 11 Physics 2B
Discuss the dangers of an electric shock from both a 240 volt AC mains supply and various DC voltages, from appliances, on the muscles of the body?
Describe the functions of circuit breakers, fuses, earthing, double insulation and other safety devices in the home
(i) Find out how a circuit breakers work (ii) Find out how RCD work. Discover and discuss the purpose of each of these devices
(1) Discuss the dangers of an electric shock from both a 240 volt AC mains supply and various DC voltages, from appliances, on the muscles of the body? The severity of injury from electrical shock depends on the amount of electrical current and the length of time the current passes through the body. For example, 1/10 of an ampere (amp) of electricity going through the body for just 2 seconds is enough to cause death. The amount of internal current a person can withstand and still be able to control the muscles of the arm and hand can be less than 10 mill amperes (milliamps or mA). Currents above 10 mA can paralyse or "freeze" muscles. When this "freezing" happens, a person is no longer able to release a tool, wire, or other object. In fact, the electrified object may be held even more tightly, resulting in longer exposure to the shocking current. For this reason, hand-held tools that give a shock can be very dangerous. If you can't let go of the tool, current continues through your body for a longer time, which can lead to respiratory paralysis (the muscles that control breathing cannot move). You stop breathing for a period of time. People have stopped breathing when shocked with currents from voltages as low as 49 volts. Usually, it takes about 30 mA of current to cause respiratory paralysis.
Currents greater than 75 mA may cause ventricular fibrillation (very rapid, ineffective heartbeat). This condition will cause death within a few minutes unless a special device called a defibrillator is used to save the victim. Heart paralysis occurs at 4 amps, which means the heart does not pump at all. Tissue is burned with currents greater than 5 amps. Effects of Electrical Current* on the Body
| Just a faint tingle.
| Slight shock felt. Disturbing, but not painful. Most people can "let go." However, strong involuntary movements can cause injuries.
| 6-25 milliamps (women)†
9-30 milliamps (men)
| Painful shock. Muscular control is lost. This is the range where "freezing currents" start. It may not be possible to "let go."
| 50-150 milliamps
| Extremely painful shock, respiratory arrest (breathing stops), severe muscle contractions. Flexor muscles may cause holding on; extensor muscles may cause intense pushing away. Death is possible.
| 1,000-4,300 milliamps (1-4.3 amps)
| Ventricular fibrillation (heart pumping action not rhythmic) occurs. Muscles contract; nerve damage occurs. Death is likely.
| 10,000 milliamps (10 amps)
| Cardiac arrest and severe burns occur. Death is probable.
| 15,000 milliamps (15 amps)
| Lowest overcurrent at which a typical fuse or circuit breaker opens a circuit!
An electric shock from an DC electrical system is more severe than an electric shock from a AC electrical system. This is because when electrocuted by an AC current the magnitude of the current fluctuates from high to low therefore, when in contact you will receive the shock for a certain period of time until the magnitude drops you can remove the victim. Whereas for a DC current electric shock does not fluctuate this when electrocuted the person will continue to have current pass through them which is most likely going to result in death.
(2) Describe the functions of circuit breakers, fuses, earthing, double insulation and other safety devices in the home The Fuse: Fuses are made from low melting point wires which will melt (blow)...
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