The strength of current depends on both the voltage and on the resistance in a circuit. A current of 50ma is dangerous for a man, it may result in an electric shock. One gets an electric shock in case one touches live conductors when the power is on. And a current of 100ma and higher is lethal. Thus, before working on a circuit, deenergize it and work on it with the power off.
Earthing system serves to protect attending personnel from electric shocks when voltage appears on parts that are normally dead. The risk of an electric shock decreases with decreasing voltage. In wet and hot atmosphere the risk of an electric shock increases. Safe voltage for circuits used in dry atmosphere is under 36V. When the power is on contacts with live conductors are dangerous for life. When a live conductor is touched with both hands the resistance of the conductor is from 10,000 to 50,000 ohms. When a live conductor is touched with one hand the resistance is much higher. The higher is the body resistance, the smaller is the current that flows through the body. Take it into consideration and work with one hand if the power is on!
Thus measures are taken to protect attending personnel from contacts with live parts of installations under voltage.
The danger of electric shock disappears provided the metal parts of installations under voltage are connected with ground by means of safety earthing. Connecting to ground is made by means of measuring devices. The faulty parts should be detected, eliminated, and replaced by new ones.
III. Remember these words:
earthing, to touch, to attend, consideration, to disappear, to connect, check, to detect, to eliminate, faulty, wet, installation.
IV. Translate the following word combinations:
power decrease, dry air, hot atmosphere, dead conductor, live conductors, power on position, power off position, safety earthing system, dangerous decrease of voltage, live conductor contact.
V. Fill in the gaps using “to detect”, “to appear”, “to disappear”, “to decrease”.
1. When the resistance increases, the risk of electric shock …
2. Faults in electric installations are … by means of special devices.
3. Electric power … only on live conductors with power on.
4. When the device is switched off electric power …