Safety Earthing System. Electric Shock

The strength of current depends on both the voltage and the resistance in a circuit. A current of 50 mA is dangerous for a man and a current of 100 mA and higher is lethal.

a) Contact with an ungrounded motor frame

b) Contact with a grounded motor frame

Fig. 19

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 electric shock increases. Safe voltage for circuits used in dry atmosphere is under 36 V. When the power is on, contacts with live conductors are dangerous for life. Thus, measures are taken to protect attending personnel from contacts with live parts of installations under voltage.

The danger of an 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 earthing electrodes which are connected directly with ground.

The insulation resistance of any installation should be regularly controlled by means of measuring devices. The faulty parts should be detected, eliminated, and replaced by new ones.

 

7. Complete the sentences using the correct variant:

1. Earthing system serves	a) as protection from an electric shock. b) as connection with ground.
2. Voltage appears on	a) dead parts. b) live parts.
3. Contact with live conductors is	a) dangerous. b) safe.
4. Connection to ground is made	a) by means of wire conductors. b) by means of earthing electrodes.
5. Danger of an electric shock disappears if the frame	a) is earthed. b) is unearthed.

 

8. Complete the sentences using while. Follow the model on page 13.

1. The insulation resistance of a faulty unearthed frame is extremely low ….

2. Danger of an electric shock disappears when the faulty parts are earthed ….

3. One should work on the circuit when the power is off. One should not work on the circuit when the power is ….

4. Contact with dead conductors is safe ….

5. In dry air the risk of an electric shock decreases ….

 

9. Answer these questions:

1. What does an earthing system serve for?

2. What parts are termed dead (live)?

3. In what air does the risk of an electric shock decrease?

4. By what means is connection to ground made?

5. What does an electric shock result from?

6. Is a current of 50 mA dangerous for a man?

7. Is wet and hot atmosphere dangerous for the attending personnel?

8. Does the risk of an electric shock decrease with increasing current?

 

Read the text and write four questions about it. Ask your groupmates to answer them.

A man can get an electric shock when he comes into contact with the electric fish. One of this kind is found in the tropical waters of South America: it is the electric eel. Small electric eels, one inch long, give a small shock. When the fish is 6 inches long its internal battery gives as much as 200 volts. A very big fish can generate 600 volts! When it is short-circuited, a current of one ampere can be obtained. A two-meter long eel can light a dozen 50 watt lamps. The eel’s head is positively charged and the opposite end is negatively charged.

 

Unit Twenty-Eight [28]

1. a) Cover the right column and read the English words. Translate them into Russian and check your translation.

b) Cover the left column and translate theRussian words back into English.

condition	[kənˈdɪʃn]	условие
plant	[plɑːnt]	завод
pole		полюс
torque	[tɔːk]	вращение, момент вращения
poor	[puə]	бедный, плохой
nameplate	[ˈneɪmpleɪt]	(заводская) табличка

Read the words and put down their Russian equivalents. Then translate themback into English.

[ˈɪndəstrɪ]	industry	___________________________
[ˈsəːvɪs]	service	___________________________
[ˈtrænspɔːt]	transport	___________________________
[ˈmoutə]	motor	___________________________
[ˈpræktɪkəl]	practical	___________________________
[pouˈtenʃəl]	potential	___________________________

 

3. Give the Russian for:

magnet pole	___________________________________________
different conduit	___________________________________________
machine ratings	___________________________________________
rated torque	___________________________________________
service life	___________________________________________
poor operation	___________________________________________