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.

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.

3. 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.

4. Complete the sentences using while.