Domestic Wiring Chapter 5

 EARTH ING SYSTEM

By earthing ofelectrical installations is meant the direct connection of all the metal non current

carrying parts of electrical equipments e.g., metallic framework, body of electric machines, main

switch, metallic covering of cables and conduits, distribution board, earth terminal of sockets etc.

to earth plate. The earth plate is buried in the ground which has good electrical connection to the

surrounding earth. As far as domestic wiring is concerned the electric utility provides earthing to

its energy meter. But beyond the meter, earthing is the responsibility ofthe owner ofthe house.

The owner should make arrangements ofhis own earthing system with an independent electrode.

All appliances which are given connection through the socket outlet e.g., refrigerator, washing

machine, drier machine, table fan etc. should be earthed through earth terminal of the socket outlet.

A safe grounding system should provide the following :

(✓ A means to carry and dissipate electric currents into ground under normal and abnormal operating conditions so that not only the equipment should remain safe, the continuity of supply should also be maintained.

✓ The personnel working on or around the equipment should not experience any electric shock.

We will discuss here in detail the second aspect of grounding i.e., 

Personnel safety

For this it is important to understand the electrical characteristics of the most important part of the circuit i.e., the human body.

It is to be noted that human body experiences shock due to flow of current through the body and not due to the voltage level. 

If the human body is properly insulated he or she can even hold bare high voltage wire. 

It is a usual sight to see birds sitting on the wire of h.t. lines.

However, if the birds span the two HT wire simultaneously, it will turn into a dead short circuit. 

In general, shock currents are classified based on severity of the shock they cause e.g.,

currents which produce direct physiological harm are termed primary shock currents, whereas,those that do not cause direct physiological but may cause involuntary muscular reactions are called secondary shock currents. 

It is to be noted that the threshold value of current which gives a tingling sensation on the hand or finger when touching an electrically live device differs from person to person. 

For a normal healthy person a current of 1 mA is the threshold value to produce tingling sensation. 

Currents of about 10 to 30 mA can cause lack of muscular control.

In most of the cases a current of 100 mA will cause ventricular fibrillation. 

Currents of higher magnitude may even completely stop the functioning of heart or may cause severe electrical burns. 

Ventricular fibrillation is that condition when the heart beats in an abnormal and

ineffective manner with fatal consequences. Therefore, the threshold value ofcurrent is the main

criterion for proper grounding ofdevices. Under threshold condition, there is rapid uncoordinated

contractions ofthe ventricles ofthe heart thereby the synchronisation between the heart beat and pulse beat is lost. 

When individual muscle fibres contract independently, they are said to be fibrillating. 

Fibrillation can occur in the atrial or ventricular muscles. During ventricular fibrillation the ECG (Electrocardiogram) is totally disorganised.

A human heart can be considered as a muscle operating rhythmically due to a nerve pulse that provides the heart beat. 

Therefore, when an external signal due to electric current is sent into the heart which will have different frequency, different from that of the normal heart, it disturbs the rhythmic flow of operation of heart. 

This condition of operation is known as ventricular fibrillation or arrhythmic operation of heart. 

Once this arrhythmic condition is set up, it is difficult to stop. 

It usually requires injection of another electric current to stop the fibrillation and reestablish the normal rhythmic.

It is to be noted that the effect ofelectric shock currents not only depends upon the physiological features of a person but the psychologic factors also play an important part. The effects of

electric current on vital human organs not only depend upon the magnitude of currents but it

depends upon the duration and frequency of the current. Humans are more vulnerable to electric

shock current at 50-60 Hz. The human body can withstand slightly higher current at 25 Hz and

almost five times larger for d.c. current. Similarly at frequencies of 1 KHz or 10 KHz even larger

currents can be tolerated. In case oflightning (where the frequency is very high and duration is

in µ secs) the human body can withstand very high currents in terms of several hundreds of amperes.

When the human body becomes a part ofthe electric circuit the current that passes through

it, is limited by the body resistance and the resistance of the earthing system of the device to

which the person comes in contact. The human body resistance differs from person to person and

also it is different when two feet or between one hand and one feet or between two hands. 

Generally two types of potentials are important from electric shock point of view. These are

✓ Touch potential and

✓  Step potential

When a person touches the electrically live part of a device, the potential between the hand and the foot is known as touch potential. 

However the potential between the two feet ofthe person when walking on an electrically conducting and live surface is known as step potential. 

Usually, the step potentials are smaller in magnitude as compared to the touch potentials.

For grounding a device, a galvanised steel wire is connected to it's body and the other end of the wire is connected to a steel rod which is put into the ground upto a certain depth. 

The pit contains moisturised mixture of salt and charcoal to provide better conductivity. The steel rod or

any other electrode is placed in this mixture. Sometimes more than one rod are used to improve

the grounding i.e., reduce the earthing resistance. The earthing resistance also depends upon the

resistivity ofthe soil. In transmission lines, tower footing resistance is reduced by using radial or

parallel galvanised steel wires in the ground. These are known as counterpoise wires. The acceptable value ofgrounding resistance varies upon its application e.g., for domestic appliances, the

acceptable value is about 1 ohm and for substation equipment a 5 ohm resistance is acceptable. It

is seen that the thickness ofthe rod does not play a major role in reducing the ground resistance

as does the length of the rod. Therefore, it is better to use thin but long rods or many small rods.

Some ofthe safety measures to avoid electric shock are given below :

✓ See that all the metallic parts ofvarious equipments and appliances are properly earthed.

✓ Before energising the domestic wiring its insulation must be checked.

✓ The phase wire must be connected through the switch.

✓ Never try to handle or operate any electrical appliances with wet hands or standing on a wet floor.

✓ Make it a habit to switch offthe supply and then pull out the plug whenever you have finished with a particular gadget.