The 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 HT 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 according to this these are divided into two types:

Primary Shock Currents

The currents which produce direct physiological harm are termed primary shock currents.

Secondary Shock Currents

The currents which 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.

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 rhythm.

Typical effects of Electric shock currents on humans

50 Hz Current

Effect

0-1 mA

No sensation

0-3 mA

Mild sensation

3-5 mA

Pain or annoyance

5-10 mA

Painful shock

10-15 mA

Local muscle contraction

30-50 mA

Breathing difficult, can cause unconsciousness

50-100 mA

Possible ventricular fibrillation of heart

100-200 mA

Ventricular fibrillation of heart certain.

It is to be noted that the effect of electric 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 of lightning (where the frequency is very high and duration is in ยต secs) the human body can withstand very high currents in terms of several hundreds of amperes.