ELECTRIC SHOCK
An electric shock or accident is normally determined by the frequency, magnitude, and duration of the current flowing through the vital parts of a human body, generally referred to as the heart. When the current passes through the heart it results in ventricular fibrillation, and when the current flows through the nerve center it causes suppression in the respiratory center and results in exhaustion of the cardiovascular muscles. The respiratory spasm shrinkage which is caused by an electric shock can cause asphyxiation.
Table of Contents
EFFECTS OF ELECTRIC SHOCK ON HUMAN BODY
Ventricular Fibrillation (VF)
It is a heart rhythm disturbance also called arrhythmia where the heart’s lower chambers or ventricles quiver or fibrillate instead of contracting. This restricts the heart from pumping blood effectively, which can quickly lead to sudden cardiac arrest if not treated immediately.
Asphyxiation
It refers to the condition of being deprived of essential oxygen, which can lead to unconsciousness or death if not addressed quickly. It occurs when the body is unable to get sufficient oxygen, because of blocked airways, and external pressure on the chest.
DESIGN FACTOR AND CALCULATION FOR HUMAN SAFETY
Normally, the minimum current a human can feel is about 1mA, and to sustain 9–25mA of current for a human is rather painful. It makes the muscles lose control and it becomes difficult or impossible to unclamp a charged object held in the hand. When the current is increased, the shrinkage of muscle may cause dyspnea or shortness of breath.
However under a greater current, which can inhibit breathing issues and result in the occurrence of heart fibrillation and cause death. This phenomenon is not permanent and it disappears when the current is interrupted, unless the shrinkage of muscle is quite serious which stops the breathing. This happens only when the duration of the current subjected is longer than several minutes, but modern protective relays and circuit breakers can interrupt the electric shock fault current within a fraction of a second or even a shorter time, so that asphyxiation caused by muscle shrinkage may not happen.
Therefore, the critical value of an electric shock current causing ventricular fibrillation is the main issue that is considered in the safety design of a grounding device.
Classification of current according to human response
Many scholars have studied the permissible body current limit for humans. For an AC current with a frequency of 50 Hz, the current can be classified into three categories according to human responses:
Sensory current:
This is the minimum current that a human adult can feel. For a male, it is 1.1 mA, and for a female, it is 0.7 mA.
Extrication current:
It is the maximum current that can be extricated by a human adult after an electric shock. It is also related to gender, for a male the limit is 16mA, and for a female adult, the limit is 10.5mA.
Fatal current:
This is the minimum current that threatens life in the shortest time, by causing ventricular fibrillation or asphyxiation.
Allowable Body Current
The magnitude of the current plays a very vital role in electric shock on the human body. However, when judging the risk of a current flowing through the body, the duration must also be considered.
According to analysis, when the duration is very short, the current that a human can tolerate and which does not cause ventricular fibrillation is:
I2k t = K
where the unit of IK, the allowable body current, is A and K is an energy coefficient related to the weight of the human body. The energy coefficients of 50 and 70 kg humans are K50 = 0.0135 and K70 = 0.0247 respectively. Therefore, we can obtain the allowable body current IK50 for a 50 kg human
IK50 = 0.116 / √t , [√0.0135 = 0.116 ]
According to the calculation, the result from the equation, the permitted current corresponding to 1 s duration is 116mA, while that corresponding to 0.1 s is 367mA and that corresponding to 0.05 s is 519 mA.
The allowable current IK70 for a 70 kg human is
IK70 = 0.157 / √t , [√0.0247 = 0.157 ]
Allowable Body Voltage
Multiplying the allowable current IK by the body resistance RB, we can obtain the allowable body voltage USV
USV = IK RB
In a solidly grounded system, a single-phase or two-phase ground fault cannot exist for a long time, so substituting the Equation of body current into the above equation, the short-time allowable body voltage for a 50 kg human is
USV50 = 0.116 RB / √t
Also, the allowable voltage for a 70 kg human can be obtained
USV70 = 0.157 RB / √t
If we assume the body resistance RB = 1000 ohm and time, t = 1 s, the allowable body voltage in 1 s for 50 and 70 kg humans are 116 and 157 V, respectively.
