Range of Tolerable Current for AC Substation Grounding

This article is about range of electric current which human body can tolerate and what factors depends upon.

Range of Tolerable Current by Human Body for AC Substation Grounding

There are three factors to be considerable for electrical current passed by human body.

1. Range of Tolerable Current
2. Effect of Frequency
3. Effects of Magnitude & Frequency
4. Importance of High Speed Fault Clearing

Duration, magnitude and frequency of the current are main factor which define the effects of electrical current which human body can tolerate.

Effect of Frequencies at Human Body

• Human bodies are very vulnerable to 50 or 60 Hertz frequencies.
• Even 0.1 ampere current is sufficient to cause death.
• 25 Hz frequency slightly can tolerate our human body and five times higher direct current.
• Currents having 3000 to 10000 Hertz can also be tolerated.
• Human Body also can tolerate very high current like lightning surge in rear cases.

Effects of Magnitude and Duration

The most common physiological effects of electric current on the body, stated in order of increasing current magnitude, are perception, muscular contraction, unconsciousness, fibrillation of the heart, respiratory nerve blockage, and burning.

Current of 1 mA is generally recognized as the threshold of perception, that is, the current mangitude at which a person is just able to detect a slight tingling sensation in his hands or fingertips caused by the passing current.

Currents of 1-6 mA, often termed let-go currents, though unpleasant to sus­tain, generally do not impair the ability of a person holding an energized object to control his muscles and release it. Dalziel’s classic experiment with 28 women and 134 men provides data indicating an average let-go current of 10.5 mA for women and 16mA for men, and 6 mA and 9mA as the respective threshold values.

Read Also: Safety in Grounding IEEE STD 80 | Basic Problems

Importance of High-Speed Fault Clearing

Considering the significance of fault duration both in terms of Equation 3 and implicitly as an accident-exposure factor, high-speed clearing of ground faults is advantageous for two reasons:
(1) The probability of electric shock is greatly reduced by fast fault clearing time, in contrast to situations in which fault currents could persist for several minutes or possibly hours
(2) Both tests and experience show that the chance of severe injury or death is greatly reduced if the duration of a current flow through the body is very brief; the allowed current value may therefore be based on the clearing time of primary protective devices, or that of the back-up protection

The questions were asked on different Internet browsers, Engineers can reply for these on comment section to help others.

1. How much current human body can bear?
2. What is Safe voltage and current for human body?
3. How many amperes can a human sustain?
4. What is human body resistance value?
5. What happens when current passes through human body?
6. What problem does come after current pass in our body.

All the points you can get be answered if you check following document and read throughly