Electric Power Distribution -- Grounding, earthing and transient overvoltage protection: Electric shock and human beings

Electric shock and sensitive ground leakage protection

There are four major factors, which determine the seriousness of an electric shock:

1. Path taken by the electric current through the body

2. Amount of current

3. Time the current is flowing

4. The bodies electrical resistance.

The most dangerous and most common path is through the heart. Persons are not normally accidentally electrocuted between phases or phase to neutral, almost all accidents are phase to ground.

FIG. 8. Dangerous current flows.

FIG. shows the four stages of the effect of a current flow through the body:

1. Perception - tingling - about 1 mA

2. Let-go threshold level - about 10 mA

3. Non-let-go threshold level - 16 mA

4. Constriction of the therasic muscles - death by asphyxiation and ventricular fibrillation - about 70-100 mA.

FIG. 9. Effects of current flow through the body

FIG. shows the normal electrocardiogram - one pulse beat - at 80 bpm = 750 ms.

• QRS phase -- normal pumping action

• T-phase - refractory or rest phase -- about 150 ms.

Death could occur if within this very short period of 150 ms a current flow was at the fibrillation level.

FIG. the resistance of the human body -- hand to hand or hand to foot.

FIG. 10. Electrocardiagram

FIG. 11. Resistance of human body

Consider an example of a man working, possibly perspiring, he touches a conductor at 300 V (525V phase to ground). 300 V divided by 1000 ohm = 300 mA! It’s important to remember that it’s current that kills, not voltage.

Sensitive ground leakage protection:

FIG. the operation of the core balance leakage device. This shows a single-phase system but would be similar for three-phase, 3w or three phase, 4w systems.

FIG. 12. Principles of core balance protection