PART III- Voltage Surges, Over- voltages, Circuit Interrupters and Grounding Practices
Chapter 21. Grounding theory and ground fault protection schemes
• Protection of a domestic or an industrial single phase system
• Ground fault on an LV system
• Ground fault protection in hazardous areas
• Ground leakage in an HV system
• Core-balanced current transformers (CBCTs)
• Ground fault (G/F) protection schemes
• Grounding systems and their choices
Protection of a domestic or an industrial single-phase system
Effects of current passing through a human body and the body’s tolerable limits
An electric current, rather than voltage, through a human body may cause shock and can damage vital organs of the body as follows:
1. It may cause muscular contraction, unconsciousness, fibrillation of the heart, respiratory nerve blockage and burning. These are all functions of body weight. The muscular contraction makes it difficult to release an energized object if held by the hand and can also make the breathing difficult. The heart, being the most vulnerable organ of a human body, is damaged most, mainly by ventricular fibrillation, which may result in an immediate arrest of the blood circulation (electrocution). In Table 21.1 we provide likely intensities of body currents when lasting for more than a heartbeat (nearly 60–300 ms or 3–15 cycles for a 50 Hz system).
2. It is generally seen that a human body can sustain a much higher current at a lightning or switching frequency (5 kHz or above) due to the extremely short duration of such surges (30 ms or less).
The current can pass through the heart, when the current passes through hand to hand, or through one hand and a foot. Current flowing between one foot to the other may not be considered dangerous, but may cause muscular contraction and pain. The subsequent body fall, if it occurs, may, however, be fatal as now the current can also flow through the hand involving the heart. Ground fault protection emphasizes keeping the fault current below the fibrillation threshold and for a period of less than a heartbeat, in the range of 60–300 ms. It has been established that the electric shock energy which a human body can endure, without damage has a relationship with the leakage current through the body and its duration.