POWER ELECTRONICS HANDBOOK
The basics of ground-fault interruption WILL DELSMAN
NK TECHNOLOGIES
Not all ground-fault interrupters are designed to protect living beings. Specialized industrial versions are optimized for maintaining the health of machinery and processes.
Just a little current can kill
GROUND FAULTS ARISE when current flows from an energized conductor to ground inadvertently. The return path of the fault current is through living beings or equipment touching the
8000
grounding system. Ground fault detection is critical to protecting people and animals from shock or death.
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At 1 mA you feel a slight tingle. At 5 mA you feel a slight shock, not painful but disturbing. The average individual can let go, but involuntary reactions can lead to injuries. At 6–25 mA there is painful shock, and muscular control is lost. The 9–30 mA level is called the freezing current or “letgo” range. At this level many humans cannot get their muscles to work, and they can’t open their hand to let go of a live conductor. At 50–150 mA there will be extreme pain, respiratory arrest, and severe muscular contractions. The individual cannot let go, and death is possible. At 1,000–4,300 mA there is ventricular fibrillation (the pumping action to the heart ceases). Muscular contraction and nerve damage occur. Death is most likely. At 10,000+ mA there will be cardiac arrest with severe burns and probable death.
DESIGN WORLD — EE NETWORK
2 • 2020
800 Current milliamperes
It doesn’t take much ground-fault current to cause harm. Extensive research in the 1960s determined the amount of current and voltage needed to cause ventricular fibrillation (where a heart stops beating) in humans. These studies found that as little as 70 mA through the heart was enough to cause fibrillation. The refinement of transistor technology provided a means of sensing currents as low as 0.003 A (3 mA) to energize a relay that would decouple the power supply. OSHA documents spell out the general relationship between the amount of current received and the reaction when current flows from the hand to the foot for just one second.
500 and higher Probably fatal
150
50 30 20 9 5 1
20-55 possibly fatal 30 standard Equipment protection 5-20 can’t let go Slight shock felt Barely feel tingle
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