Questions: Electrical Safety in Engineering

Question 1

Why do GFCI outlets protect against electrocution while standard circuit breakers do not?

• A. GFCIs are faster
• B. GFCIs detect the tiny current imbalance that occurs when current leaks through a person, while circuit breakers only trip at much higher currents (15-20 A)
• C. GFCIs use special wiring
• D. Circuit breakers are outdated technology

Answer: (B)

A GFCI monitors the current flowing out on the hot wire and returning on the neutral wire. If even 5 mA is 'missing' (flowing through a person to ground instead of returning on the neutral), the GFCI trips in about 25 milliseconds. A 15A circuit breaker would not notice 5 mA of leakage.

Question 2

A 12-volt car battery is mostly safe to work around because the voltage is low.

• T. True
• F. False

Answer: False

While 12V is unlikely to cause a shock through dry skin, a car battery can deliver hundreds of amps through a short circuit. A dropped wrench across the terminals can weld itself in place, cause severe burns, spray molten metal, or ignite hydrogen gas produced by the battery.

Question 3

What does 'grounding' a circuit mean and why does it improve safety?

Model answer: Grounding connects the metal chassis or enclosure of an electrical device to the earth through a low-resistance wire (the ground wire). If a fault causes the hot wire to contact the metal chassis, the ground wire provides a low-resistance path for current to flow back to the panel, tripping the circuit breaker. Without grounding, the chassis would become energized and anyone touching it would receive a shock.

Grounding works because current follows the path of least resistance. The ground wire's resistance is much lower than the human body's resistance, so fault current flows through the wire instead of through a person. This is why three-prong plugs exist -- the third prong is the ground connection.