CET CET Power Supplies and Electrical Safety 2 — Questions and Answers
Question 1: What current level through the human body is generally considered the threshold for causing ventricular fibrillation?
- 100 mA (Correct answer)
- 1 mA
- 10 mA
- 1 A
Correct answer: 100 mA
Approximately 100 mA through the chest region for more than a fraction of a second can induce ventricular fibrillation, which is fatal without defibrillation.
Question 2: What does a Ground Fault Circuit Interrupter (GFCI) outlet protect against?
- Electric shock from current leaking to ground through a person (Correct answer)
- Overloading from excessive current draw
- Voltage surges from lightning
- Short circuits in connected devices
Correct answer: Electric shock from current leaking to ground through a person
A GFCI monitors the difference between hot and neutral currents; if more than ~5 mA is unaccounted for (leaking through a person), it trips within milliseconds.
Question 3: What is the function of a Metal Oxide Varistor (MOV) in a surge protector?
- Clamp transient overvoltages by conducting heavily when voltage exceeds a threshold (Correct answer)
- Filter high-frequency noise from the power line
- Regulate output voltage to a fixed level
- Block reverse polarity from damaging equipment
Correct answer: Clamp transient overvoltages by conducting heavily when voltage exceeds a threshold
An MOV has a highly non-linear resistance that drops sharply at its clamping voltage, diverting surge energy away from protected equipment.
Question 4: What IEC safety class designates equipment with double insulation that requires no protective earth ground connection?
- Class II (Correct answer)
- Class I
- Class III
- Class 0
Correct answer: Class II
IEC Class II equipment uses double or reinforced insulation and bears the square-within-a-square symbol; no earth ground pin is needed.
Question 5: Why are bleeder resistors used in high-voltage power supply circuits?
- Discharge filter capacitors when power is removed, preventing electric shock (Correct answer)
- Increase output current capacity
- Improve voltage regulation under load
- Reduce AC ripple at the output
Correct answer: Discharge filter capacitors when power is removed, preventing electric shock
Bleeder resistors provide a discharge path for stored capacitor charge so the output voltage decays to a safe level when the supply is turned off.
Question 6: Which grounding method bonds the equipment chassis to earth ground to protect a user from electric shock if an internal fault energizes the chassis?
- Protective earth grounding (Correct answer)
- Signal grounding
- Floating ground
- Virtual ground
Correct answer: Protective earth grounding
Protective earth grounding connects the chassis to earth so a fault current flows to ground and trips the breaker rather than passing through the user.
CET CET Power Supplies and Electrical Safety 2
What current level through the human body is generally considered the threshold for causing ventricular fibrillation?