CBET Electronics and Electrical Safety 2 — Questions and Answers
Question 1: What is the maximum allowable leakage current for a Type CF (cardiac floating) applied part according to IEC 60601-1?
- 500 microamps
- 100 microamps
- 10 microamps (Correct answer)
- 50 microamps
Correct answer: 10 microamps
Type CF applied parts must have a maximum patient leakage current of 10 microamps under normal conditions to prevent microshock.
IEC 60601-1 defines three types of applied parts: Type B (body), Type BF (body floating), and Type CF (cardiac floating). The 10 microamp limit exists because current applied directly to the myocardium can cause ventricular fibrillation at much lower levels than surface current.
Question 2: In a hospital electrical system, what is the purpose of an isolation transformer in an isolated power system?
- To step down voltage for equipment
- To prevent ground-fault current from flowing through patients (Correct answer)
- To increase available current capacity
- To convert AC to DC power
Correct answer: To prevent ground-fault current from flowing through patients
An isolation transformer prevents ground-fault current from flowing through patients by electrically isolating the secondary circuit from ground.
If one conductor faults to ground in an isolated system, no current flows because there is no return path. A Line Isolation Monitor (LIM) continuously monitors impedance between each line and ground, alarming when total hazard current exceeds 5 milliamps.
Question 3: What component in an electrical circuit allows current to flow in only one direction?
- Resistor
- Capacitor
- Diode (Correct answer)
- Inductor
Correct answer: Diode
A diode is a semiconductor device that allows current to flow in one direction (forward bias) while blocking current in the reverse direction.
Diodes are fundamental in medical equipment power supplies (rectification), protection circuits (preventing reverse polarity), and signal processing. Biomedical technicians encounter diodes in defibrillator circuits, power supply troubleshooting, and LED indicator circuits.
Question 4: When measuring the resistance of a component with a multimeter, why must the circuit power be turned off first?
- To protect the component from damage
- Because the multimeter supplies its own test current and external voltage will cause inaccurate readings (Correct answer)
- To prevent electrical shock to the technician
- To avoid damaging the circuit board traces
Correct answer: Because the multimeter supplies its own test current and external voltage will cause inaccurate readings
A multimeter in resistance mode supplies a small known current and measures the resulting voltage to calculate resistance. External circuit voltage would produce incorrect readings and could damage the meter.
This is particularly critical in medical equipment containing capacitors (like defibrillators) that can retain charge even after power-off. Proper discharge procedures must be followed before resistance measurements.
Question 5: What is the purpose of a ground wire (green or green/yellow) in a medical device power cord?
- To carry the return current during normal operation
- To provide a low-impedance fault current path to trip the circuit breaker (Correct answer)
- To filter electromagnetic interference
- To supply backup power during outages
Correct answer: To provide a low-impedance fault current path to trip the circuit breaker
The ground wire provides a low-impedance path for fault current to flow back to the source, tripping the circuit breaker quickly and protecting patients and staff.
Biomedical technicians must verify ground wire integrity during safety inspections by measuring ground wire resistance (must be less than 0.1-0.2 ohms from plug ground pin to any exposed metal surface). A broken ground wire is an immediate safety hazard.
Question 6: Which of the following best describes the function of an optocoupler (optoisolator) in medical equipment?
- Amplifying weak biological signals
- Transmitting signals between circuits while maintaining electrical isolation (Correct answer)
- Converting optical signals to radio frequency
- Measuring light intensity for phototherapy
Correct answer: Transmitting signals between circuits while maintaining electrical isolation
An optocoupler uses an LED and photodetector to transmit signals between electrically isolated circuits, maintaining patient safety.
In medical equipment, optocouplers isolate patient-connected circuits from mains-powered circuits. They appear in patient monitors, infusion pumps, and communication interfaces. Biomedical technicians encountering isolation failures should check optocoupler function.
CBET Electronics and Electrical Safety 2
What is the maximum allowable leakage current for a Type CF (cardiac floating) applied part according to IEC 60601-1?