CHBT Electrical Safety and Biomedical Engineering Principles 2 — Questions and Answers
Question 1: When performing preventive maintenance on a hemodialysis machine, what electrical test measures the resistance between the equipment chassis and the ground pin of the power cord?
- Insulation resistance test
- Ground continuity test (Correct answer)
- Leakage current test
- Dielectric strength test
Correct answer: Ground continuity test
The ground continuity (protective earth resistance) test measures the resistance of the safety ground path from the equipment chassis to the power cord ground pin, which should typically be less than 0.1 ohm.
Question 2: What is the function of a ground fault circuit interrupter (GFCI) in a hemodialysis clinical environment?
- It regulates voltage to dialysis machines
- It interrupts power when current imbalance between hot and neutral exceeds ~5 mA (Correct answer)
- It isolates patient circuits from building power
- It monitors dialysate conductivity
Correct answer: It interrupts power when current imbalance between hot and neutral exceeds ~5 mA
A GFCI detects a current difference of approximately 4–6 mA between the hot and neutral conductors, indicating current leaking to ground, and interrupts the circuit within milliseconds.
Question 3: According to Ohm's Law, if a dialysis machine has a leakage current pathway with a resistance of 500,000 ohms and is connected to a 120V supply, what is the leakage current?
- 24 milliamperes
- 240 microamperes (Correct answer)
- 24 microamperes
- 2.4 milliamperes
Correct answer: 240 microamperes
Using I = V/R: 120V ÷ 500,000Ω = 0.00024 A = 240 microamperes, which exceeds the 100 µA AAMI limit.
Question 4: Which NFPA standard specifically governs electrical safety requirements in health care facilities, including dialysis units?
- NFPA 70 (NEC)
- NFPA 99 (Correct answer)
- NFPA 101
- NFPA 13
Correct answer: NFPA 99
NFPA 99 (Health Care Facilities Code) establishes electrical safety requirements specific to health care environments, including patient care areas such as dialysis units.
Question 5: What is the primary biomedical engineering reason for using isolated power systems rather than standard grounded power in dialysis patient care areas?
- To provide higher voltage for dialysis machines
- To prevent a single ground fault from causing power interruption or patient electrocution (Correct answer)
- To reduce energy consumption of equipment
- To allow use of three-phase electrical loads
Correct answer: To prevent a single ground fault from causing power interruption or patient electrocution
In an isolated power system, a single line-to-ground fault does not complete a circuit, preventing shock to a grounded patient and allowing continuous operation while the fault is located.
Question 6: During biomedical equipment inspection, an insulation resistance test on a dialysis machine reads 0.5 megaohm. What does this indicate?
- Excellent insulation integrity
- Acceptable insulation within normal range
- Degraded insulation that may pose a safety hazard (Correct answer)
- Normal reading for all dialysis machines
Correct answer: Degraded insulation that may pose a safety hazard
Medical equipment insulation should typically measure at least 1–2 megaohms; a reading of 0.5 megaohm suggests insulation breakdown and potential leakage current risk requiring further evaluation.
Question 7: Which type of electrical isolation transformer provides the safest power for patient-connected medical equipment in a dialysis unit?
- Autotransformer
- Step-down transformer
- Medical-grade isolation transformer (Correct answer)
- Ferroresonant transformer
Correct answer: Medical-grade isolation transformer
Medical-grade isolation transformers are specifically designed with low interwinding capacitance and meet NFPA 99 requirements, providing electrical isolation while minimizing hazard current in patient environments.
CHBT Electrical Safety and Biomedical Engineering Principles 2
When performing preventive maintenance on a hemodialysis machine, what electrical test measures the resistance between the equipment chassis and the ground pin of the power cord?