Electronics and Electrical Safety Flashcards

According to NFPA 99, what is the maximum allowable chassis leakage current for a new, cord-connected, non-patient care-related electrical appliance used within the patient care vicinity?

Answer: 500 microamps

NFPA 99 (2005 edition) states that the chassis leakage current for facility-owned, non-patient care-related appliances, such as housekeeping or maintenance equipment, shall not exceed 500 microamps when used in the patient care vicinity. The limit for patient-care related appliances is generally lower, at 300 microamps.

A biomedical equipment technician is performing an electrical safety inspection on a portable medical device. The resistance measurement between the ground pin of the attachment plug and the device's metal chassis should be less than what value to be considered safe?

Answer: 0.5 ohms

To ensure a reliable path to earth ground in case of a fault, the resistance between the chassis and the ground pin of the power cord must be very low. According to NFPA 99, this resistance must be less than 0.5 ohms. A low-resistance ground path is critical for shunting fault currents safely to the ground, preventing the chassis from becoming energized.

Which of the following describes the primary danger associated with microshock?

Answer: A very small current applied directly to or near the heart, potentially causing ventricular fibrillation.

Microshock is a hazard specific to electrically susceptible patients, where a very small current (as low as 10-20 microamps) passes directly through the heart via an internal conductor, such as a pacemaker lead or saline-filled catheter. This is dangerous because the current bypasses the high resistance of the skin and can induce ventricular fibrillation at levels that would be imperceptible on the body's surface (macroshock).

A newly installed, permanently wired (fixed) piece of medical imaging equipment is being tested. Before any grounds are connected, what is the maximum allowable ground leakage current according to NFPA 99 standards?

Answer: 10 milliamps

For permanently wired equipment, NFPA 99 allows for a higher leakage current limit compared to portable devices. The ground leakage current, tested before the permanent ground is connected, must not exceed 10.0 mA. This test is typically performed on new installations.

What is the primary function of an isolation transformer in medical equipment used in a hospital operating room?

Answer: To create an ungrounded electrical system that reduces the risk of macroshock from a single fault.

Isolation transformers are crucial for electrical safety in critical care areas like operating rooms. They create a 'floating' or ungrounded secondary circuit. In a standard grounded system, touching a single live wire while grounded can cause a severe shock. With an isolated system, touching one of the secondary conductors while grounded will not complete a circuit, thus preventing a macroshock. This significantly enhances patient and staff safety.

When testing a medical device with patient-applied parts (like ECG leads), the international standard IEC 60601-1 establishes specific, very low leakage current limits. This is primarily to protect against which hazard?

Answer: Microshock

The IEC 60601-1 standard sets stringent leakage current limits for patient-applied parts, often in the microamp range, to protect electrically susceptible patients from microshock. Because these leads provide a direct or low-impedance path to the patient's heart, even a tiny amount of current could be fatal. The standard for intracardiac connectors is as low as 10 µA.