FYS 4250 Kap.14 Electrical safety

Electric current, perception- and hazard thresholds 10 000 device related patient injuries in USA each year Three phenomenon with current through the body: Electric stimulation of excitable tissue (nerve & muscle) Resistive heating of tissue Electrochemical burns and tissue damage for direct current and very high voltages Perception = tingling sensation

Electrovibration (1uA 50Hz)

Statistical scattering of sensitivity for electric current

Frequency dependency and spread of the let-go threshold Let go current rais below 10 Hz, muscles can partially relax during part of each cycle

Ventricular fibrillation thresholds in animals Duration of applied current is an important factor.

Macro- og microshock Macroshock 100 mA Microshock 10 uA

Electromedical devices must be CE-marked (Europe)

Patient near PC (B-apparatus)

American electric power distribution

4-leads (norske), og 5-leads (europeiske) electrical circuits fra ”Medisinsk-teknisk sikkerhet på sykehus” Grimnes&Jensen 2003 Norwegian elctrical network is floating with respect to ground = IT network (No current dropout for the first ground short circuit European TN.

Isolated system

Macroshock due to ground fault

Microshock, two error situations

Ventricular fibrillation and pump failure vs catheter area

Micro-shock A device with a ground fault that does not open the circuit breaker. 5 A flow to the distribution panel on the ground wire. Ground wire have 0.1 ohm resistance -> 500 mV could appear across the patient. Resistance of a patient’s body and liquid filled catheter is less than 50 kOhm -> current = 10 uA

System for grounding

Ground fault circuit-interupter Disconnect the source of electric power when a ground fault greater than 6 mA occurs. Senses the difference between the hot conductor and neutral conductor Trip time varies inversely with the magnitude of the ground-fault current

Floating Patient

Electrical isolation Perfect isolation barrier modeled by the isolation capacitance and resistance. IMRR = isolation mode rejection ratio (potential between the input common and the output common)

Electric isolation

Isolated pressure transducer

Receptacles tests

Ground pin to chassis test Should not exceed 0.2 Ohm

Chassis leakage cuttent test

Patient leakage current test

Leakage current between patient leads

Isolation test for AC isolation Equipment manufacturer only, may be omitted for testing in health care facilities