Electrical Safety. Hsiao-Lung Chan, Ph.D. Dept Electrical Engineering Chang Gung University, Taiwan

Transcription

1 Electrical Safety Hsiao-Lung Chan, Ph.D. Dept Electrical Engineering Chang Gung University, Taiwan

2 Electro-medical equipment safety standard International electrotechnical commission IEC601 Standard endorsed by Health Industry Manufacturers Association (HEMA) National Electrical Manufacturers Association (NEMA) Food and Drug Administration (FDA) Safety 2

3 Physiological effect of electricity Electrolysis Direct current passing between electrodes Small ulcers will be formed beneath the electrodes under a 100μA direct current for a few minutes. IEC601 define direct current with frequency 0.1 Hz. IEC601 limits direct current between electrodes within 10μA Safety 3

4 Physiological effect of electricity (cont.) Neural stimulation Typical membrane potential of nerve is 80 mv. If the potential is reversed for 20 μs, the neuron is stimulated and action potential is propagated. Sensory nerve stimulation pain be felt Motor nerve stimulation muscle contraction Stimulation becomes increasing difficult above 1 khz Major hazards of stimulation Heart muscle Coordinated pumping activity is disrupted Skeletal muscle Breathing difficulty, etc. Safety 4

5 Physiological effect of electricity (cont.) Macroshock hazard Stimulation through the skin Figure from J. Enderle, Introduction to Biomedical Engineering, Academic Press, Safety 5

6 Physiological effect of electricity (cont.) Stimulation through the skin (cont.) An alternating current at 50/60 Hz is applied: Perception Skeletal muscle Contraction Ventricular fibrillation Heat restart after current removed (defibrillator) 1mA 15mA 100mA 500mA Increasing painful and breath difficult Let-go current: maximum current at which subject can withdrawal voluntarily Safety 6

7 Safety 7

8 Safety 8

9 Physiological effect of electricity (cont.) Stimulation through the skin (cont.) Depend on skin impedance: If a 240V is applied to skin Dry skin (10 ~ 100 kω): 25 ~ 2.5 ma Gripped connection (1kΩ): 250 ma Wet skin (100Ω): 2.5 A IEC61 limits the current to skin within 0.5 ma with a single fault in equipment Safety 9

10 Macroshock hazard: electrical fault Grounded case Black-color Hot Neutral Electrical Fault Circuit 100kΩ Skin Contact 500Ω Ground Water Pipe Earth ground Ground Green-color White-color Chassis Safety 10

11 Macroshock hazard (cont.) Ungrounded case or broken ground Ground Water Pipe Earth ground Hot Neutral x Broken Ground Electrical Fault Circuit Chassis 100kΩ Skin Contact 500Ω Safety 11

12 Physiological effect of electricity (cont.) Microshock harzard Direct stimulation of the heart Figure from J. Enderle, Introduction to Biomedical Engineering, Academic Press, Safety 12

13 Physiological effect of electricity (cont.) Direct stimulation of the heart (cont.) Cardiac catheter 100μA can cause ventricular fibrillation IEC61 limits the current to heart within 10μA under normal operating condition, and 50μA with a single fault. Safety 13

14 Microshock harzard: leakage current Leakage current pathway: intact ground Hot Neutral Circuit Catheter 500Ω Ground Ground Water Pipe I leakage C dv dt Chassis Earth ground Safety 14

15 Microshock harzard (cont.) Leakage current pathway: broken ground Hot Neutral x Broken Ground Circuit Chassis Catheter 500Ω Ground Water Pipe Earth ground Safety 15

16 Classification of equipment Class I equipment Hot Neutral Ground Circuit Metal box Safety 16

17 Classification of equipment Class IIA equipment Hot Neutral Circuit Two completely separate insulating box Many domestic equipments are double insulated, like hair dryers, etc. No metal part, subject is safe when fault condition. Safety 17

18 Patient protection Power distribution Grounding system Isolated power distributed system Ground-fault circuit interrupt Safety 18

19 Grounding system From JG. Webster, Medical Instrumentation, application and design, 3 rd, Houghton Mifflin, 2000.

20 Isolated power system Hot L1 L2 Neutral Ground Ground Water Pipe Earth ground No current can flow from L1 or L2 to ground In fact, some current does flow due to imperfection of transformer. Safety 20

21 Ground-fault circuit interrupt (GFCI) The same number of turns but in opposite directions Normal condition: I H I N Detected by sensing amplifier and shut off the power Fault condition: I H I N From J. Enderle, Introduction to Biomedical Engineering, Academic Press, 2000.

22 Reference BH.Brown, et al, Medical Physics and Biomedical Engineering, IOP Publishing Ltd, J. Enderle, Introduction to Biomedical Engineering, Academic Press, JG. Webster, Medical Instrumentation, application and design, 3rd, Houghton Mifflin, JJ.Carr, JM.Brown, Introduction to Biomedical Equipment Technology, 3rd Edition, Prentice-Hall, Safety 22