Electrical Safety

1. Electrical Safety • History of (electrical) safety • Safety in the Hospital • Role of Clinical Engineer • Rules and regulations • Food and Drug Administration (FDA), Underwriter’s Laboratory (UL), Federal Communication (FCC) • Principles and examples of safety

2. Physiological Response to Electricity • Electrical shock from devices in the home to lightening • Macroshock (lightning, instruments) • Microshock (implantable catheters, leakage) • Susceptibility Parameters – Frequency, Duration, Body weight, Point-of-entry

3. Frequency Susceptibility • Note: the thresholds for preception, let-go currents.

4. Duration/Body Weight Susceptibility • Strength-duration curve is a very important plot, applicable to shocks, pacemaker/defibrillator type devices, etc. • Note the chronaxie point (at the lower left corner of each curve)

5. Current vs. Frequency • Difference in DC vs AC shocks • Note that the threshold is lowest at the powerline frequencies! • Very high threshold/low sensitivity at high frequencies used in many medical applications • impediance check, respiration, communications.

6. Isolated-Power Systems Transformer isolation Transformer isolation used in hospital based clinical instruments: Isolation of power supply and also of the instrument/circuit

7. Isolated-Power Systems Optical Isolation Simple LED, photodiode circuit Advanced circuit for linearization from the book Optical isolation is used in simple instrument/circuit designs; e.g. interfacing ECG amplifier to the outside world; optical path provides break of electrical continuity – usually interrupting path to ground for the leakage current

8. Micro- vs. Macroshock Hazards Points of Entry • Microshock occurs because there is a low resistance invasive path to the heart • Note the path to ground in any microshock circuit • Microshock due to ventricular fibrillation • Lowered safety in hospital, bathroom…

9. Sources of Microshock • Failure of power outlet,cord • Failure of transformer • Failure of catheter/lead • Ground path and ground loop leakage

10. Sources of Macroshock • Direct contact to “Live” power supply • “Live” instrument chassis • Failed transformer • Electrical spark/discharge • Lightning

11. Conductive Paths to the Heart • Pacemaker Leads • Epi- or endocardial electrodes • Intracardiac electrogram (EGM) electrodes • Liquid-filled Catheters for: • Blood pressure monitoring • Blood sampling • Drug/dye injection • Ground loop and inadvertent leakage path

12. Safety Codes & Standards • NFPA 99 • Standard for Health Care Facilities • National Electrical Code • Article 517: Health Care Facilities • Association for the Advancement of Medical Instrumentation (AAMI) • Developed American National Standard on “Safe Current Limits for Electromedical Apparatus”

13. Leakage Current Limits

14. Protection Circuits Pole-to-opposite pole. Figure 2A depicts the circuit path when a man simultaneously touches both poles of the floating or isolated circuit. In this situation, there is no insulation. The current is limited only by the impedance of the body. http://www.ewh.ieee.org/soc/emcs/pstc/TechSpk/floating.html

15. Protection Circuits If we interpose an insulating barrier between one pole of the floating circuit and the man, then we can define that barrier as Basic Insulation. In the event of failure of that Basic Insulation, there is no electric shock current in the man. If we extend that same insulation such that it is interposed between the OPPOSITE pole and the man, then we can define the OPPOSITE pole portion of the insulation as Supplementary Insulation (because it provides insulation against the SECOND body connection). http://www.ewh.ieee.org/soc/emcs/pstc/TechSpk/floating.html

16. References • Webster, JG (1998). Medical Instrumentation. John Wiley & Sons, Inc., New York, NY. Chapters 1 & 14. • Nute, R (1998). Floating circuits: protection against electric shock. In Technically Speaking section of Product Safety Technical Committee Newsletter (online). • http://www.ewh.ieee.org/soc/emcs/pstc/TechSpk/

17. Systolic pressure Dichrotic notch Distolic pressure Problems • Describe a sensor or a measurement system in which accuracy is important. In contrast, describe a sensor or a measurement in which precision is important. • A temperature sensor, such as a thermistor can be described by a first order system. Write down the general equation for a first order system (you can write a differential equation or a transfer function). • Plot the output of the first order system in response to a step change in temperature. • A blood pressure sensor is described by a second order system. Write down the general equation for a second order system (you can write a differential equation or a transfer function). • Plot the output of the second order underdamped pressure system in response to a blood pressure signal.

18. PROBLEMS • Draw the circuit of a differentiator –OR– integrator. • Now, through circuit analysis show why that circuit works like a differentiator/integrator (i.e. derive the relationship between output and input of the amplifier to show using your equation that the input signal is differentiated/integrated. • Next, obtain the frequency response of this circuit. E.g. you can derive a transfer function (output over input) in the Laplace form, substitute s=jw and then show the frequency response. • Point out one major advantage and one major disadvantage of an analog differentiator/integrator over a digital/software version. • Describe (very briefly) a biomedical instrumentation application of an integrator or differentiator.