Electrical Safety

Medical Instrumentation Application and Design, 4th Edition, Chapter 14

John G. Webster, Univ. of Wisconsin, MadisonISBN: 978-0-471-67600-3

Taught Matter of Lectures

IntroductionBasic Theory of MeasurementsBeginnings of Basic SensorsSensors [MEMS]Signals and NoiseAmplifiers of SignalsConnection and Protection of SignalsData Acquisition and Data Converters Electric Safety in Medical Systems

Electrical SafetyAgenda

• Introduction• Physiological Effects of Electricity• Susceptibility Parameters• Distribution of Electric Power• Macroshock Hazards• Microshock Hazards• Electrical-Safety Codes and Standards• Approaches to Protection Against Shock

• Power Distribution• Equipment Design

• Testing the Electric System• Tests of Electric Appliances• Conclusion

IntroductionES as elemetary protection

• Medical technology has improved health care in ALL medical specialties, with rising complexity

• More than 10,000 device-related patient injuries• Most patient injuries are attributable to improper use• Medical personnel rarely read user manuals until a problem has

occurred

Result: Medical instrumentation safety• Safe Design• Safe Use

... Safe EVERYTHING (one of the most regulated industrial market)

ES is one of the basic protection mechanisms for patient, operater, and third persons and part of this chapter

Physiological effectsCurrent can heal and harm

• For a physiological effect the body must become part of an electrical circuit

• Three phenomena occur when el. current flows1. El. stimulation of excitable tissue (muscle, nerve)2. Resistive heating of tissue3. Electrochemical burns

• Further consideration are based on the following parameters• Human body with contact to el. circuit at left and right hand• Body weight: 70 kg• Applied current time: 1 s to 3 s• Current frequency: 60 Hz

Figure 14.1 Physiological effects of electricity Threshold or estimated mean values are given for each effect in a 70 kg human for a 1 to 3 s exposure to 60 Hz current applied via copper wires grasped by the hands.

Physiological effectsCurrent can heal and harm

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Physiological effectsThreshold of Perception (1)

• Current density is just large enough to excite nerve endings in the skin

• Subject feels tingling sensation• Lowes values with moistered hands (decreases contact resistance)

• 0.5 mA at 60 Hz• 2 mA to 10 mA DC• The subject meight feel a slight warming

Physiological effectsLet-go Current (2)

• The let-go current is defined as the maximal current at which the subject can withdraw voluntarily

• For higher current nerves and muscles are vigorously stimulated• Involuntary contraction or reflex withdrawals may cause secondary

physical injuries (falling off the ladder)• The minimal threshold for the let-go current is 6 mA

Physiological effectsRespiratory Paralysis, Pain, Fatigue (3)

• Higher current causes involuntary contraction of muscles and stimulation of nerves what can lead to pain and cause fatigue

Example: stimulation of respiratory muscles lead to involuntary contraction with the result of asphyxiation if current is not interrupted

Of course, today’s ethics commission would never allow these experiments on human beings.

Physiological effectsVentricular Fibrillation (4)

• The heart is especially susceptible to electric current.• Just 75 mA to 400 mA (AC) can rapidly disorganize the cardiac

rhythm and death occurs within minutes• Only a brief high-current pulse from a defibrillator can depolarize all

the cells of the heart muscle simultaneously• Within the U.S. occur approximately 1,000 death per year due to cord-

connected appliances

Physiological effectsSustained Myocardial Contraction (5)

• When current is high enough to stimulate the entire heart muscle, it stops beating

• Usually the heart-beat ensues when the current is interrupted• Minimal currents range from 1 A to 6 A (AC), like used in defibrillators

Physiological effectsBurns and Physical Injury (6)

• Resistive heating cause burns • Current can puncture the skin• Brain and nerve tissue may lose all functional excitability • Simultaneously stimulated muscles may contract strong enough to

pull the attachment away from the bone or bread the bone

Susceptibility ParametersIntroduction

The current needed to produce each effect depends on these parameters• Threshold of Perception and Let-Go Variability• Frequency• Duration• Body Weight (and gender)• Points of Entry

• Macroshock• Microshock

Figure 14.2 Distributions of perception thresholds and let-go currents These data depend on surface area of contact (moistened hand grasping AWG No. 8 copper wire). (Replotted from C. F. Dalziel, "Electric Shock," Advances in Biomedical Engineering, edited by J. H. U. Brown and J. F. Dickson III, 1973 3, 223-248.)

Susceptibility ParametersVariability of threshold and Let-go current

• Let-go current versus frequencyMinimal let-go currents occur for commercial power-line frequencies (50 Hz to 60

Hz)For frequencies below 10 Hz let-go current rises again (muscle can relax)

Figure 14.3 Let-go current versus frequencyPercentile values indicate variability of let-go current among individuals. Let-go currents for women are about two-thirds the values for men. (Reproduced, with permission, from C. F. Dalziel, "Electric Shock," Advances in Biomedical Engineering, edited by J. H. U. Brown and J. F. Dickson III, 1973, 3, 223–248.)

Susceptibility ParametersFrequency

Geddes and Baker (1989) presented the excitation behavior of myocardial cells by a lumped parallel RC circuit that represents the resistance and capacitance of the cell membrane.

This model determines the cell excitation thresholds that exceed about 20 mV for varying rectangular pulse duration d by assigning the rheobase currents Ir and cell membrane time constant τ=RC.

The strength-duration equation

For a short duration:Stimulation current Id is inversely related to the pulse duration d (Figure

14.4)

Susceptibility ParametersDuration

Figure 14.4 Normalized analytical strength–duration curve for current I, charge Q, and energy U. The x axis shows the normalized duration of d/τ (From Geddes, L. A., and L. E. Baker, Principles of Applied Biomedical Instrumentation, 3rd ed. New York: John Wiley & Sons, 1989).

Susceptibility ParametersDuration

• Several studies (animals) show a clear dependency of the fibrillating current to the body weight (Figure 14.5)50 mA rms for 6kg dogs to 130 mA rms for 24 kg dogs

Figure 14.5 Fibrillation current versus shock duration. Thresholds for ventricular fibrillation in animals for 60 Hz ac current. Duration of current (0.2 to 5 s) and weight of animal body were varied. (From L. A. Geddes, IEEE Trans. Biomed. Eng., 1973, 20, 465–468. Copyright 1973 by the Institute of Electrical and Electronics Engineers. Reproduced with permission.)

Susceptibility ParametersBody weight

Figure 14.6 Effect of entry points on current distribution (a) Macroshock, externally applied current spreads throughout the body, (b) Microshock, all the current applied through an intracardiac catheter flows through the heart. (From F. J. Weibell, "Electrical Safety in the Hospital," Annals of Biomedical Engineering, 1974, 2, 126–148.)

• Macroshock: only a small fraction of the total current flows through the heart. Magnitude to harm the heart is far greater

• Microshock: all the current applied flows through the heart

Susceptibility ParametersPoints of Entry

• 60 Hz for 5 s to a ventricular pacing catheter during implantable cardioverter-defibrillator implant testing in 40 patients

• ResultIntermittent capture with a minimum current of 20 µAContinuous capture with a minimum current of 49 µA

• Resulting RegulationThe widely accepted safety limit to prevent microshocks is 10 µA

Susceptibility ParametersPoints of Entry - Example

Figure 14.7. Percentile plot of thresholds for continuous capture and VF (or sustained VT). Cumulative percent of patients is shown on abscissa and root-mean-square AC current (in µA) on ordinate. Squares denote unipolar data; circles, bipolar data. Solid symbols identify data from patients in whom the only clinical arrhythmia was atrial fibrillation (AF). Top, Thresholds for continuous capture. Current strength of 50 µA caused continuous capture in 5 patients (12%) with unipolar AC and in 9 (22%) with bipolar AC (P=0.49). Bottom, Thresholds for sustained VT/VF. These plots do not reach 100% because sustained-VT/VF thresholds exceeded maximum output of stimulator in 6 patients (15%) with bipolar AC and 8 (20%) with unipolar AC. From Swerdlow, C. D., W. H. Olson, M. E. O’Connor, D. M. Gallik, R. A. Malkin, M. Laks, “Cardiovascular collapse caused by electrocardiographically silent 60-Hz intracardiac leakage current – Implications for electrical safety.” Circulation., 1999, 99, 2559–2564.

Susceptibility ParametersPoints of Entry - Example

Distribution of Electric PowerIntroduction

• Electric Power is needed in health-care facilities not only for medical devices but also for any other electrical equipment like lightning, air condition, telephone, television etc.

• BUTMedical devices underlie special safety regulations as they might stay in special contact to and with patients, applicants and third persons

1. Overvoltage protection2. Special ground

• Example A lightning causes an overvoltage at the public power supply. The overvoltage is transferred directly to the patients heart by applied ECG-Electrodes. => Over voltage protection

Figure 14.8 Simplified electric-power distribution for 115 V circuits. Power frequency is 60 Hz.

• Health-care facilities need an additional (green) ground path for all receptacles redundant to the metal (white) ground path

Distribution of Electric PowerEl. power-distribution from grid to receptacles

Figure 14.9 Power-isolation-transformer system with a line-isolation monitor to detect ground faults.

• Isolation transformer like this example protect systems from ground faults

• The Line-isolation monitor must be used to detect the occurrence of ground faults

Distribution of Electric PowerIsolated-power systems

Macroshock Hazards

Macroshock = current spreads through the body

• Two factors reduce danger in case of an electric shock1. High skin resistance (15kOhm to 1 MOhm at 1 cm2)2. Spatial distribution

• Many medical devices• Reduce the skin resistance with ionic gel (good electrode contact), or• Bypass the natural protection by bypassing the skin (thermometer in the mouth,

intravenous catheters, etc.)• Many fluids conduct electricity (blood, urine, intravenous solution,

etc.)

Result• Patients in medical-care facilities are much more susceptible to

macroshocks

• Ground fault with short circuit to a metal chassisa. not grounded chassis macroshockb. grounded chassis safe

Figure 14.10 Macroshock due to a ground fault from hot line to equipment cases for (a) ungrounded cases and (b) grounded chassis.

Macroshock HazardsProtection

Microshock Hazards

Microshock = all current flows through the heart

• Microshock accidents generally result from • leakage-currents in line-operated equipment• differences in voltage between grounded conductive surfaces due to large

currents in the grounding system• Microshock currents can flow either into or out of the electric

connection to the heart

Result• Patient is only in danger of microshock if there is some electric

connection to the heart

• Leakage-current flowsa. through the ground wire – no microshock occursb. through the patient if he touches the chassis and has a grounded catheter etc. c. through the patient if he is touching ground and has a connected catheter etc.

Figure 14.11 Microshock leakage-current pathways. Assume 100 μA of leakage current from the power line to the instrument chassis, (a) Intact ground, and 99.8 μA flows through the ground, (b) Broken ground, and 100 μA flows through the heart, (c) Broken ground, and 100 μA flows through the heart in the opposite direction.

Microshock HazardsProtection

Microshock HazardsConductive Path to the Heart

Specific types of electric connections to the heart can be identified• Epicardial or endocardial electrodes (i.e. temporary externalized

pacemakers)< 1 Ohm

• Electrodes for intracardiac electrogramm (ECT)< 1 Ohm

• Liquid-filled catheters placed in the heart (i.e. measure blood preassure, withdraw blood samples, inject substances, etc.)usually 50 kOhm to 1 MOhm

Internal resistance of the body is about 300 Ohm

Microshock HazardsConductive Path to the Heart

Figure 14.11Microshock Assume 100leakage-current pathways. μA of leakage current from the power line to the instrument chassis, (a) Intact ground, and 99.8 μA flows through the ground, (b) Broken ground, and 100 μA flows through the heart, (c) Broken ground, and 100 μA flows through the heart in the opposite direction.

•• Ventricular fibrillation and pump failure thresholds vs. electrode area

• Patient is connected to:• ECG monitor that grounds the

right-leg electrode to reduce 60 Hz interferce

• Blood-pressure monitor that monitors the left-ventricular blood-pressure

Figure 14.13 (a) Large ground-fault current raises the potential of one ground connection to the patient. The microshock current can then flow out through a catheter connected to a different ground, (b) Equivalent circuit. Only power-system grounds are shown.

Microshock HazardsExample of patient in the intensive-care unit

Electrical-Safety Codes and Standards

Basic Approaches to Protection against Shock

There are two fundamental methods of protecting patients against shock1. Complete isolation and insulation from all grounded objects and all

sources of electric current2. Same potential of all conducting surfaces within reach of the patients

• Neither approach can be fully achieved in most practical environments, so some combination must usually suffice

• Protection must include patient, applicants and third party persons

• Low-resistance grounding system carry currents up to circuit-breaker ratings by keeping all conductive surfaces on the same potential (refer to Figure 14.10 and 14.11)

• Patient-equipment grounding point• Reference grounding point• Connections for other patient-equipment

Figure 14.14 Grounding systemAll the receptacle grounds and conductive surfaces in the vicinity of the patient are connected to the patient-equipment grounding point. Each patient-equipment grounding point is connected to the reference grounding point that makes a single connection to the building ground.

Protection: Power DistributionGrounding System

• Ground-fault circuit interrupters disconnect the source power when a ground fault greater than about 6 mA occurs

• GFCI senses differences in the in- an outgoing current• Most GFCI use differential transformer and solid-state circuitry• Most GFCI are protectors against macroshocks as they are usually

not as sensitive as 10 µA or the medical equipment has a fault current greater than that

Protection: Power DistributionGround-Fault Circuit Interrupter (GFCI)

Protection: Power DistributionExample of a GFCI

Figure 14.15 Ground-fault circuit interrupters (a) Schematic diagram of a solid-state GFCI (three wire, two pole, 6 mA). (b) Ground-fault current versus trip time for a GFCI. [Part (a) is from C. F. Dalziel, "Electric Shock," in Advances in Biomedical Engineering, edited by J. H. U. Brown and J. F. Dickson III, 1973, 3: 223–248.]

Protection: Equipment DesignIntroduction

• Most failures of equipment ground occur at the ground contact or in the plug and cable

• Molded plugs should be avoided because of invisible breaks• Strain-relief devices are recommended• No use of three-prong-to-two-prong adapters (cheater adapters)

• Reduction of leakage current• Special use of low-leakage power cords• Capacitance-minimized design (special layout-design and usage of insulation)• Maximized impedance from patient leads to hot conductors and from patient

leads to chassis ground• Double-Insulated equipment

• Interconnection of all conducting surfaces• Separate layer of insulation to prevent contact with conductive surfaces (e.g.

non conductive chassis, switch levers, knobs, etc.)• Operation at low voltages• Electrical isolation

Protection: Equipment DesignIntroduction

• Operation at low voltages (< 10 V)• Reduction of risk of macroshock with reduced operation voltage• Risk of microshock still exists

• Electrical isolation with isolation amplifiers• Isolation amplifiers break the ohmic continuity of electric signals• Isolation amplifiers use different voltage sources and different grounds• Isolation voltage νiso is rated from 1 kV to 10 kV without breakdown and

described by the isolation-mode rejection ration (IMMR) (lightning example form the beginning)

• Next slides describe isolation amplifiers

Three fundamental design methods 1. Transformer isolation

• Frequency-modulated or pulse-width-modulated carrier signal with small signal bandwidths

• Possibility to transmit energy and/or information2. Optical isolation

• Uses LED on source-side and photodiode on output-side• Very fast signal transmission possible, but no energy

3. Capacitive isolation

Protection: Equipment DesignElectric Isolation

Figure 14.16 Electrical isolation of patient leads to biopotential amplifiers (a) General model for an isolation amplifier, (b) Transformer isolation amplifier (Courtesy of Analog Devices, Inc., AD202). (c) Simplified equivalent circuit for an optical isolator (Copyright © 1989 Burr-Brown Corporation. Reprinted in whole or in part, with the permission of Burr-Brown Corporation. Burr Brown ISO100). (d) Capacitively coupled isolation amplifier (Horowitz and Hill, Art of Electronics, Cambridge Univ. Press, Burr Brown ISO106).

Protection: Equipment DesignElectric Isolation Transformer

Protection: Equipment DesignElectric Isolation optical and capacitive methods

• The best way to minimize hazards of microshocks is to isolate or eliminate electric connections to the heart

• Fully insulated connectors for external cardiac pacemakers powered by batteries

• Blood-pressure sensors with triple insulation between the column of liquid, the sensor case, and the electric connections

• Catheters with conductive walls all the way inside the patient to distribute the shock throughout the body (enlarged surface)

Protection: Equipment DesignGood Practice for isolated Heart Connections

Testing the Electric SystemIntroduction

• Test equipment: electrical-safety analyzers• Testing the electric system

• Receptacles• Grounding system• Isolated power system

• Testing the electric appliance• Ground-pin-to-chassis resistance• Chassis leakage current• Leakage current in patient leads

Testing the Electric SystemElectrical-Safety Analyzers

• Wide product range of el.-safety analyzers is availablee.g. http://www.electricalsafetyanalyzers.com

• Medical-facility power systems• Medical appliances• Medical devices• Special use-cases• …

• Products range from simple conversion box to computerized automatic measurement systems

Testing the Electric SystemTesting Receptacles

Figure 14.17 Three-LED receptacle tester Ordinary silicon diodes prevent damaging reverse-LED currents, and resistors limit current. The LEDs are ON for line voltages from about 20 V rms to greater than 240 V rms, so these devices should not be used to measure line voltage.

Testing the Electric SystemGround-pin-to-Chassis Resistance

• Resistance between the ground pin of the plug and the equipment chassis and exposed metal objects should not exceed 0.15 Ohm during the life of the appliance

Figure 14.18 Ground-pin-to-chassis resistance test

Testing the Electric SystemChassis Leakage Current

Figure 14.19 (a) Chassis leakage-current test, (b) Current-meter circuit to be used for measuring leakage current. It has an input impedance of 1 kΩ and a frequency characteristic that is flat to 1 kHz, drops at the rate of 20 dB/decade to 100 kHz, and then remains flat to 1 MHz or higher. (Reprinted with permission from NFPA 99-2005, "Health Care Facilities," Copyright ©2005, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the complete and official position of the National Fire Protection Association, on the referenced subject, which is represented only by the standard in its entirety.)

Testing the Electric SystemLeakage Current in Patient Leads (1)

Leakage current from patient leads to ground

Figure 14.20 Test for leakage current from patient leads to ground (Reprinted with permission from NFPA 99-2005, "Health Care Facilities," Copyright ©2005, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the complete and official position of the National Fire Protection Association, on the referenced subject, which is represented only by the standard in its entirety.)

Testing the Electric SystemLeakage Current in Patient Leads (2)

Leakage current between patient leads

Figure 14.21 Test for leakage current between patient leads (Reprinted with permission from NFPA 99-2005, "Health Care Facilities," Copyright © 2005, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the complete and official position of the National Fire Protetion Association, on the referenced subject, which is represented only by the standard in its entirety.)

Testing the Electric SystemLeakage Current in Patient Leads (3)

AC isolation current

Figure 14.22 Test for ac isolation current (Reprinted with permission from NFPA 99-2005, "Health Care Facilities," Copyright © 2005, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the complete and official position of the National Fire Protection Association, on the referenced subject, which is represented only by the standard in its entirety.)

Conclusion

• Adequate electrical safety in health-care facilities and systems can be achieved at moderate costs by combining:

1. Good power-distribution system2. Well designed equipment3. Periodic maintenance and testing of power systems and equipment4. Modest training program for medical personnel

With special thanks…

…to the author of the corresponding book: John G. Webster

Further readingMedical Instrumentation Application and Design, 4th Edition, Chapter 14John G. Webster, Univ. of Wisconsin, MadisonISBN: 978-0-471-67600-3