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Electrical Safety
Medical Instrumentation Application and Design, 4th
Edition, Chapter 14
John G. Webster, Univ. of Wisconsin, Madison
ISBN: 978-0-471-67600-3
Taught Matter of Lectures
Introduction
Basic Theory of Measurements
Beginnings of Basic Sensors
Sensors [MEMS]
Signals and Noise
Amplifiers of Signals
Connection and Protection of Signals
Data Acquisition and Data Converters
Electric Safety in Medical Systems
Electrical Safety
Agenda
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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
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Power Distribution
•
Equipment Design
Testing the Electric System
Tests of Electric Appliances
Conclusion
Introduction
ES as elemetary protection
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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 effects
Current can heal and harm
•
•
For a physiological effect the body must become part of an electrical
circuit
Three phenomena occur when el. current flows
1. El. stimulation of excitable tissue (muscle, nerve)
2. Resistive heating of tissue
3. Electrochemical burns
•
Further consideration are based on the following parameters
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Human body with contact to el. circuit at left and right hand
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Body weight: 70 kg
•
Applied current time: 1 s to 3 s
•
Current frequency: 60 Hz
Physiological effects
Current can heal and harm
6
5
4
3
2
1
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 effects
Threshold 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 effects
Let-go Current (2)
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•
•
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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 effects
Respiratory 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 effects
Ventricular Fibrillation (4)
•
•
•
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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 cordconnected appliances
Physiological effects
Sustained Myocardial Contraction (5)
•
•
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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 effects
Burns and Physical Injury (6)
•
•
•
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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 Parameters
Introduction
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
Susceptibility Parameters
Variability of threshold and Let-go current
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 Parameters
Frequency
•
Let-go current versus frequency
Minimal 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 frequency
Percentile 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 Parameters
Duration
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 Parameters
Duration
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 Parameters
Body weight
•
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 Parameters
Points of Entry
•
•
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
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.)
Susceptibility Parameters
Points of Entry - Example
•
60 Hz for 5 s to a ventricular pacing catheter during implantable
cardioverter-defibrillator implant testing in 40 patients
•
Result
Intermittent capture with a minimum current of 20 µA
Continuous capture with a minimum current of 49 µA
•
Resulting Regulation
The widely accepted safety limit to prevent microshocks is 10 µA
Susceptibility Parameters
Points 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.
Distribution of Electric Power
Introduction
•
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.
•
BUT
Medical devices underlie special safety regulations as they might stay
in special contact to and with patients, applicants and third persons
1. Overvoltage protection
2. 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
Distribution of Electric Power
El. power-distribution from grid to receptacles
•
Health-care facilities need an additional (green) ground path for all
receptacles redundant to the metal (white) ground path
Figure 14.8 Simplified electric-power distribution for 115 V circuits. Power frequency is 60
Hz.
Distribution of Electric Power
Isolated-power systems
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Isolation transformer like this example protect systems from ground
faults
The Line-isolation monitor must be used to detect the occurrence of
ground faults
Figure 14.9 Power-isolation-transformer system with a line-isolation monitor to detect
ground faults.
Macroshock Hazards
Macroshock = current spreads through the body
•
Two factors reduce danger in case of an electric shock
1. High skin resistance (15kOhm to 1 MOhm at 1 cm2)
2. Spatial distribution
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Many medical devices
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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
Macroshock Hazards
Protection
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Ground fault with short circuit to a metal chassis
a. not grounded chassis Æ macroshock
b. grounded chassis Æ safe
Figure 14.10 Macroshock due to a
ground fault from hot line to
equipment cases for (a) ungrounded
cases and (b) grounded chassis.
Microshock Hazards
Microshock = all current flows through the heart
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Microshock accidents generally result from
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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
Microshock Hazards
Protection
•
Leakage-current flows
a. through the ground wire – no microshock occurs
b. 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 Hazards
Conductive 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 Hazards
Conductive Path to the Heart
••
Ventricular fibrillation and pump failure thresholds vs. electrode area
Figure 14.11Microshock Assume 100
leakage-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.
Microshock Hazards
Example of patient in the intensive-care unit
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Patient is connected to:
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ECG monitor that grounds the
right-leg electrode to reduce
60 Hz interferce
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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.
Electrical-Safety Codes and Standards
Basic Approaches to Protection against Shock
There are two fundamental methods of protecting patients against shock
1. Complete isolation and insulation from all grounded objects and all
sources of electric current
2. 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
Protection: Power Distribution
Grounding System
•
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 system
All the receptacle grounds and conductive surfaces in
the vicinity of the patient are connected to the
patient-equipment grounding point. Each patientequipment grounding point is connected to the
reference grounding point that makes a single
connection to the building ground.
Protection: Power Distribution
Ground-Fault Circuit Interrupter (GFCI)
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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 Distribution
Example 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 Design
Introduction
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•
•
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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
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Strain-relief devices are recommended
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No use of three-prong-to-two-prong adapters (cheater adapters)
Reduction of leakage current
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Special use of low-leakage power cords
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Capacitance-minimized design (special layout-design and usage of insulation)
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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 Design
Introduction
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•
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
Protection: Equipment Design
Electric Isolation
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 information
2. Optical isolation
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Uses LED on source-side and photodiode on output-side
•
Very fast signal transmission possible, but no energy
3. Capacitive isolation
Protection: Equipment Design
Electric Isolation Æ Transformer
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 Design
Electric Isolation Æoptical and capacitive methods
Protection: Equipment Design
Good Practice for isolated Heart Connections
•
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)
Testing the Electric System
Introduction
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•
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Test equipment: electrical-safety analyzers
Testing the electric system
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Receptacles
•
Grounding system
•
Isolated power system
Testing the electric appliance
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Ground-pin-to-chassis resistance
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Chassis leakage current
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Leakage current in patient leads
Testing the Electric System
Electrical-Safety Analyzers
•
Wide product range of el.-safety analyzers is available
e.g. http://www.electricalsafetyanalyzers.com
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Medical-facility power systems
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Medical appliances
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Medical devices
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Special use-cases
•
…
Products range from simple conversion box to computerized
automatic measurement systems
Testing the Electric System
Testing 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 System
Ground-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 System
Chassis Leakage Current
Figure 14.19 (a) Chassis leakagecurrent 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 System
Leakage 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 System
Leakage 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 992005, "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 System
Leakage 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 system
2. Well designed equipment
3. Periodic maintenance and testing of power systems and equipment
4. Modest training program for medical personnel
With special thanks…
…to the author of the corresponding book: John G. Webster
Further reading
Medical Instrumentation Application and Design, 4th Edition, Chapter 14
John G. Webster, Univ. of Wisconsin, Madison
ISBN: 978-0-471-67600-3