Electrical Safety - 8 (1)

8/9/2019 Electrical Safety - 8 (1)

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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


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Taught Matter of Lectures

Introduction

Basic Theory of Measurements

Beginnings of Basic Sensors

Sensors [MEMS]

Signals and Noise

Amplifiers of SignalsConnection and Protection of Signals

Data Acquisition and Data Converters

Electric Safety in Medical Systems


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Electrical Safety

Agenda

• 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


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Introduction

ES 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


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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

• 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


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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.


Current can heal and harm

6

5

43

21


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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


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Let-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


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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.


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Ventricular 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


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Sustained 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


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Burns 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


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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


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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.)


Variability of threshold and Let-go current


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• 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 currentamong 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.)


Frequency


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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)


Duration


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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, Principlesof Applied Biomedical Instrumentation, 3rd ed. New York: John Wiley & Sons, 1989).


Duration


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• 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.)


Body weight


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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 anintracardiac catheter flows

through the heart. (From F.

J. Weibell, "Electrical

Safety in the Hospital,"

Annals of BiomedicalEngineering, 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


Points of Entry


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• 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


Points of Entry - Example


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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.


Points of Entry - Example


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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 persons1. 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


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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


El. power-distribution from grid to receptacles


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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 ofground faults


Isolated-power systems


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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

• 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 tomacroshocks


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• Ground fault with short circuit to a metal chassis

a. not grounded chassis macroshock

b. 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 Hazards

Protection


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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


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• 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 leakagecurrent 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 μAflows through the heart in the opposite

direction.

Microshock Hazards

Protection


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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 bloodpreassure, withdraw blood samples, inject substances, etc.)

usually 50 kOhm to 1 MOhm

Internal resistance of the body is about 300 Ohm


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Microshock Hazards

Conductive Path to the Heart

Figure 14.11Microshock Assume 100

leakage-current pathways. μA of leakage

current from the power line to the

instrument chassis, (a) Intact ground, and99.8 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


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• Patient is connected to:

• ECG monitor that grounds the

right-leg electrode to reduce60 Hz interferce

• Blood-pressure monitor that

monitors the left-ventricular

blood-pressure

Figure 14.13 (a) Large ground-faultcurrent 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 Hazards

Example of patient in the intensive-care unit


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Electrical-Safety Codes and Standards


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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


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• 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 inthe 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 Distribution

Grounding System


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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


Ground-Fault Circuit Interrupter (GFCI)


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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 versustrip 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.]


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Protection: Equipment Design

Introduction

• 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


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Introduction

• 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 thebeginning)

• Next slides describe isolation amplifiers


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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

• Uses LED on source-side and photodiode on output-side

• Very fast signal transmission possible, but no energy

3. Capacitive isolation


Electric Isolation


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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 circuitfor an optical isolator (Copyright

© 1989 Burr-Brown

Corporation. Reprinted in whole

or in part, with the permission of

Burr-Brown Corporation. BurrBrown ISO100). (d) Capacitively

coupled isolation amplifier

(Horowitz and Hill, Art of

Electronics, Cambridge Univ.

Press, Burr Brown ISO106).


Electric Isolation Transformer


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Protection: Equipment DesignElectric Isolation optical and capacitive methods


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• 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 bybatteries

• 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 theshock throughout the body (enlarged surface)


Good Practice for isolated Heart Connections


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Testing the Electric System

Introduction

• 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


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Electrical-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


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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 linevoltage.


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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


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Chassis Leakage Current

Figure 14.19 (a) Chassis leakage-

current test, (b) Current-meter circuitto be used for measuring leakage

current. It has an input impedance of 1

k Ω and a frequency characteristic that

is flat to 1 kHz, drops at the rate of 20

dB/decade to 100 kHz, and thenremains 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 thestandard in its entirety.)


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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.)


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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.)


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AC isolation current

Figure 14.22 Test for ac isolationcurrent (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.)


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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 equipment

3. Periodic maintenance and testing of power systems and equipment

4. Modest training program for medical personnel


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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

Documents

Electrical Safety - 8 (1)