1. Chapter 14 Electrical Safety
References:
1. Walter H. Olson, “Chapter 14: Electrical Safety,” in J. G. Webster (ed.), Medical Instrumentation: Application and Design, 3rd ed.. New York, Wiley, 1998, pp. 623658.
2. David Prutchi and Michael Norris, “Design of safe medical device prototypes,” in Design and Development of Medical Electronic Instrumentation: A Practical Perspective of the Design, Construction, and Test of Medical Devices. New York: Wiley, 2005, pp. 97146.

2. The environment
Patients are exposed to more hazards in medical environments than the typical home or workplace

3. Statistic 10,000 device-related patient injuries per year in the US
Reason:
Improper use of devices 
Inadequate training & lack of experience
Increased complexity of medical devices
Utilization of medical devices in more procedures
User manuals are seldom read

4. Fail-safe design: Because all devices eventually fail.
Name one device that will never go wrong.
Fail-safe design: Because all devices eventually fail.
Safe design + safe use
Concept: Everything that can go wrong will eventually go wrong.

5. chemicals, drugs, microorganisms, Vermin, waste, sound,
Hazards
hazard 危險; 危害物; 危險之源
Sources of hazards:
fire, air, earth, water,
chemicals, drugs, microorganisms,
Vermin, waste, sound,
electricity, natural and unnatural disasters, surroundings,
gravity, mechanical stress, people responsible for acts of omission and commission,
radiation from x rays, ultrasound, magnets,
ultraviolet light, microwaves, and lasers.
vermin 害蟲(指鼠、蝨等); 寄生蟲
disaster災難

6. Action
* Electrically isolated patient connections of the medical equipment
* Education on safety for medical personnel
* Medical-equipment testing procedures

7. This chapter
Contents:
 physiological effects of electric current
 shock hazard
 methods of protection
 electrical-safety standards
 electrical-safety testing procedures.
objectives:
 to understand the possible hazards;
 to incorporate safety features
into the medical instrument design

8. 14.1 Physiological effects of electricity
Electrical current through biological tissue:
(1) Stimulation of excitable tissue
(2) Resistive heating
(3) Electrochemical burns and tissue damage (DC)

9. Various levels of physiological effects
Perception:
Exciting nerve endings in the skin
A tingling sensation
AC 60 Hz: threshold = 0.5 mA (moistened hands);
DC: threshold = 2-10 mA, slight warming of the skin;
Let-go:
Vigorous stimulation of nerves and muscles
Pain and fatigue
Involuntary muscular contractions
Reflex withdrawals
Let go current ≡the max current for voluntary withdrawal
The minimal threshold for the let go current = 6 mA

10. Various levels of physiological effects (cont.)
Respiratory paralysis, pain, and fatigue:
Involuntary contraction of respiratory muscles
 (1) Asphyxiation 窒息
Respiratory arrest: mA
(2) Pain and fatigue

11. Various levels of physiological effects (cont.)
VF:
The normal propagation of action potential is disrupted
Major cause of death due to electric shock
1000 deaths per year in the USA
Threshold for VF = mA
Doesn’t stop until defibrillator is used
(Defibrillation: A brief high-current pulse depolarizes all the myocardial cells simultaneously.)
Sustained myocardial contraction:
A normal rhythm ensues after removing the current
Threshold = 1-6 A
Burns:
Usually on the skin at the entry points
Physical injury:
Can puncture the skin if V > 240 V
Brain and nervous tissue will lose excitability when high currents pass through them
Muscular contraction can be strong enough to pull the muscle attachment away from the bone.

12. Let go current:
You = 100 mA
Your friend = 10 mA
If the current = 20 mA
Can you escape?
Can your friend escape?
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.

13. 14.2 Important Susceptibility Parameters

14. (1) Current magnitude
Figure 14.2 Distrubutions 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 IIII, 1973, 3, )

15. (2) Frequency
Figure 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 IIII, 1973, 3, )

16. (3) Duration
Figure 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, Copyright 1973 by the Institute of Electrical and Electronics Engineers. Reproduced with permission.)

17. Electromuscular Incapacitation Device (EMD)
Electrical devices include any weapons that use the effects of electricity to incapacitate (使無能力) the target. There are a variety of different devices but their principle of operation is the same. They are battery powered and use a low current, high voltage impulse shock for incapacitation. Other terms for these devices include
Conducted Energy Device (CED)
Electro-Muscular Disruption (EMD) Device
Human Electro-Muscular Incapacitation (HEMI) Device.
Source:

18. Strength-duration curve

19. Strength-duration equation
Rheobase 基本電位,(引起刺激之最低電位).
Chronaxie 時值,(引起肌肉收縮最少所需之電流時間).
rhe(o)- word element [Gr.], electric current; flow (as of fluids).
incapacitate 使無能力

20. Stimulation threshold:
= 3.5 uC/cm2 of charge transfer density (large amplitude, < 100 us, directly to the heart)
Fibrillation stimulation threshold : single-beat stimulation threshold
= 20:1 ~ 30:1 (electrode on the heart)
= 10:1 ~ 15:1 (electrodes on the chest)

21. (4) Points of entry Current threshold for VF:
LA-RA > LL-RA or LL-LA
Two points on the same hands > Two points on two hands

22. (4) Points of entry (cont.)
Figure 14.5 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, )

23. Current threshold for microshock (of dogs): 20 μA (total current)
Current threshold for microshock (of human): μA (total current)
Safety limit to prevent microshock: 10 μA

24. (5) Body weight
Threshold: increases with body weight

25. Threshold: increases with body weight

27. 14.3 Distribution of Electric Power
Electric power in health-care facilities for:
Medical instruments, lighting, maintenance appliances, patient conveniences (TV, hair curler, and electric toothbrushes), clocks, nurse call buttons, etc.
240 V for heavy-duty devices:
Air conditioner, electric dryers, x-ray machines
This section:
Safe distribution of power
in health-care facilities

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

29. NEC (National Electric Code) 1996, 2006
* All receptacles be grounded by a separate insulated (green) copper conductor (Article )
* The maximal potentials permitted between any two exposed conductive surfaces in the vicinity of the patient (Article )
<1> General-care areas – 500 mV
<2> Critical-care areas – 40 mV
receptacle (電源)插座

30. Unsafe Critical-care areas < 40 mV (Article 517-15)
Patients are intentionally exposed to electric devices
Single patient-grounding point
Periodical testing of ground continuity
Receptacles: > 6 single or 3 duplex
Unsafe

31. General-care areas
< 500 mV
Receptacles: > 4 single > 4 duplex
Each rceptacle must be grounded
Patients are incidentally exposed to electric devices

32. Ground faults
Ground fault
A short circuit between the hot conductor and ground that injects large currents into the grounding system

33. The circuit breaker will open due to ground fault

34. Isolated power systems
In isolated power systems,
large currents into the grounding system
* will occur when there are double ground faults
* will not occur when there is a single ground fault
Isolated power systems are used in applications where
loss of power supply cannot be tolerated.
NEC requirement: isolated-power systems in operating rooms and locations where flammable anesthetics are used or stored

35. Isolated power systems
C = 0.3 F, Zc = 8.8 Hz
C = 0.01 F, Zc = 26.5 Hz
Line-isolation monitor (dynamic ground detector): to detect the occurrence of the first fault from either conductor to ground
“Isolation” transformer： means to isolate the two conductors from ground

37. Isolated power systems
Figure Power-isolation-transformer system with a line-isolation monitor to detect ground faults.

39. Emergency power systems
Article 517, NEC (1990, 2006)
The emergency electric system for health-care facilities：
Requirement  automatically restoring power within 10 s after interruption

40. Skin resistance   Susceptibility to VF  ? ?
14.4 Macroshock Hazards
Skin resistance   Susceptibility to VF  ? ?
Skin resistance:
Dry: k-1 M/cm2
Wet or broken:  1%
Internal resistance:

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

42. 14.4 Macroshock Hazards (cont)
Possible causes:
* Failures of insulation
* Shorted components
* Mechanical failures
* Strain and abuse of power cords, plugs, and receptacles
* Spilling of fluids (blood, urine, intravenous solutions, baby formulas)

43. 14.5 Microshock Hazards
When there are direct electric connections to the patient’s heart
Causes:
Leakage current in line-operated equipment
Potential difference between grounded conductive surfaces

44. Direct electric connections to the patient’s heart
Microshock-susceptible situations:
Epicardial/endocardial electrodes of externalized temporary cardiac pacemakers
Electrodes for introcardiac electrogram measuring /stimulation devices
Liquid-filled catheters placed in the heart to measure blood pressure, withdraw blood samples, inject substances such as dye or drugs into the heart, etc. (Internal R of catheter = 50 k to 1 M, much higher than the two cases above)

45. It only requires a leakage current It doesn’t require a ground fault
to induce a microshock.
(b) With a grounded electric connection to the heart, the patient may receive a microshock while touching the chassis whose ground wire is broken.
(c) There is a connection from the chassis to the patient’s heart. There is also a connection to ground anywhere on the body. The broken ground of the chassis could cause a microshock.
Figure 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.

46. Examples of possible microshock incidents
Microchock via temporary transvenous pacemaker
A figure from the second edition

47. Electrode Surface Area
Electrode area 
 VF probability ? ?
Current density is the key factor that causes VF.
(Total current) Threshold of VF
Figure Thresholds of ventricular fibrillation and pump failure versus catheter area in dogs.

48. Conductive Surfaces
It only requires a small potential difference between two conductive surface
It doesn’t require a leakage current
To induce a microshock.
Electrode area   current density 
 VF more probable
Electrode area   electrode resistance  current magnitude 
 VF less probable

49. Example 14.2 Conclusion: Macroshock & microshock cause VF
Macroshock vs Microshock
Macroshock:
Minimum current I for VF = 75 mA
Cross-sectional area A of the heart = 10 am  10 cm
Current density J = 75 mA/100 cm2 = 7.5 A/mm2
Microshock:
A = 90 mm2
I = 1000 μA
J = 1000 μA/90 mm2 = 11.1 μA/mm2
Conclusion: Macroshock & microshock cause VF
by the same mechanism
Figure Thresholds of ventricular fibrillation and pump failure versus catheter area in dogs.

50. Ground Potential Differences
Solution:
(1) A single patient grounding point
for all devices used in the vicinity of each patient
(2) Electrical isolation for all patient leads
Figure (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.

51. 14.6 Electrical-safety Codes and Standards
A code =法規, 行為準則,規範;
a document that contains only mandatory requirements
uses “shall”
is cast in a form suitable for adoption into law by an authority that has jurisdiction
Explanations in a code must appear only in fine-print notes, footnotes, and appendices.
A standard:
contains only mandatory requirements
Compliance tends to be voluntary.
More detailed notes and explanations are given.
A manual or guide:
doesn’t contain mandatory requirements
Informative and tutorial

52. 14.6 Electrical-safety Codes and Standards
An arduous history of the development, adoption, and use of standards and codes for electrical safety in health-care facilities:
1. Explosions and fires resulting from electric ignition of flammable anesthetics
2. Microshock scare, 1970s, led to impractical proposals
3. Many years of debating over implicit requirements for isolated-power systems and very low-leakage current
4. NFPA and ANSI/AAMI ES standards were adopted.
arduous 艱辛的

53. 14.6 Electrical-safety Codes and Standards
NFPA = National Fire Protection Association
NASI = American National Standards Institute
AAMI = The Association for the Advancement of Medical Instrumentation
IEC = International Electrotechnical Commission)
The NFPA
ANSI/AAMI ES1-1985
The NFPA 99Standard for Health Care Facilities2005
(Evolved from 12 NFPA documents that were combined in 1984 and revised every 3 years)
Electric equipment, gas, vacuum, environmental systems and materials
The requirements for patient-care-related electric appliances
The performance, maintenance, and testing of electrical equipment
The performance, maintenance, and testing with regard to safety
The safe use of high-frequency (100 kHz to microwave frequencies) electricity

54. 14.6 Electrical-safety Codes and Standards
The National Electrical Code  2006, Article 517Health Care Facilities
A. General; B. Wiring design and protection; C. Essential electrical system; D. Inhalation anesthetizing locations; E. X-ray installations; F. Communications, signaling systems, data systems, fire-protective signaling systems, and systems less than 120 Volts, nominal; G. Isolated power systems
ANSI/AAMI ES1  1993 Safe Current Limits for Electromedical Apparatus
Chassis and patient lead leakage currents
NFPA = National Fire Protection Association
NASI = American National Standards Institute
AAMI = The Association for the Advancement of Medical Instrumentation
IEC = International Electrotechnical Commission)

55. Table 14.1 Limits on Leakage Current for Electric Appliances
IEC (2006) standard
Allows a “patient auxiliary current” up to 100 uA at not less than 0.1 Hz to permit amplifier bias currents and impedance plethysmorgraphy if the current is not intended to produce a physiological effect.
Table Limits on Leakage Current for Electric Appliances
Electric Appliance
Chasis Leakage, uA
Patient-lead Leakage, uA
not intended to contact patients
100 NA
not intended to contact patients and single fault
500
with nonisolated patient leads 10
with nonisolated patient leads and single fault
300
with isolated patient lead
with isolated lead and single fault 50
plethysmorgraphy 體積變化描記

56. 14.6 Electrical-safety Codes and Standards
Code vs standard
Code
Standard
Definition
Mandatory
yes
Compliance
Explanation

57. 14.7 Basic Approaches to Protection Against Shock
Method 1: Isolate and insulate patients from grounded objects and electrical sources
Method 2: Maintain at the same potential all conductive surfaces within the patient’s reach
Neither can be fully achieved in most practical environments, so some combination of them must usually suffice.

58. 14.7 Basic Approaches to Protection Against Shock
Whom to protect?
Patients with accessible electrical connections to the heart.
(哀莫大於心死)
2. Patients with
reduced skin resistance (e.g., coupled to electrodes)
invasive connections (e.g., intravenous catheters)
exposure to wet conditions (e.g., dialysis, i.e., 洗腎)
3. Patients
4. Visitors and staff
Cost-benefit ratio:
safety vs purchase cost + maintenance cost

59. 14.8 Protection: Power Distribution
Grounding system:
Essential requirement for protecting patients from both macroshock and microshock:
Low-resistance grounds that can carry current up to circuit-breaker ratings
(1) Macroshock: (2) Microshock:

60. 14.8 Protection: Power Distribution (cont.)
Grounding system: (cont.)
A good grounding system protects patients by :
(1) Keeping all conductive surfaces and receptacle grounds in the patient’s environment at the same potential
(2) Protect the patients from ground faults at other locations (e.g., Fig )
Grounding system, hierarchy of (see Fig )
Patient-equipment grounding point
Reference grounding point
Building service ground

61. Figure Grounding system All 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.

62. Figure Grounding system All 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.

63. Figure Grounding system All 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.

64. 14.8 Protection: Power Distribution (cont.)
Grounding system: (cont.)
Grounding system:
(1) connection  0.15 
(2) diff. voltage (between receptacle ground & conductive surface)  40 mV
(3) The patient-equipment grounding point is connected individually to all receptacle grounds, metal beds, metal door and window frames, water pipes, and any conductive surfaces.
(4) Each patient-equipment grounding point
individually connected to a reference grounding point
connected to the building service ground

65. 14.8 Protection: Power Distribution (cont.)
Isolated power-distribution system:
* High cost
* Only necessary in locations where flammable anesthetics are used

66. 14.8 Protection: Power Distribution (cont.)
Ground fault circuit interrupters (GFCI):
Function:
If |Ihot conductor  Ineutral conductor| 6 mA
 Disconnect the electrical power source
(to prevent macroshock)
= 0 ideally
* Not sensitive enough to interrupt microshock level of leakage current.
* Primarily for macroshock protection.
Note:
Microshock doesn’t need ground fault; leakage current can cause microshock.
Microshock level of leakage current < macroshock level of leakage current

67. 14.8 Protection: Power Distribution (cont.)
Ground fault circuit interrupters (GFCI): (cont.)
NEC (1996):
There shall be GFCIs in circuits serving
bathrooms, garages, outdoor receptacles, swimming pools, construction sites
NFPA 99:
There shall be GFCIs in wet location, especially hydrotherapy areas, (where continuity of power is not essential).
In patient-care areas, circuits should not include GFCIs,
because: the loss of power to life-support equipment (due to GFCIs) may be more hazardous to the patient than most small ground faults would be.
GFCI ($10) vs isolated power-distribution system ($2000)
This is attractive if brief power interruptions can be tolerated.

68. Figure 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," Advances in Biomedical Engineering, edited by J. H. U. Brown and J. F. Dickson IIII, 1973, 3, )

70. N G H
Electric device

71. GFCI
When power is interrupted by a GFCI, the manual reset button on the GFCI must be pushed to restore power.
Most GFCIs have a momentary pushbutton that create a safe ground fault to test the interrupter.
Example 14.3
Most GFCIs have a momentary pushbutton that create a safe ground fault to test the interrupter. On Figure 14.15, design the modification to permit this test.
Answer

72. 14.9 Protection: Equipment Design
Reliable grounding for equipment
* Power cords: Hard Service (SO, ST, STO), Junior Hard Service (SJO, SJT, SJTO)
Plugs: Avoid molded (一體成形) plugs (because of 40% to 85% invisible breaks within 1 to 10 years)
Strain-relief devices
Cord-storage compartment – to reduce cord damage
Be careful of three-prong-to-two-prong adaptor
Reduction of leakage current (between chassis and patient leads)
*Low-leakage power cords  < 1.0 μA/m are available
Capacitor between the hot conductors and the chassis  can be reduced thr layout and insulating
Impedance from patient leads to hot conductors or chassis  must be maximized
Double-insulated equipment
Insulating material for the secondary insulation
Safe even when spilled
insulation

73. 14.9 Protection: Equipment Design
Operation at low voltage (Use battery, < 10 V)
* e.g., inhalation-anesthetizing locations
Electrical isolation
. Different voltage sources and different grounds on each side
. Isolation amplifiers: ohmic isolation > 10 M, isolation-mode voltage > 1000 V, CMRR > 100 dB
Transformer/optical/capacitive isolation

74. CM
CMRR
SIG
ISO
Error
Isolation
barrier
Capacitance
and resistance - +
Input common
(a)
*IMRR in v/v
Output
common o =
IMRR
Gain RF
IMRR* ~
Isolation barrier
Input
control
Output -V +V
+o - + o = i RK RG
CR3
CR1
CR2 i2 i i1 AI
AII i3
RK = 1M W
(c) ~ 1 2 - +
(b)
+ 15 V DC o
Power
return
25 kHz
- 7.5 V
+ISO
Out
SIG
ISO
+ 7.5 V
In com In
In + FB
5 V
F.S.
Oscillator
Signal
Mod
Rect and
filter
Demod
± 5 V
AD202 Hi Lo
Isolation barrier
Frequency-to-
voltage converter
(phase-locked loop)
Osc Q
± 15 V (Receiver)
15 V (Driver)
Isolation
barrier
(d)
3 pF
Freq
control
Analog
signal out, o
signal in, i
Figure 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 (c) 1989 Burr-Brown Corporation. Burr Brown ISO100) (d) Capacitively coupled isolation amplifier (Horowitz and Hill, Art of Electronics, Cambridge Univ. Press. Burr Brown ISO106).

75. 14.9 Protection: Equipment Design
Isolated heart connections
Cardiac pacemakers powered by battery
Blood-pressure sensors with triple insulation
Catheters with conductive walls
Quiz: Which is safer? A catheter with insulated wall or one with conductive wall? Why?
Ans:

76. 14.9 Protection: Equipment Design
Isolated heart connections
Cardiac pacemakers powered by battery
Blood-pressure sensors with triple insulation
Catheters with conductive walls
Quiz: Which is safer? A catheter with insulated wall or one with conductive wall? Why?
Ans:

77. 14.10 Electrical-Safety Analyzer
Line Voltage
Cord Resistance
Case Leakage Current
Earth Leakage Current
Leads Leakage Current
Instrument Current
Insulation resistance
Earth resistance
Point-to-Point Measurement

78. 14.11 Testing the Electric System
Electrical safety relies on the integrity of the power connection.

79. 3-LED Receptacle Tester
Hot
Ground
Neutral
Open
The four possible states:
Hot (120 V)
GND
Open
Neutral
Possible practical cases: 43 = 64 cases
3-LED receptacle tester: 23 = 8 cases

80. 120 V
0 V X ☼ ◌
120 V
0 V ☼ ◌ X
0 V
120 V ☼
Figure 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.

81. Description (in textbook)
LEDs
Prongs 1 2 3
Hot
(Black)
Ground (Green)
Neutral
(White)
1. Hot open (or all hot) ○ O G N H
2. Neutral open ☼
3. No possible wiring ―
4. Ground open
5. Hot/ground reversed
6. Correct (or ground/neutral reversed)
7. Hot/neutral reversed
8. Hot open and neutral/hot
Note:
The “correct wiring” is when Black = Hot, Green = Ground, and White = Neutral.

82. Tests of Grounding System in Patient-Care Areas
NFPA 99:
Between ground and receptacle:
< 0.1 ohm, new construction
< 0.2 ohm, existing construction
NFPA 99:
< 20 mV, new construction
< 40 mV, critical area
, existing construction
< 500 mV, general area

83. Tests of Isolated Power Systems
Alarm
Yes No
3.7 mA mA
Total hazardous current
(leakage current + LIM current)

84. 14.12 Tests of Electric Appliances
Leakage current is the current that could flow from the point where a person makes contact with an appliance, through that person's body, and back to ground (or some other point). [Jim Richards]
The leakage tests are commonly performed as the final production tests on medical appliances.
During a leakage test, the appliance is powered up under operating conditions.
The leakage test is not a common production test for most non-medical electrical appliances.
Various leakage tests are different in how or where the human body comes into contact with a medical appliance.
The measuring device (such as a current meter in Figure 14.19) in a leakage test simulates the impedance of the human body.
Tests include (1) normal power application (hot to 120 V and neutral to 0 V), (2) reverse power application (hot and neutral reversed), (3) normal power with single fault, and (4) reverse power with single fault .
Tests on single faults are essential because it is a problem that could occur. Two or more faults are unlike to happen, so tests on them are not considered necessary.
Source: Jim Richards, Medical Electronics Ensuring Compliance with Product Safety Tests, Reference Guide (Compliance Engineering Magazine ) ,

85. Ground-Pin-to-Chassis Resistance
Figure Ground-pin-to-chassis resistance test

86. Chassis Leakage Current
Appliance power switch
(use both OFF and ON positions)
Open switch
for appliances
not intended to
contact a patient
Grounding-contact
switch (use in
OPEN position)
Polarity- reversing
switch (use both
positions)
Appliance
H (black) H
To exposed conductive
surface or if none, then 10 by
20 cm metal foil in contact
with the exposed surface
120 V N
N (white) G
G (green)
Insulating surface I
Building
ground
Current meter
H = hot
N = neutral (grounded)
G = grounding conductor
Test circuit
This connection is at service entrance or on supply side of separately derived system.
simulates the human body
I < 500 μA for facility owned housekeeping and maintenance appliances
I < 300 μA for appliances intended for use in the patient vicinity
(a)
Input of
test load
Leakage current
being measured
1400 W
100
Millivoltmeter 15 F
900
(b) mV
Figure (a) Chassis leakage-current test. (b) Current –meter circuit to be used for measuring leakage current. It has an input impedance of 1 k 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 , "Health Care Facilities," Copyright © 1996, National Fire Protection Association, Quincy, MA 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.)

87. Ammeter Design
An ammeter is placed in series with a circuit element to measure the electric current flow through it. The meter must be designed offer very little resistance to the current so that it does not appreciably change the circuit it is measuring. To accomplish this, a small resistor is placed in parallel with the galvanometer to shunt most of the current around the galvanometer. Its value is chosen so that when the design current flows through the meter it will deflect to its full-scale reading. A galvanometer full-scale current is very small: on the order of milliamperes.
galvanometer 檢流計(測驗微小電流、電壓、電量),
Source:

88. The input impedance of the current meter in Figure 14.18
% Matlab code for ploting the input impedance of current meter
R1 = 900; R2 = 100; R3 = 1400; R4 = 15; C1 = 0.10e-6;
f = [1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8]
Z_volmeter = 1./(j*2*pi.*f*C1) + R4
Z_total = R1 + 1./(1/R2 +1./(R3 + Z_volmeter))
Z_total_mag = abs(Z_total)
Z_volmeter_mag = abs(Z_volmeter);
subplot(2,1,1);
semilogx(f,Z_total_mag);
title('Input impedance of current meter');
xlabel = ('Frequency, Hz'); ylabel = ('Magnitude, ohm');
subplot(2,1,2);
semilogx(f,Z_volmeter_mag);
title('Impedance across volmeter');

89. Lakage current in Patient Leads
Figure Test for leakage current from patient leads to ground (Reprinted with permission from NFPA , "Health Care Facilities," Copyright © 1996, National Fire Protection Association, Quincy, MA 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.)

90. Test for leakage current between patient leads
Figure Test for leakage current between patient leads (Reprinted with permission from NFPA , "Health Care Facilities," Copyright © 1996, National Fire Protection Association, Quincy, MA 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.)akage current between patient leads

91. Test for ac isolation current
Figure Test for ac isolation current (Reprinted with permission from NFPA , "Health Care Facilities," Copyright © 1996, National Fire Protection Association, Quincy, MA 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.)

92. To achieve adequate electrical safety in health-care facilities：
CONCLUSION
To achieve adequate electrical safety in health-care facilities：
. A good power distribution system
. Careful selection of well designed equipment
. Periodical testing of power systems and equipment
. Modest training program for medical personnel

93. Have you learned anything?
Ch 12 medical imaging
MRI: magnetic resonance image
ECG
Is there anything I should add to this course?
Applications, current flow through the body