2. TOPICS TO BE COVERED
Safety in the clinical environment: Electrical safety
Physiological effects of electricity
Susceptibility parameters
Distribution of electrical power
Isolated power systems
Macroshock hazards
Microshock hazards
Electrical safety codes and standards
Protection
Power distribution
Ground fault circuit interrupters (GFCI)
Equipment design
Electrical safety analyzers / Testing electrical systems
3. Safety in Clinical Environment
Electrical hazards
Electrical shocks (micro and macro) due to equipment failure, failure of
power delivery systems, ground failures, burns, fire, etc.
Mechanical hazards
mobility aids, transfer devices, prosthetic devices, mechanical assist
devices, patient support devices
Environmental hazards
Solid wastes, noise, utilities (natural gas), building structures, etc.
Biological hazards
Infection control, viral outbreak, isolation, decontamination, sterilization,
waste disposal issues
Radiation hazards
Use of radioactive materials, radiation devices (X-ray, CT), exposure
control
4. Electrical Safety
Many sources of energy, potentially hazardous substances,
instruments and procedures
Use of fire, compressed air, water, chemicals, drugs, microorganisms,
waste, sound, electricity, radiation, natural and unnatural disaster,
negligence, sources of radiation, etc.
Medical procedures expose patients to increased risks of hazards due
to skin and membranes being penetrated / altered
10,000 device related injuries in the US every year! Typically due to
Improper use
Inadequate training
Lack of experience
Improper (lack of) use of manuals
Device failure
5. Physiological Effects of Electricity
For electricity to have an effect on the human body:
An electrical potential difference must be present
The individual must be part of the electrical circuit, that is, a current must
enter the body at one point and leave it at some other point.
However, what causes the physiological effect is NOT voltage, but
rather CURRENT.
A high voltage (K.103V) applied over a large impedance (rough skin) may
not cause much (any) damage
A low voltage applied over very small impedances (heart tissue) may cause
grave consequences (ventricular fibrillation)
The magnitude of the current is equal to the applied voltage divided
by the total effective impedance the current faces; skin : largest.
Electricity can have one of three effects:
Electrical stimulation of excitable tissue (muscles, nerve)
Resistive heating of tissue
Electrical burns / tissue damage for direct current and high voltages
6. Physiological Effects of Electricity
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.
Dry skin impedance:93 kΩ / cm2
Electrode gel on skin: 10.8 kΩ / cm2
Penetrated skin: 200 Ω / cm2
The real physiological effect depends on the actual path of the current
7. Threshold of perception: The minimal current that an individual can detect. For AC (with
wet hands) can be as small as 0.5 mA at 60 Hz. For DC, 2 ~10 mA
Let-go current: The maximal current at which the subject can voluntarily withdraw.
6~100mA, at which involuntary muscle contractions, reflex withdrawals, secondary physical
effects (falling, hitting head) may also occur. Minimal threshold is 6mA.
Respiratory Paralysis / Pain / Fatigue At high currents, involuntary contractions of
respiratory muscles can cause asphyxiation / respiratory arrest (18-22 ma), if the current is not
interrupted. Strong involuntary contraction of other muscles can cause pain and fatigue
Ventricular fibrillation 75 ~ 400 mA can cause heart muscles to contract uncontrollably,
altering the normal propagation of the electrical activity of the heart. HR can raise up to 300
bpm, rapid, disorganized and too high to pump any meaningful amount of blood ventricular
fibrillation. Normal rhythm can only return using a defibrillator
Sustained myocardial contraction / Burns and physical injury At 1 ~6 A, the
entire heart muscle contracts and heart stops beating. This will not cause irreversible tissue
damage, however, as normal rhythm will return once the current is removed. At or after 10A,
however, burns can occur, particularly at points of entry and exit because skin resistance is high.
Physiological Effects of Electricity
8. Important Susceptibility Parameters
Threshold and let-go current variability
Distributions of perception thresholds and let-go currents These data depend on surface area of
contact, moistened hand grasping AWG No. 8 copper wire, 70 kg human, 60Hz, 1~3 s. of exposure
Threshold of perception
for
Women-0.7mA mean
9. Important Susceptibility Parameters
Frequency
Note that the minimal let-
go current happens at the
precise frequency of
commercial power-line,
50-60Hz.
Let-go current rises below
10 Hz and above several
hundred Hz.
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.
10. Important Susceptibility Parameters
Duration
The longer the duration, the smaller
the current at which ventricular
fibrillation occurs
Shock must occur long enough to
coincide with the most vulnerable
period occurring during the T wave.
Weight
Fibrillation threshold increases with
body weight (from 50mA for 6kg
dogs to 130 mA for 24 kg dogs)
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.
11. Important Susceptibility Parameters
Points of entry
The magnitude of the current required to fibrillate the heart is far greater if the
current is not applied directly to heart; externally applied current loses much of
its amplitude due to current distributions. Large, externally applied currents cause
macroshock.
If catheters are used, the natural protection provided by the skin (15 kΩ ~ 2
MΩ) is bypassed, greatly reducing the amount of current req’d to cause
fibrillation. Even smallest currents (80 ~ 600 μA), causing microshock, may
result in fibrillation. Safety limit for microshocks is 10 μA.
The precise point of entry, even externally is very important: If both points of
entry and exit are on the same extremity, the risk of fibrillation is greatly reduced
even at high currents (e.g. for dog the current req’d for fibrillation through Lead
I (LA-RA) electrodes is higher than for Leads II (LL-RA) and III (LL-LA).
12. Important Susceptibility Parameters
Points of entry
Effect of entry points on current distribution (a) Macroshock, externally applied current spreads through-
out the body. (b) Microshock, all the current applied through an intracardiac catheter flows through the heart.
14. Distribution of Power
If electrical devices were perfect, only two wires
would be adequate (hot and return), with all power
confined to these two wires. However, there are
two major departures from this ideal case:
• A fault may occur, through miswiring,
component failure, etc., causing an electrical
potential between an exposed surface (metal
casing of the device) and a grounded surface
(wet floor, metal case of another device etc.)
Any person who bridges these two surfaces is
subject to macroshock.
• Even if a fault does not occur, imperfect insulation or electromagnetic
coupling (capacitive or inductive) may produce an electrical potential
relative to the ground. A susceptible patient providing a path for this
leakage current to flow to the ground is subject to microshock.
15. GROUND!
The additional line that is connected directly to the earth-ground provides the
following:
In case of a fault (short circuit between hot conductor and metal casing), a
large current will use the path through the ground wire (instead of the patient)
and not only protect the patient, but also cause the circuit breaker to open.
The ability of the grounding system to conduct high currents to ground is
crucial for this to work!
If there is no fault, the ground wire serves to conduct the leakage current
safely back to the electrical power source – again, as long as the grounding
system provides a low-resistance pathway to the ground
Leakage current recommended by ECRI are established to prevent injury in
case the grounding system fails and a patient touches an electrically active
surface (10 ~ 100 μA).
16. Isolated Power Distribution
Not grounded !
• Normally, when there is a ground-fault from hot wire to ground, a large current is drawn
causing a potential hazard, as the device will stop functioning when the circuit breakers
open !
• This can be prevented by using the isolated system, which separates ground from neutral,
making neutral and hot electrically identical. A single ground-fault will not cause large
currents, as long as both hot conductors are initially isolated from ground!
17. Isolated Power Distribution
Not grounded !
• In fact, in such an isolated system, if a single ground-fault occurs, the system simply
reverts back to the normal ground-referenced system.
•A line isolation monitor is used with such system that continuously monitors for the
first ground fault, during which case it simply informs the operators to fix the problem.
•The single ground fault does NOT constitute a hazard!
18. Macroshock
Most electrical devices have a metal cabinet, which constitutes a hazard, in
case of an insulation failure or shortened component between the hot
power lead and the chassis. There is then 115 ~ 230 V between the chassis
and any other grounded object.
The first line of defense available to patients is their skin.
The outer layer provides 15 kΩ to 1 MΩ depending on the part of the body,
moisture and sweat present, 1% of that of dry skin if skin is wet or broken.
Internal resistance of the body is 200Ω for each limb, and 100Ω for the trunk,
thus internal body resistance between any two limbs is about 500Ω (somewhat
higher for obese people due to high resistivity of the adipose tissue)!
Any medical procedure that reduces or eliminates the skin resistance increases
the risk of electrical shock, including biopotential electrode gel, electronic
thermometers placed in ears, mouth, rectum, intravenous catheters, etc.
A third wire, grounded to earth, can greatly reduce the effect of
macroshock, as the resistance of that path would be much smaller than
even that of internal body resistance!
19. Macroshock Hazards
• Direct faults between the hot conductor and the ground is not common,
and technically speaking, ground connection is not necessary during
normal operation.
•In fact, a ground fault will not be detected during normal operation of
the device, only when someone touches it, the hazard becomes known.
•Therefore, the continuity of ground wire in devices and receptacles must
be tested periodically.
21. Microshock Hazards
Small currents inevitably flow between adjacent insulated
conductors at different potentials leakage currents
which flow through stray capacitances, insulation, dust and
moisture
Leakage current flowing to the chassis flows safely to
the ground, if a low-resistance ground wire is available.
22. Microshock Hazards
• If ground wire is broken, the chassis
potential rises above the ground; a
patient who has a grounded
connection to the heart (e.g. through a
catheter) receives a microshock if
he/she touches the chassis.
• If there is a connection from the
chassis to the patient’s heart, and a
connection to the ground anywhere in
the body, this also causes microshock.
• Note that the hazard for
microshock only exists if there is a
direct connection to the heart.
•Otherwise, even the internal
resistance of the body is high enough
top prevent the microshocks.
23. Microshock via
Ground Potentials
•Microshocks can also occur if
different devices are not at the
exact same ground potential.
•In fact, the microshock can
occur even when a device that
does not connected to the
patient has a ground fault!
•A fairly common ground wire
resistance of 0.1Ω can easily
cause a 500mV potential
difference if initiated due to, 5A
of ground fault.
•If the patient resistance is less
than 50kΩ, this would cause an
above safe current of 10μA
24. Safety Codes & Standards
Limits on leakage current are instituted and regulated by the safety
codes instituted in part by the National Fire Protection Association
(NFPA), American National Standards Institute (ANSI), Association
for the Advancement of Medical Instrumentation (AAMI), and
Emergency Care Research Institute (ECRI).
25. Basic Approaches to
Shock Protection
There are two major ways to protect patients from
shocks:
Completely isolate and insulate patient from all grounded
object and all sources of electric current
Keep all conductive surfaces within reach of the patient at the
same voltage
Neither of these can be fully achieved some
combination of these two
Grounding system
Isolated power-distribution system
Ground-fault circuit interrupters (GFCI)
26. Grounding Systems
Low resistance (0.15 Ω) ground that can
carry currents up to the circuit-breaker
ratings protects patients by keeping all
conductive surfaces and receptacle
grounds at the same potential.
Protects patients from
• Macroshocks
• Microshocks
• Ground faults elsewhere
The difference between the receptacle
grounds and other surface should not
exceed 40 mV
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.
27. Isolated Power Systems
A good equipotential grounding system cannot eliminate large
current that may result from major ground-faults (which are
rather rare).
Isolated power systems can protect against such major (single)
ground faults
Provide considerable protection against macroshocks,
particularly around wet conditions
However, they are expensive
Used only at locations where flammable anesthetics are used.
Additional minor protection against microshocks does not
justify the high cost of these systems to be used everywhere in
the clinical environment
28. Ground – Fault
Circuit Interrupters (GFCI)
Disconnects source of electric current when a ground fault greater than about 6 mA occurs!
•When there is no fault, Ihot=Ineutral. The GFCI detects the difference between these two
currents. If the difference is above a threshold, that means the rest of the current must
be flowing through elsewhere, either the chassis or the patient !!!.
•The detection is done through the monitoring the voltage induced by the two coils (hot
and neutral) in the differential transformer!
29. GFCI
•The National Electric Code (NEC - 1996) requires that all
circuits serving bathrooms, garages, outdoor receptacles,
swimming pools and construction sites be fitted with
GFCI.
•GFCI protect against major ground faults only, not
against microshocks.
•Patient care areas are typically not fitted with GFCI, since
the loss of power to life support equipment can also be
equally deadly!
30. Protection through
Equipment Design
Strain-relief devices for cords is recommended, where cord enters the
equipment and connection between the cord and plug
Reduction of leakage current through proper layout and insulation to minimize
the capacitance between all hot conductors and the chassis
Double insulation is used to prevent the contact of the patient with the chassis
or any other conducting surface (outer case being insulating material, plastic
knobs, etc.)
Operation at low voltages; solid state devices operating at <10V are far less
likely to cause macroshocks
Electrical isolation in circuit design
31. Electrical Isolation
CM
CM
CMRR
SIG ISO
ISO
Error
Isolation
barrier
Isolation
Capacitance
and resistance
-
+
-
+
Input common
(a)
*IMRR in v/v
Output
common o =
CM
CMRR
ISO
IMRR
SIG± Gain
±
RF
ISO
IMRR*
~
~
~
~
~
Error
• Main features of an isolation amplifier:
• High ohmic isolation between input and output (>10MΩ)
• High isolation mode voltage (>1000V)
• High common mode rejection ration (>100 dB)
32. Transformer Isolation Amplifiers
-
+
(b)
+ 15 V DC
o
Power
return
25 kHz
- 7.5 V
+ISO
Out
SIG
-ISO
Out
+ 7.5 V
In com
In-
In +
FB
±
5 V
F.S.
Oscillator
25 kHz
Signal
Mod
Rect. &
filter
Power
Demod
± 5 V
F.S.
AD202
Hi
Lo
33. Optical Isolation Amplifier
Isolation barrier
Input
control
Output
control
-V
+V
+o
-
+
o = i
RK
RG
CR3 CR1 CR2
i2
i
i
i
i1
RG
AI AII
i3
i2
o
RK = 1M W
-
+
-
+
+
-
(c)
~
1 2
35. Electrical Safety Analyzers
Wiring / Receptacle Testing
Three LED receptacle tester:
Simple device used to test common wiring problems (can detect only 8 of possible 64
states)
Will not detect ground/neutral reversal, or when ground/neutral are hot and hot is
grounded (GFCI would detect the latter)
36. Electrical Safety Analyzers
Testing Electrical Appliances
Ground-pin-to-chassis resistance: Should be <0.15Ω during
the life of the appliance
Ground-pin-to-chassis resistance test
37. Electrical Safety Analyzers
Testing Electrical Appliances
Chassis leakage current: The leakage current should not exceed 500μA with single fault for
devices not intended for patient contact, and not exceed 300 μA for those that are intended for
patient contact.
To exposed conductive
surface or if none, then 10 by
20 cm metal foil in contact
with the exposed surface
Insulating surface
Current meter
I
Test circuit
Open switch
for appliances
not intended to
contact a patient
Grounding-contact
switch (use in
OPEN position)
Polarity- reversing
switch (use both
positions)
Appliance power switch
(use both OFF and ON positions)
This connection
is at service
entrance or on
supply side of
separately derived
system
Building
ground
H (black)
N (white)
Appliance
G
N
H
H = hot
N = neutral (grounded)
G = grounding conductor
I < 500 μA for facility owned housekeeping and maintenance appliances
I > 300 μA for appliances intended for use in the patient vicinity
120 V
G (green)
Internal
Circuitry
38. Electrical Safety Analyzers
Testing Electrical Appliances
Leakage current in patient leads:
Potentially most damaging leakage is the one with patient
leads, since they typically have low impedance patient
contacts
Current should be restricted to 50μA for non-isolated leads
and to 10 μA for isolated leads (used with catheters /
electrodes that make connection to the heart)
Leakage current between any pair of leads, or between a
single lead and other patient connections should also be
controlled
Leakage in case of line voltage appearing on the patient
should also be restricted.
41. Leakage Current Testers
Test for ac isolation current
Isolation current is
the current that
passes through
patient leads to
ground if and when
line voltage appears
on the patient.
This should also be
limited to 50μA
