Download Electrical Safety

Biomedical Instrumentation I
Electrical Safety
This Week
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Safety in the clinical environment: Electrical safety
Physiological effects of electricity
Susceptibility parameters
Distribution of electrical power
 Isolated power systems
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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
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 (MRI, CT, PET), exposure
control
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
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 another.
 However, what causes the physiological effect is NOT voltage, but
rather CURRENT.
 A high voltage (KV,(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 simply 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
Physiological Effects of
Electricity
The real physiological effect depends on the actual path of the current
Dry skin impedance:93 kΩ / cm2
Electrode gel on skin: 10.8 kΩ / cm2
Penetrated skin: 200 Ω / cm2
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 of
Electricity
 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 ~ 100
mA, at which involuntary muscle contractions, reflex withdrawals, secondary physical effects
(falling, hitting head) may also occur
 Respiratory Paralysis / Pain / Fatigue At as low as 20 mA, involuntary contractions of
respiratory muscles can cause asphyxiation / respiratory arrest, 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.
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
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. Letgo currents for women are about twothirds the values for men.
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.
Important Susceptibility
Parameters
 Points of entry
 The magnitude of the current required to fibrillate the heart is far greater if the
current is 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. the current req’d for fibrillation through Lead I (LA-RA)
electrodes is higher than for Leads II (LL-RA) and III (LL-LA).
Important Susceptibility
Parameters
 Points of entry
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.
Distribution of
Electrical Power
(230 V)
Simplified electric-power distribution for 115 V circuits. Power frequency is 60 Hz
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.
• The additional ground line provides a good line of defense! (how / why?)
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).
Isolated Power Distribution
Not grounded !
• In
fact, in such
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stop functioning when the circuit breakers open !
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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 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 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 then even that of
internal body resistance!
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, ground wire in devices and receptacles must be periodically
tested.
Macroshock Hazards
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.
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 s/he 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.
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 a 500mV
potential difference if
initiated due to a, say
5A of ground fault.
If the patient resistance is less
then 50kΩ, this would cause an
above safe current of 10μA
Microshock Via
Ground Potentials
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).
Basic Approaches to
Shock Protection
 There are two major ways to protect patients from shocks:
 Completely isolate and insulate patient from all sources of electric current
 Keep all conductive surfaces within reach of the patient at the same
voltage
 Neither can be fully achieved  some combination of these two
 Grounding system
 Isolated power-distribution system
 Ground-fault circuit interrupters (GFCI)
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 be no more then 40 mV)
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.
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
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!
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.
Note that 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!
Protection through
Equipment Design
 Strain-relief devices for cords, where cord enters the equipment and
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 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
Electrical Isolation
Isolation
barrier
CM
Error
CMRR
~
SIG
ISO
IMRR*
RF
Error
~
-
~
ISO
+
+
CM
Isolation
Capacitance
and resistance
~
~
Input common
(a)
ISO
Output
common
o
=
SIG ±
*IMRR in v/v
• 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)
CM
CMRR
±
ISO
IMRR
Gain
Transformer Isolation
Amplifiers
FB
AD202
In In +
SIG
In com
+ISO
Out
-ISO
Out
(b)
Signal
+
Demod
Hi
Mod
±
±5V
F.S.
5V
F.S.
Lo
o
Power
+ 7.5 V
- 7.5 V
Rect and
filter
Oscillator
25 kHz
25 kHz
+ 15 V DC
Power
return
Optical Isolation Amplifier
Isolation barrier
CR3
i
CR1
i
-
AI
+
i
+V
i2
i3
AII
+
+
o
-V
Input
control
(c)
2
+o
i1
~
RK = 1M W
i2
1
RG
+
CR2
o
= i
RK
Output
RG
control
-
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)
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
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
Appliance power switch
patient contact.
(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
N
120 V
N (white)
G (green)
Building
ground
This connection
is at service
entrance or on
supply side of
separately derived
system
G
I
Current meter
Internal
Circuitry
H (black)
To exposed conductive
surface or if none, then 10 by
20 cm metal foil in contact
with the exposed surface
Insulating surface
H = hot
N = neutral (grounded)
G = grounding conductor
Test circuit
I < 500 μA for facility Ðowned housekeeping and maintenance appliances
I > 300 μA for appliances intended for use in the patient vicinity
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.
Leakage current Testers
Test for leakage current from
patient leads to ground
Leakage Current testers
Test for leakage current
between patient leads
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
And this concludes…