Electrical Safety, shock, earthing, grounding, types of the circuits and shocks, protection devices, ICU protection

1. Compiled By: Prof. G. B. Rathod
Email: ghansyam.rathod@bvmengineering.ac.in
Electrical Hazards & Patient safety in Bio-
medical equipment

2. Significance of Electrical Danger
 In a clinical environment patient is exposed to various risks,
more than a typical workplace or at home
 Frequent invasive (blood) operations - penetration through skin
or mucous membranes
 Presence of potentially hazardous chemicals and substances -
anaesthetics, medicines, medical gases
 Sources of infection - particularly “hospital infection”
 Various sources of energy that penetrate into or through the
patient: current, voltage, ionizing and non-ionizing radiation,
sound and ultrasound, electric and magnetic field, UV radiation,
lasers, microwave radiation, mechanical stress, etc.
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3. Physiological Effect of Electrical Current
• For a physiological effect the body must become part of an electrical circuit
• Three phenomena occur when electrical current flows
 Electrical stimulation of excitable tissue (muscle, nerve)
 Resistive heating of tissue
 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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4. Physiological Effect of Electrical Current
Fig.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.
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5. Physiological Effect of Electrical Current
• Current density is just large enough to excite nerve
endings in the skin
• Subject feels tingling sensation
• Lowes values with moisture hands (decreases contact
resistance)
 0.5 mA at 60 Hz
 2 mA to 10 mA DC
 The subject might feel a slight warming
• 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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6. Physiological Effect of Electrical Current
 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.
 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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7. Physiological Effect of Electrical Current
• 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
• 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 break the bone
• Lets see how electrical shock can harm
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8. Microshock vs. macroshock
•Macroshock is caused by current passing through the body through the skin, and the current that can
cause harmful effects is relatively high and significantly depends on the point of contact
• Microshock is caused by the passage of relatively low current, but the source of electricity was
brought directly into contact with the heart, for example, during cardiac catheterization (catheter in
the heart is a diagnostic procedure). Electricity sufficient to induce fibrillation is of n x 10μA. Another
point of contact can be anywhere on the body (eg limbs). Current of 10 μA is considered safe limit to
prevent microshock
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9. 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.
 Medical devices underlie special safety regulations as they might stay in special
contact to and with patients, applicants and third persons
1. Overvoltage protection
2. Special ground
 For 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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10. Electric power-distribution from grid to receptacles
 Health-care facilities need an additional (green) ground path for all receptacles
redundant to the metal (white) ground path
Simplified electric-power
distribution for 115 V circuits.
Power frequency is 60
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11. Macroshock Hazards Protection
Figure: Macroshock due to a ground fault from hot line to equipment chasses for (a)
ungrounded chasses and (b) grounded chassis.
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12. 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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13. Microshock Hazards Protection
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,
• Leakage-current flows through the ground wire – no microshock occurs
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14. Microshock Hazards Protection
(b) Broken
ground, and
100 μA flows
through the
heart,
• Through the patient if he touches the chassis and has a grounded catheter etc.
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15. Microshock Hazards Protection
(c) Broken ground, and
100 μA flows through
the heart in the opposite
direction.
• Through the patient if he is touching ground and has a connected catheter etc
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16. Microshock Hazards
Example of patient in the intensive-care unit
• Patient is connected to:
• ECG monitor that grounds the
right-leg electrode to reduce 60 Hz
interferce
• Blood-pressure monitor that
monitors the left-ventricular
blood-pressure
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,
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17. Microshock Hazards
Example of patient in the intensive-care unit
Figure : (b) Equivalent circuit. Only
power-system grounds are shown.
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18. Electrical-Safety Codes and Standards
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19. 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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20. Protection: Power Distribution Grounding System
• Low-resistance grounding system carry currents up to
circuit-breaker ratings by keeping all conductive surfaces
on the same potential
 Patient-equipment grounding point
 Reference grounding point
 Connections for other patient-equipment
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21. Protection: Power Distribution Grounding System
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.
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22. Protection: Power Distribution Ground-Fault Circuit
Interrupter (GFCI)
• 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.
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23. Protection: Power Distribution Ground-Fault Circuit
Interrupter (GFCI)
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," in Advances in Biomedical Engineering, edited by J. H. U.
Brown and J. F. Dickson III, 1973, 3: 223–248.]
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24. 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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25. Protection: Equipment Design 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 (IMRR)
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26. Protection: Equipment Design Electric Isolation
 Three fundamental design methods
1. Transformer isolation
 Frequency-modulated or pulse-width-modulated carrier signal with small signal
bandwidths
 Possibility to transmit energy and/or information
2. Optical isolation
 Uses LED on source-side and photodiode on output-side
 Very fast signal transmission possible, but no energy
3. Capacitive isolation
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27. Reference:
 Book: John G.Webster,“Medical Instrumentation Application
and Design”,WSEWiley India Private Limited, 3rd Edition,
2012
 Online Source: Link: Click here
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28. Thank You
Scan the QR Code for more information
Prof.G.B.Rathod, EC Dept.BVM-EC453
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