Download introduction to power system safety for medical

INTRODUCTION TO
POWER SYSTEM SAFETY FOR
MEDICAL DEVICES
Part 2: Introduce isolated power
system.
Discuss the advantages of isolated
power system in healthcare institution
1
History
During the 1920s and ‘30s, the number of fires
and explosions in operating rooms grew at an
alarming rate. Authorities determined the major
causes of these accidents fell into two
categories:
Man-made electricity
Static electricity (75% of recorded incidents)
In 1939, experts began studying these
conditions in an attempt to produce a safety
standard. The advent of World War II delayed
the study’s results until 1944, when the National
Fire Protection
Agency (NFPA) published “Safe Practices in
Hospital Operating Rooms.”
2
History (cont’d)
The early standards were not generally adopted in new
hospital construction until 1947. It soon became apparent
these initial standards fell short of providing the necessary
guidelines for construction of rooms in which combustible
agents would be used.
NFPA appointed a committee to revise the 1944 standards.
In 1949, this committee published a new standard, NFPA
No. 56, the basis for our current standards.
The National Electrical Code (NEC) of 1959 firmly
established the need for ungrounded isolated distribution
systems in areas where combustible gases are used.
In the same year, the NEC incorporated the NFPA
standards into the code. The NFPA No. 56A–Standard for
the Use of Inhalation Anesthetics, received major revisions
in 1970, 1971, 1973, and 1978.
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History (cont’d)
In 1982, NFPA No. 56A was incorporated into a new
standard, NFPA No. 99—Health Care Facilities.
The new document includes the text of several other
documents, such as:
NFPA-3M • 56HM
56K • 56B
76A • 56C
76B • 56D
76 • 56G
The material originally covered by NFPA 56A is now
located in Chapter 3 of NFPA No. 99, which was updated
in 1984, 1987, 1990, 1993, 1996, and 1999, and in
Chapter 4, which was updated in 2002.
4
History (cont’d)
The increased use of electronic diagnostic and treatment
equipment, and the corresponding increase in electrical
hazards, has resulted in the use of isolated
ungrounded systems in new areas of the hospital since
1971.
These new hazards were first recognized in NFPA
bulletin No. 76BM, published in 1971.
Isolating systems are now commonly used for
protection against electrical shock in many areas,
among them:
Intensive care units (ICUs)
Coronary care units (CCUs)
Emergency departments
Special procedure rooms
Cardiovascular laboratories
Dialysis units
Various wet locations
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Electrical Hazards In Hospitals
The major contributors to hospital electrical accidents
are faulty equipment and wiring. Electrical accidents
fall into three categories:
Fires
Burns
Shock
Electrical shock is produced by current, not voltage.
It is not the amount of voltage a person is exposed to,
but rather the amount of current transmitted through
the person’s body that determines the intensity of a
shock.
The human body acts as a large resistor to current
flow.
The average adult exhibits a resistance between
100,000 ohms (Ω) and 1,000,000 Ω, measured hand to
hand. The resistance depends on the body mass and
moisture content.
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Electrical Hazards In Hospitals
(cont’d)
The threshold of perception for an average adult is 1
milliampere (mA). This amount of current will produce a
slight tingling feeling through the fingertips.
Between 10 and 20 mA, the person experiences muscle
contractions and finds it more difficult to release his or
her hand from an electrode.
An externally applied current of 50 mA causes pain,
possibly fainting, and exhaustion.
An increase to 100 mA will cause ventricular fibrillation.
The hazardous levels of current for many patients are
amazingly smaller. The most susceptible patient is the
one exposed to externalized conductors, diagnostic
catheters, or other electric contact to or near the heart.
7
Electrical Hazards In Hospitals
(cont’d)
Surgical techniques bypass the patient’s body resistance
and expose the patient to electrical current from
surrounding equipment.
The highest risk is to patients undergoing surgery within
the thoracic cavity.
Increased use of such equipment as heart monitors, dye
injectors, and cardiac catheters increases the threat of
electrocution when used within the circulatory system.
Other factors contributing to electrical susceptibility are
patients with hypokalemia, acidosis, elevated
catecholamine levels, hypoxemia, and the presence of
digitalis.
Adult patients with cardiac arrhythmias can be
electrocuted through the misuse of pacemakers
connected directly to the myocardium.
8
Electrical Hazards In Hospitals
(cont’d)
Infants are more susceptible to electric shock because of
their smaller mass, and thus lower body resistance.
Much has been written about current levels considered
lethal for catheterized and surgical patients.
Considerable controversy exists about the actual danger
level for a patient who has a direct electrical connection
to his or her heart.
The minimum claimed hazard level seems to be 10
microamperes (μA) with a maximum level given at 180
μA.
Whatever the correct level, between 10 and 180 μA, it is
still only a fraction of the level hazardous to medical
attendants serving the patient.
It is believed that approximately 1,000 Ω of resistance
lies between the patient’s heart and external body parts.
9
Electrical Hazards In Hospitals
(cont’d)
All of this information leads us to the conclusion
that the patient environment is a prime target for
electrical accidents.
Nowhere else can one find these elements:
lowered body resistance, more electrical
equipment, and conductors such as blood, urine,
saline, and water.
The combination of these elements presents a
challenge to increase electrical safety
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Leakage Currents
Electric equipment operating in the patient vicinity,
even though operating perfectly, may still be
hazardous to the patient.
This is because every piece of electrical equipment
produces a leakage current.
The leakage consists of any current, including
capacitively coupled current, not intended to be
applied to a patient, but which may pass from
exposed metal parts of an appliance to ground or to
other accessible parts of an appliance.
Normally, this current is shunted around the patient
via the ground conductor in the power cord. However,
as this current increases, it can become a hazard to
the patient.
11
Leakage Currents (cont’d)
Isolated systems are now commonly used to protect
against electrical shock in many areas, among them:
Intensive care units (ICUs)
Coronary care units (CCUs)
Emergency departments
Special procedure rooms
Cardiovascular laboratories
Dialysis units
Various wet locations
Without proper use of grounding, leakage currents
could reach values of 1,000 μA before the problem is
perceived.
On the other hand, a leakage current of 10 to 180 μA
can injure the patient.
Ventricular fibrillation can occur from exposure to this
leakage current.
12
Leakage Currents (cont’d)
• The following figure illustrates the origin and
path of leakage current.
13
Leakage Currents (cont’d)
Failure to use the
grounding conductor in
power cords causes a
dangerous electrical
hazard.
This commonly results
from using two-prong
plugs and receptacles,
improper use of adapters,
use of two-wire extension
cords, and the use of
damaged electrical cords
or plugs.
The following figure
illustrates these hazards.
Electrical hazards
14
Answers for electrical hazards
There are no perfect electrical systems or infallible equipment
to eliminate hospital electrical accidents.
However, careful planning on the part of the consulting
engineer, architect, contractor, and hospital personnel can
reduce electrical hazards to nearly zero.
Hospital electrical equipment receives much physical abuse;
therefore, it must be properly maintained to provide electrical
safety for patients and staff.
Procedures for electrical safety should include the following:
Check all wall power receptacles and their polarities regularly.
Routinely verify that conductive surfaces are grounded in all patient
areas.
Request that patient electrical devices such as toothbrushes and
shavers be battery powered.
Use completely sealed and insulated remote controls for use in patient
beds.
Use bedrails made of plastic or covered in insulating material.
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Codes and Standards
It would not be practical to attempt to reproduce the codes
and standards that affect the application of isolated
distribution systems in hospitals  Codes are continually
refined and updated, with frequent amendments between
major publications.
All hospitals should have copies of the current standards for
reference; the design engineer must have this information
available.
Obtain copies of all standards referenced in this bulletin from
the National Fire Protection Association, Batterymarch Park,
Quincy, MA 02269.
This subtopic briefly covers the sections of codes and
standards that apply to hospital isolated ungrounded
distribution systems. This subtopic only covers a few of the
important points within these standards. A thorough study of
applicable codes and standards is required to effectively
design a project. (hand-out will be given for this subtopic)
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Codes and Standards (cont’d)
Before a designer can
choose the proper electrical
distribution system for a
hospital, the governing
body of the hospital must
inform the designer about
the location’s use.
This requires close
coordination with the
medical staff of the facility,
to ensure the designer
understands current
medical procedures as well
as possible future
procedures.
17
Isolated power system (IPS)
• were first introduced into the hospital
environment as a means of reducing the
risk of explosions in operating rooms and
any other area where flammable
anesthetizing agents are used.
18
Advantages that IPS offer
1.
2.
3.
4.
Reduced shock hazard
Continuity of power
Something for nothing — noise reduction
Advance warning of equipment failure
It is said that Isolated Power supplies is
special protection against electrical shock
in "wet locations" or any area where the
interruption of power cannot be tolerated.
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1. Reduced shock hazard
A. THE GROUNDED SYSTEM
• Diagram 1 shows the schematic of a conventionally
grounded system. The neutral of the transformer is
bonded to ground, which, if adequately sized, will
provide an equipotential bond between the neutral and
ground conductors. As the diagram shows, normally we
would expect 0 Volts from ground to neutral and 120
Volts from the line conductor to either ground or neutral.
Diagram 1: Conventional
grounded system with a
typical 1000 Ohm (Ω) person
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Reduced shock hazard (cont’d)
• If we assume that a person has a body resistance of
1000 Ohms, and comes into contact with the live
conductor, we can expect the following result, as shown
in the equivalent circuit, Diagram 2.
Diagram 2: Schematic representation of 1000 Ohm(Ω) person
in contact with live conductor
21
Reduced shock hazard (cont’d)
• A current of 120 mA would flow from the line conductor
— via the 1000 Ohm person — and return to the neutral
via the very low impedance neutral-ground connection.
This 120 mA could prove extremely dangerous for our
1000 Ohm person.
• NOTE: Should our person have a reduced ohmic
resistance, due to excessive moisture or internal body
connections, we could expect potentially lethal current to
flow. (In this example, system capacitance has been
neglected as its impedance value is many times that of
the neutral-ground bonding.)
22
Reduced shock hazard (cont’d)
B. THE ISOLATED POWER SYSTEM (IPS)
• Diagram 3 shows the schematic representation of an
Isolated Power System. The IPS is a system in which
the transformer neutral-ground connection has been
deliberately omitted. In this example we will examine
why our 1000 Ohm person is greatly protected from
potentially lethal shock hazards. We will first consider
the situation of the pure IPS, without externally
connected equipment (externally connected equipment
only increases the system net capacitance and leakage
resistance).
Diagram 3: Isolated Power
System with our 1000
Ohm(Ω) person
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Reduced shock hazard (cont’d)
Diagram 4 assumes a typical equally distributed, balanced
capacitive system where the small leakage current, 50 microamps,
flow from L1, via C1, through the ground, and returns to L2 via C2.
NOTE: On a correctly installed system there will be a very small
leakage resistance in parallel with the system net capacitance, but
this value, being so low, may be neglected for the purpose of this
example.
We can measure the voltage drop across the system capacitance
by using a high impedance voltmeter. In a balanced system as
shown, we can expect to measure 60 Volts from each line to
ground. (Leakage current may at this time be measured by
connecting a mA or microamp meter from either L1, or L2 to
ground.)
NOTE: This is not recommended on a grounded system!
Diagram 4: Schematic
representation an Isolated
Power System
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Reduced shock hazard (cont’d)
• We may now examine our circuit parameters more
closely by using Ohm's Law and Diagram 5.
Diagram 5: Reduced schematic of an Isolated
Power System
The impedance value of C1 and C2 is 1.2 x 106 Ohms, giving a lineto-ground capacitance value of:
25
Reduced shock hazard (cont’d)
• Now we can calculate what happens should our 1000 Ohm person
come into contact with either L1 or L2. This situation may be
represented by the equivalent circuit shown in Diagram 6.
Diagram 6: 1000 Ohm person in contact with one line conductor & ground
We can now calculate the voltage that will be present across our 1000 Ohm person
First we calculate the total leakage current from L1 to L2.
1.2 10 6 1000
C1 in parallel with 1000 Ohms =
 999 Ohms
1201000
We may round this number to 1000 Ohms, showing that the impedance
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of our person has in fact completely shunted the C1 capacitance.
Reduced shock hazard (cont’d)
• We may now reduce our equivalent circuit as follows:
Diagram 7: Reduced schematic of
Isolated Power System with 1000
Ohm person in contact with line
conductor
Leakage current is now :
120 V
 100 mA
1201000
Our 1000 Ohm person coming into contact with L1 has approximately doubled
the leakage current to 100 mA, which is still an extremely low level.
Voltage across the person wou ld be :
1000 120
 0.1 V
1201000
1.2 10 6 120
Voltage across C 2 :
 119.9 V
1201000
Current passing through our 1000 Ohm person :
120 V
 100 mA
1201000
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Reduced shock hazard (cont’d)
Let's compare the grounded system vs. IPS with our grounded
1000 Ohm person in contact with the line conductor in each
system.
V. Person = Voltage across our 1000 Ohm person
I. Person = Current flowing through our 1000 Ohm person
V. Person
I. Person
Grounded
120V
20mA
Isolated Power System
0.1V (with 50 microamp leakage)
100 mA (with 50 microamp initial leakage)
(5.0V and 5mA theoretical maximum
values with a LIM reading of 5mA)
Clearly, the Isolated Power System offers considerably greater
protection to the operator and patient.
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Reduced shock hazard (cont’d)
C. THE LINE ISOLATION MONITOR
The line isolation monitor is a device which continually
monitors the impedance (resistance and capacitance)
from all lines (single and three phase) to ground and
indicates what current could flow to a patient of body
resistance 1000 Ohms, should the patient come into
contact with a line conductor (i.e. defective equipment).
A note on interpretation of the LIM reading: many
variables exist to exactly what current could flow to the
patient:
The value of 1000 Ohms may vary between less than 100 Ohms
to 20,000 Ohms depending on the condition of the patient
(moisture content, muscle condition, dry skin, etc.).
Parallel leakage return paths will also bypass a portion of the
leakage current from the patient.
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Reduced shock hazard (cont’d)
D. CONCLUSION
• We have examined how the Isolated Power System can help
protect the patient from electrical shock hazards. We have made
these calculations first order, and as simple as possible so that only
a basic knowledge of Ohm's Law is sufficient to understand the
system concepts.
• The principles are no different outside the operating room. Only the
definition of "wet location" is present to recommend that IPS is the
better solution — and then only as far as continuity of supply is
concerned.
• ICU and CCU areas where the patient may be connected to several
pieces of equipment — all which contain their respective leakages,
both resistive and capacitive, greatly add to the possibility of
hazardous leakage currents flowing. We must never neglect the fact
that a leakage on a grounded system will return via the low
impedance of the parallel paths to ground — for example, our 1000
Ohm person.
• The Isolated Power System does not have this low impedance
connection. It has a high impedance capacitive/resistive return path.
This provides an additional layer of safety to protect both operators
30
and patient alike.
2. Continuity of Supply
Probably the strongest argument for the application of
isolated power is where continuity of supply is paramount.
Article 517-20(a) of the 1993 National Electrical Code states
that 15 and 20 ampere, 125 Volt, single phase receptacles
supplying wet locations shall be provided with ground fault
circuit interrupters if interruption of power under fault
conditions can be tolerated, or an isolated power system, if
such interruption cannot be tolerated.
With isolated power systems at one fault to ground the
circuit breaker does not trip, maintaining power to the
equipment. This gives the hospital personnel a choice of
what to do since the equipment may be supporting the
patient's life. At the same time during such an occurrence,
the Line Isolation Monitor would clearly alarm as to the fault
condition so that action may be taken.
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3. Something For Nothing — Noise
Reduction
The increased use of sensitive electronic systems in the
hospital environment has created a growing need to
supply these systems with "clean" voltage, free of noise
and transients. Many types of data storage and
monitoring equipment may be extremely sensitive to line
transients and line noise which is frequently present on
voltage feeders.
The Isolated Power System contains a high quality
shielded isolation transformer which provides a
convenient and effective means of greatly reducing or
even eliminating line-to-line and line-to-ground noise on
voltage feeders.
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Something For Nothing — Noise
Reduction (cont’d)
Many manufacturers of voltage-sensitive equipment have
recognized the problem created by transients and noise on their
equipment's input line and have provided a measure of protection as
an integral part of their equipment. This protection, however, may
not be adequate for frequent or serious disturbances.
Although the primary reason for Isolated Power System design and
installation was not to achieve this noise reduction, but to provide a
low leakage secondary power system, we must consider the "builtin" advantages of this system again when comparing isolated
power with conventionally grounded systems.
It is required that all hospital biomedical equipment that may come
into contact with the patient be periodically tested for leakage
currents. As previously mentioned, many large hospitals may have
well in excess of 10,000 pieces of equipment. This creates a
nightmare situation for the hospital electrician/biomedical engineer
or whoever is in charge of equipment testing.
33
4. Advanced Warning of Equipment
Failure (cont’d)
As with all testing, the values of leakage found at the time of test
were just that. A few minutes, hours, or even a few days later, the
equipment may have been exposed to environmental conditions
which caused further decline in insulation integrity. Liquid spillage,
cable damage, equipment misuse, or just plain heat aging through
continued usage are all factors which may contribute to the decline
in the insulation value.
Any help that can be given to let the equipment operator know that
his/her device is in a correct and safe condition is a benefit to both
patient and operator alike.
Isolated Power Systems contain a Line Isolation Monitor (LIM).
This device continually monitors all potential parallel leakage paths
to ground. The LIM monitors all circuitry from the isolation
transformer via circuit breakers and power and ground modules and
finally to each piece of connected equipment.
Should a faulty piece of equipment be plugged into any of the output
sockets of the Isolated Power System, then this would immediately
cause the LIM to alarm, a warning that such an event had just
occurred.
34
4. Advanced Warning of Equipment
Failure (cont’d)
The operator can now choose to remove the equipment or
continue to use it by taking extra caution that this may cause
a serious hazard to the patient or operator should either come
into contact with the other line conductor.
If we compare the same occurrence on a grounded system,
and use as an example, a line-to-ground fault, then we are
back to our colorful situation as described in Section 2.
When initially energized, a piece of equipment may operate
correctly and be in a safe condition. However, during
operation, malfunction may occur due to spillage of liquid or
cable damage (cart running over power cable or equipment
being dropped). In any case, the result would be the same.
The level of electrical safety has now been reduced. Such
occurrences on an Isolated Power System would, as
described above, result in the LIM alarming and a warning
being issued of potential fault hazard.
35
Conclusion
Isolated Power Systems provide many advantages and
levels of protection over conventional grounded systems.
The grounded system is excellent where unskilled
operators or electrically unknowledgeable people come
into contact with everyday equipment.
It's operation is simple and any failure generally results
in equipment or circuit disconnection, very quickly within
less than 1 cycle. However, grounded power systems
are a potential killer should a grounded person come into
contact with a line conductor.
How many of us has never received some form of
electric shock?
Isolated Power Systems are the best solution for
reliable and safe power, particularly in the hospital
environment.
36