Download Importance of Isolated System

Importance of Requirement of Isolated System
in Hospitals
Isolated systems were first introduced in Hospitals World wide as to prevent Fire and Explosions in operating areas
where inflammable gases were used for Anesthesia.
It is general feeling that as of today inflammable gases are not used in hospitals, why still isolation system is
recommended or mandatory? Presently so many electrical and electronic gadgets are used for benefit of patient.
Hence shock hazards to patient is increased. Today medical and nursing are progressively dependent on electrical
apparatus for saving life of a hospitalized patients. Presently more cardiac operations are performed, In some of
which patient’s life is dependent on artificial stimulate and regulate heart action. At the same time equipment,
Surgeon, Nurse may be standing on prepping solution, blood, urine and other conductive fluids that reduces the
passage of unintended electrical current. Isolated system reduces the ignition hazard from arcs and sparks between
line conductor and grounded metal and mitigates hazard os shock or burn from electric current flowing through
body to ground. Reverence to these facts may be found in the Standards for Health Care Facilities NFPA 99 and
USA National electrical code NFPA 70.
It is the purpose of this writeup to explain the other advantages that isolated power system offer such as:
1. Reduced shock hazard
2. Continuity of power
3. Noise reduction
4. Advance waning of equipment failure
Please note the copy of NFPA NEWS Brief referring to electrical shocks in operating rooms which have installed
grounded power.
In conclusion Isolated Power Systems supplies special protection against electrical shock in “wet Locations”or any
area where the interruption of power cannot be tolerated.
1. Reduced shock hazard
A- Grounded system:
Transformer
Phase
=
L2
0V
Neutra
1000 ohm
230 V
L1
Conventional grounded system
with typical 1000 ohm person
Ground
Fig. 1: Grounded System
Fig. 1 above shows schematic of conventional grounded system. The neutral of the
transformer is bonded to ground, which, if adequately sized will provide equipotential bond
between the neutral and ground conductors. As the fig. Shows, normally we would expect
0 volt from ground to neutral and 230 volts fronm the conductor to either neutral or ground.
If we assume that the person has a body resistance of 1000 W and comes in contact with
line conductor the following result is expected as shown in the equivalent circuit Fig. 2.
SWITCHTR
TR NICS
Medicare Company
Page 03
1000 ohm Person
230 =230 mA
V = -----I=—
R 1000
Fig. 1: Schematic representation of 1000 W person in contact with line conductor
The current of 230 mA would flow from the line conductor - via the 1000 W person - and return to neutral
via the vary low impedance neutral ground connection. This 230 mA could prove fatal to our 1000 W person.
NOTE: should our person have a reduced ohmic resistance, due to excessive moisture or internal body
connection, we could expect potentially lethal current to flow. (In this example system capacitance has been
neglected as the impedance value is is very high compared to neutral ground bonding)
B. The Isolated System (IPS)
Transformer
Phase
=
L2
Neutra
C1
C2
1000 ohm
L1
Isolated Power system
with typical 1000 ohm person
Ground
Fig. 3: Isolated Power Ungrounded System with Typical 1000 ohm person
Fig. 3 shows the schematic representation of Isolated Power System (IPS). The IPS is a
system in which transformer neutral - ground connection is deliberately omitted. In this we
will see how our 1000 ohm person is protected against potentially lethal shock hazards.
Consider only Isolation system without any external connected equipments, which will only
increases system’s net capacitance and leakage current
C1
C2
Schematic Representation of
Isolated Power System (IPS)
Fig. 4: Repesents Schematic representation of Isolated Power System (IPS)
Fig. 4 assumes that a typical equally distributed balanced capacitive system where small
leakage current, say 50 µA allowable leakage current for medicinal transformer, flow from
L1 via C1,through ground and returns ti L2 via C2.
Page 02
SWITCHTR
TR NICS
Medicare Company7
NOTE: On the proper system there will be negligible leakage current in parallel in net capacitance due to
resistance which is not considered in this example.
Considering balanced capacitive load, C1 = C2. We can measure the voltage drop across across system
capacitance by using high impedance voltmeter. Being balance system voltage drop will be equal and 115 V from
each line to ground for 230 V system.( Leakage current can be measured by connecting
mA or µA meter from either L1 or L2 to ground.)
NOTE: This is not recommended on grounded system.
By using Ohm’s Law we will now examine circuit parameters more closely of Fig. 4 above.
Z - Impedance
V - Voltage across capacitor
115
V = ----------Z=—
= 2.3 x 10
-6 Ohms
50 x 10
I
i - current flowing (say 50 µA)
6
HENCE impedance value of C! Or C2 is 2.3 Mega ohm (MW)
Value of C1 or C2 can be worked out as:
-6
1
1
C = ------------------ =-------------------------------- = 0.001386 x10 Farads = 0.00138 mF
2 x p x F x Xc 2 x 3.142 x50 x 2.3 x10
C - capacitance
Xc - reactance = Z
F - frequency 50 HZ
Now let us concider the 1000 ohm person comes in contact with line L2. Let us calculate the voltage
C1
L2
L1
C2
1000 ohm
Person
L1
C1
L2
C2
R
100 ohm
Person
Fig. 5: 1000 ohm person in contact with one of the line
now let us calculate the voltage present across the 1000 ohm person.
6
2300000000
2.3 x 10 x1000 =----------------Total impedance as C2 in parallel with 1000 ohm =------------------------= 999.957 say 1000 W
6
2301000
(2.3 x 10 ) + 1000
The conclusion is 1000 W person shunts C2 completely
230
V = ---------------------Total leakage current from L1 to L2 is current passing through 1000 W person = ---= 100 mA
1000 + 2300000
R
Our 1000 W person coming in contact with L2 has approximately double the leakage current to 100 mA which is
still extremely low level.
1000 x230 = 0.1 V Voltage across C1 will be 229.9 V.
Voltage across the person would be ----------2301000
In Isolation Power System Line Isolation Monitor has 2 adjustable hazard levels. They are 2mA and 5 mA
SWITCHTR
TR NICS
Medicare Company
Page 03
Now let us compare Isolated Power System with Grounded Power System and our 1000 ohm person in contact
with line conductor in both cases.
V Person is the voltage across 1000 ohm person
I Person is the current passing through 1000 ohm person
Grounded
V Person
230 V
I Person
230 mA
Isolated Power system
0.1 V with 50 mA leakage current
100mA with 50 mA initial leakage current
5 mA and 5 V theoretical maximum values with LIM reading of 5 mA
2 mA and 2 V theoretical maximum values with LIM reading of 2 mA
Hence Isolated Power system considerably offers greater protection to operator and patient.
C. Line Isolator Monitor (LIM)
Line Isolator Monitor is a devise which continuously monitors impedance (capacitance and resistance) from all
lines (single phase and three phase) to ground and indicates what current would flow through a patient of body
resistance 1000 ohms, should the patient come into contact with live conductor (i. e. defective equipment)
A Note on interpretation of LIM reading many variables exists to exactly what current could flow through the
patient.
1. The value of 1000 ohms may vary between less than 100 ohms to 20000 ohms depending on the condition
of the patient (moisture content, muscle condition, dry skin etc.)
2. Parallel leakage return paths will also bypass a portion of leakage current from the patient.
D. Conclusion:
We have seen how Isolated Power System (IPS) helps to protects the patient from electrical shock hazards. We
have made these calculations of first order, and as simple as possible so that only basic knowledge of Ohm’s
Law is sufficient to understand the importance of Isolation System concept.
ICU and ICCU areas, where patient may be connected to several pieces of equipment, all which contain their
respective leakages, both resistive and capacitive, greatly add to possibility of hazardous leakage currents
flowing. We must never neglect the fact that leakage on a grounded system return via a low impedance of parallel
path to a ground - for example our 1000 ohm patient. Also patient operated in operation Theater is protected for
leakages foe a few hours remains unprotected for several days in ICU or ICCU.
The Isolated Power system (IPS) does not have low impedance connection. It has high impedance capacitive /
resistance return path. This provides additional layer of safety to protect operator and patient.
The principals are not different outside the operating room. Only the difference of “wet location”is present to
recommend that IPS is better solution and then only when continuity of supply is is concerned, as per US
standards. But in India NEC 1985 recommends department wise of hospitals. It is recommended for 14Anaesthetic Room, 15- Operating Theater, 16- Operating Preparation room, 17- operating Plaster room, 18Operating Recovery Room, 19- Out patient Operating Theater, 20- Heart Catheterization Room, 21- Intensive
Care Unit (ICU), Intensive Cardiac Care Unit (ICCU) 22- Intensive Examination Room, 23- Intensive monitoring
Room, 24- Angiographic Exmination Room, 25- Hemodialysis Room, 27- Central Monitoring Room. In addition
to this list IPS is also recommended as additional safety in NEC 1985 for 4- Delivery Room 7- Examination or
Treatment Room, 8- Labor Room, 13- Physiotherapy Room
Page 04
SWITCHTR
TR NICS
Medicare Company7
2- Continuity of Supply:
Probably strongest argument for the application of Isolated Power System (IPS) where continuity of power
supply is essential. In USA article 517-20(a)of the 1993 National Electrical Code states that single phase
receptacles suppling wet locations shall be provided with ground fault circuit interrupters (ELCB), If interruption
of power can be tolerated or Isolation Power System (IPS) if If interruption of power can not be tolerated.
With Isolated Power System ELCB does not trip maintaining power to the equipment. This gives the hospital
personnel choice of what to do since the equipment may be supporting patient’s life. At the same time during
such time Line Isolation Monitor would clearly alarm as to the fault condition so that action can be taken.
3- Noise Reduction:
The increased use of sanative electronic system in the hospital environment has created growing need to
supply these systems with “clean” voltage, free of noise and transients. Many types of data storage and
monitoring equipments may be extremely sanative to the transient and line noise which is frequently present
on voltage feeders.
The Isolated Power System (IPS) contains high quality shielded medicinal 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.
Many manufacturers of voltage sensitive equipment have recognized the problem created by transient and
noise on their equipments input line and have provided a measure of protection. The problem, however may
not be adequate to frequent or serious disturbances.
Although the primary reason for Isolated Power System (IPS) design and installation was not to archive noise
reduction but to provide a low leakage secondary power system, we must consider the “built in” advantages
of of the system again when comparing Isolated power with conventional grounded power system.
4- Advance warning of Equipment failure:
It is required that all hospital biomedical equipment that may come in contact with the patient be periodically
tested for leakage currents. Many large hospitals may have well in excess of 10,000 pieces of equipments.
This creates a nightmare situation for the hospital electrician or biomedical engineer or whoever is in charge
of equipment testing.
As with all testing the values of leakage found at the time of test were just that. A few minutes, hours or even
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 of insulation value.
Any help that can be given to let the equipment operator know that his/her device is in correct and safe
condition, is benefit to both operator and patient alike.
Isolation Power System (IPS) contain a Line Isolation Monitor (LIM). This devise continuously 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 each piece of connected equipment.
Should if faulty piece of equipment be plugged into any of the sockets of Isolated Power System (IPS), then
this would immediately cause LIM to alarm, a warning that such an event had just occurred.
The operator can now choose to remove the equipment or continue to use it by taking extra caution that this
this may cause a serious hazard to the patient or operator should either comes in contact with other line
conductor.
SWITCHTR
TR NICS
Medicare Company
Page 05
If we compare the same occurrence on a grounded system, and use as an example, a line to ground
fault, then we are back our colourful situation as described in section 2 A.
When initially energized, a piece of equipment may operate correctly and be in a safe condition.
However during operation mal function may occur due to spillage of liquid or cable damage (cart
running over power cable or equipment being dropped) In any case result would be the same The level
of electrical safety has now been reduced. Such occurrences on an Isolated Power System (IPS) would
as described above result in the LIM alarming and warning being issued of potential fault hazard.
6- Conclusion:
Isolation Power System (IPS) provide many advantages and levels of protection over conventional
grounded system. 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
one cycle. However, grounded power systems are potential killer should a grounded
person comes in contact with the line conductor. How many of us has never received
some form of electric shock ?
Isolated Power Systems (IPS) are the best solution for reliable and safe power
particularly in the hospital environment, It has been the intent of this paper to try to
explain to the decision makers, in layman’s term, some of the main benefits to having
Isolated Power System (IPS) in their faculties.
Page 06
SWITCHTR
TR NICS
Medicare Company7