Isolated Power Systems 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.
Many feel that since hospitals no longer use flammable gases, isolated power is of no benefit.
This is not true! Today medical and
nursing sciences are becoming progressively more dependent on electrical apparatus for the preservation of life of hospitalized patients.
For example, year by year more cardiac operations are performed, in some of which the patient's life depends on artificial circulation
of the blood; in other operations, life is sustained by means of electric impulses that stimulate and regulate heart action. At the same
time the equipment, doctor or nurses may be standing in prepping solution, blood, urine and other conductive fluids that greatly reduce
resistance to passage of unintended electrical current. Isolated Power reduces the ignition hazard from arcs and sparks between a live
conductor and grounded metal and mitigates the hazard of shock or burn from electric current flowing through the body to ground.
Reference to these facts may be found in the
Standard For Health Care Facilities NFPA 99 and National Electrical Code NFPA 70.
It is the purpose of this document to explain the other advantages that Isolated Power Systems offer. Such as:
- Reduced shock hazard
- Continuity of power
- Something for nothing — noise reduction
- Advance warning 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, we feel Isolated Power 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. 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.
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.
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.)
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 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!
We may now examine our circuit parameters more closely by using Ohm's Law and Diagram 5.
The impedance value of C1 and C2 is 1.2 x 106 Ohms, giving a line-to-ground capacitance value of:
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.
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.
| C1 in parallel with 1000 Ohms = |
1.2 x 106 x 1000
_____________
1201000 |
= 999 Ohms |
We may round this number to 1000 Ohms, showing that the impedance of our person has in fact completely shunted the C1 capacitance.
We may now reduce our equivalent circuit as follows:
| Leakage current is now: |
120 V
—————
1201000 |
= 100 microamps |
Our 1000 Ohm person coming into contact with L1 has approximately doubled the leakage current to 100 microamps, which is still
an extremely low level.
| Voltage across the person would be: |
1000
—————
1201000 |
x 120 = 0.1V |
| Voltage across C2 would be: |
1.2 x 106
__________
1201000 |
x 120 = 119.9V |
| Current passing through our 1000 Ohm person would be: |
120V
—————
1201000 |
= 100 Microamps |
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
|
|
Grounded |
|
Isolated Power System |
| V. Person |
|
120V |
|
0.1V (with 50 microamp leakage) |
| I. Person |
|
20mA |
|
100 microamp (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.
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.
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 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.
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.
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 "built-in" advantages of this system again when comparing
isolated power with conventionally grounded systems.
4. ADVANCED WARNING OF EQUIPMENT FAILURE
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.
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.
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.
5. 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. It has been the
intent of this paper to try to explain to the decision-makers, in layman's terms, some of the main benefits to having isolated power
in their facilities.
