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INTRODUCTION TO POWER SYSTEM SAFETY FOR MEDICAL DEVICES. Part 2: Introduce isolated power system. Discuss the advantages of isolated power system in healthcare institution. History.
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INTRODUCTION TO POWER SYSTEM SAFETY FOR MEDICAL DEVICES Part 2: Introduce isolated power system. Discuss the advantages of isolated power system in healthcare institution
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.”
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.
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.
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
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.
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.
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.
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.
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
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.
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.
Leakage Currents (cont’d) • The following figure illustrates the origin and path of leakage current.
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
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.
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)
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.
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.
Advantages that IPS offer • 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.
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
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
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.)
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
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
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 line-to-ground capacitance value of:
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. We may round this number to 1000 Ohms, showing that the impedance 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 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.
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 personI. Person = Current flowing through our 1000 Ohm person GroundedIsolated Power System V. Person 120V 0.1V (with 50 microamp leakage) I. Person 20mA 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.
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.
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 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.
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 "built-in" 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.
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.
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.
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.