Consulting-Specifying Engineer - January 2008 - (Page 42) coordinated, using a 60-amp main fuse and 20-amp sub-fuses, a short circuit would only take out the single 20-amp breaker and leave the remainder of the operating room in operation. This would still represent a problem—just not as significant of a problem. The International Electrotechnical Commission (IEC) Regulation 62, “Common Aspects of Electrical Equipment Used in Medical Practice,” indicates two critical issues for the use of isolated power systems. These two issues include the requirement for a reliable power supply and the reduction of leakage current. The definition of isolated power system in the NEC indicates “a system comprising an isolated transformer or its equivalent, a line isolation monitor, and its ungrounded circuit conductors.” The NEC also defines a line isolation monitor as a “test instrument designed to continually check the balanced and unbalanced impedance from each line of an isolated circuit to ground and equipped with a built in test circuit to exercise the alarm without adding to the leakage current hazard.” the skin. Using the equation below, the amount of fault current through the body could be 120 mA (milliamp). The equation is as follows: I = V/R = 120 / 1,000 = 120 mA Where I = current V = voltage R = resistance (impedance) With the implementation of an isolated power supply, the neutral to ground connection is removed. There will be a very small amount of current flow that is based on the systems leakage capacitance. Even if a person comes in contact with a line conductor and ground, and the capacitance between the phase and ground is shunted, the return path through the isolated neutral will greatly reduce the current flowing through the body. This reduction can be 1/1000 or less than the current in a conventional solidly grounded transformer. This could result in 0.1 mA as compared to 120 mA. In addition, a hospital patient may be in a situation that could result in the body having significantly less impedance than what is represented in the equation above. This lower impedance of the body and additional leakage capacitance from multiple pieces of medical equipment connected to the isolated power supply could result in higher levels of current running through the body. NEC 517.160 (B) (2) states, “The line isolation monitor shall be designed to have sufficient internal impedance such that, when properly connected to the isolation system, the maximum internal current that can flow through the isolation monitor, when any point of the isolation system is grounded, shall be 1 mA.” The perception level of current through the body is about 1 mA. The body typically would feel just a slight tingle at this level of current. At about 5 mA, the body can feel a disturbing pain, but the average person would probably be able to let go and pull away from the shock hazard. But this may not be the case with a sedated or incapacitated hospital patient. Painful shock would occur, and the ability to pull away would occur at between 6 mA to 25 mA. At 50 mA to 150 mA, extreme pain and respiratory arrest are possible. Remember, in the example above, 120 mA could flow through the body. In addition, the shielded isolation panel can prevent power anomalies such as voltage spikes, surges, and noise from damaging the sensitive electronic equipment typically located on operating rooms. This could increase uptime and longevity of the critical medical equipment. NEC 2005, Section 517-160, Isolated Power Systems, states the following: “Each isolated power circuit shall be controlled by a switch that has a disconnect pole in each isolated circuit conductor to simultaneously disconnect all power. Such isolation shall Leakage current can be harmful to people, and hospital patients are even more susceptible to electric shock than healthy individuals. An isolation panel typically will include an isolation panel, a circuit breaker panel, a ground bus, and a line isolation monitor. If the isolation panel becomes unintentionally grounded, the alarm will activate without disconnecting the service. The line isolation monitor is a device that continually measures the balanced and unbalanced impedance from line to ground on each line of an ungrounded electrical distribution system. This measured impedance can be displayed on the unit as hazard current. The amount of hazard current is the amount of current that would flow through a low impendance ground fault on an ungrounded electrical system. If the impedance from line to ground is higher, the amount of hazard current that could flow through the system is lower. The opposite is true if the impedance of the system is lower, the amount of hazard current that could flow through the system would be increased. Keeping patients safe Leakage current can be harmful to people, and hospital patients are even more susceptible to electric shock than healthy individuals. A patient in an operating room may not be able to react in a hazardous situation. In addition, the natural impedance of the human skin that protects against leakage current may be removed from the equation in certain circumstances, such as the insertion of a catheter or other medical procedures. An isolation panel protects the patient from leakage current and potential injury or death. In a standard transformer, the neutral is bonded to the ground and there is no impedance between the neutral and ground. Assume for the sake of calculation that a person has approximately 1,000 ohms of impedance. The actual impedance will vary among individuals as well as with the condition and moisture content of 42 Consulting-Specifying Engineer • JANUARY 2008
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