Consulting-Specifying Engineer - January 2008 - (Page 44) be accomplished by means of one or more transformers having no electrical connection between primary and secondary windings, by means of motor generator sets, or by means of suitable isolated batteries.” Additionally, per NEC 517-160 (4), Isolation Transformer, an isolation transformer shall not serve more than one operating room. Note that there are exceptions to this requirement. It is important to maintain the impedance of the electrical system as high as possible. This is achieved by limiting the physical size of the isolation transformer, limiting the number of medical devices connected to the isolation system, and by limiting the total system size. Furthermore, according to NEC 517.160 (A) (6), wire pulling compounds that increase the dielectric constant shall not be used on the conductors on the secondary side of the isolation transformer. NEC Section 517-160 (6), in the fine-print notes, states that the isolation transformer should be limited to 10 kVA or less and that section indicates, “Class A ground fault circuit interrupters trip when the current to ground has a value in the range of 4 mA to 6 mA.” Ground fault A ground fault through the body typically occurs when some part of a person’s body contacts a source of electrical current and the body provides a path for the electrical current to go to the ground, causing a severe shock. An unintentional electric path between a source of current and a grounded surface is referred to as a ground fault. Ground faults occur when current is leaking outside of its intended path; in essence, electricity is escaping to the ground. How it leaks is very significant. If your body provides a path to the ground for this current leakage, you could be burned, injured, severely shocked, or electrocuted. The ground fault circuit interrupter receptacle constantly monitors electricity flowing in a circuit, to sense any loss of current. If the current flowing through the circuit (hot conductor) differs by a small amount from the current returning (neutral conductor)—4 mA to 6 mA as indicated above—the ground fault circuit interrupter rapidly switches off power to that circuit. The ground fault circuit interrupter interrupts power quickly in order to prevent a lethal dose of electricity from flowing through the body. In this situation, the current should remain low enough that you may receive a painful shock, but you should not be electrocuted or receive a serious shock injury. The important point to note is that under a ground fault condition, the ground fault circuit interrupter will disconnect power to the critical piece of medical equipment. If a power interruption cannot be tolerated, this method of protection is not appropriate. NEC 517.1, Definitions, defines a wet location as “Wet locations are those patient care areas that are normally subject to wet conditions while patients are present. These include standing fluids on the floor or drenching of the work area, either of which condition is intimate to the patient or staff. Routine housekeeping procedures and incidental spillage of liquids do not define a wet location.” The somewhat vague definition of “wet location” in the NEC gives the engineer and the hospital some flexibility to determine if the electrical distribution system requires the use of an ungrounded power supply. It is important that the engineer and hospital officials discuss the intended procedures that will take place in the operating room before a decision is made whether or not to use isolation power supplies in the electrical distribution system for these critical installations. Keith Lane is partner/principal at Lane Coburn & Assocs., Seattle, and a member of several professional organizations, including BICSI, NFPA, 7x24 Exchange, Illuminating Engineering Society, and the Washington State Society of Healthcare Engineers. He is a member of Consulting-Specifying Engineer’s editorial advisory board. Class A ground fault circuit interrupters trip when the current to ground has a value in the range of 4 mA to 6 mA. the conductors should use insulation with low leakage to meet the impedance requirements. Note 2 states, “Minimize the length of branch circuit conductors and use conductor insulation with a dielectric constant less than 3.5 and insulation resistance greater than 6100 megohm meters (20,000 megohm ft.) at 16 C reduced leakage from line to ground, reducing hazard currents.” Wet locations Isolation panels are required for operating rooms that also are considered wet locations (as defined in the NEC) that cannot tolerate a power outage caused by a fault condition. NEC Section 517.20 (A), Receptacle and Fixed Equipment, states that “all receptacles and fixed equipment within the area of a wet location shall have ground fault circuit interrupter protection for personnel if interruption of power under fault conditions can be tolerated, or be served by an isolation power system if such interruption cannot be tolerated.” If life-support equipment is part of an operating room, then it is a forgone conclusion that a power outage cannot be tolerated. Part (B) of this section of the code for isolated power systems indicates that where an isolated power system is used, the isolated power equipment shall be listed as isolated power equipment, and the isolated power system shall be designed and installed in accordance with 517.160. A ground fault circuit interrupter is described in Section 100 of the 2005 NEC as “a device intended for the protection of personnel that functions to de-energize a circuit or portion thereof within an established period of time when a current to ground exceeds the values established for Class A devices.” The fine print note in this 44 Consulting-Specifying Engineer • JANUARY 2008
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