Electrical supply

Published on 07/02/2015 by admin

Filed under Anesthesiology

Last modified 22/04/2025

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Electrical supply

Brian A. Hall, MD

Power systems

The two types of power systems used in health care facilities include a grounded power system (GPS) and an isolated power system (IPS). Which system is used in a specific area depends upon the patient care provided at that location as well as the characteristics of the electrical system that supplies the patient care area.

Grounded power systems

The main wiring in an OR is similar to that of a typical home in the United States—a GPS—comprised of a live (hot, positive) wire carrying alternating current at 120 V (120 VAC), a neutral (cold, negative) wire that completes the circuit by transmitting the current from the house back to the power-generating station, and an earth wire, or ground wire. If a person is grounded and comes in contact with the hot limb of a GPS, an electric shock will be delivered.

In a home, ground-fault circuit interrupters (GFCIs) are installed in a GPS that is used in wet locations (e.g., bathrooms and kitchens). Likewise, in the OR, GFCIs can be placed on each circuit of a GPS to minimize the likelihood of a macroshock occurring. The circuit will “trip” in milliseconds if a current greater than 4 to 6 mA begins to flow between the live wire and any other pathway other than the neutral wire. The concern with using GFCIs in an OR, however, is that, if the circuit trips, every electrical device plugged into that circuit will lose power.

In a “dry” OR with a GPS, the live (hot) wire is connected to the narrower of the two parallel prongs of an electrical plug and can be inserted only into the wider of the two parallel holes in the outlet. The neutral wire has the same potential difference from ground as the ground wire itself (i.e., 0 V). The single pole switches used in a GPS disconnect only the “hot” wire. If a person or a patient standing or lying in water comes in contact with the neutral or ground wire, no current flows. However, if the person contacts the hot wire, possible electrocution could occur unless protected by a GFCI.

Isolated power systems

Electricity in IPS is supplied by a transformer that is separate from the power-station electrical supply. The term isolated means precisely that—isolated from earth ground; the electrons in the circuit are not seeking the path of least resistance back to the power-generating station.

Electrical plugs and receptacles in an IPS look exactly like those in a GPS (i.e., one prong is wider than the other). The difference is that, in an IPS, both wires are “hot,” and the voltage between either of the lines and ground is zero. (It is one way to differentiate IPS from GPS; in a GPS, the difference between the hot wire and ground is 120 VAC.)

Circuit breakers in the electrical panel of an IPS are double-pole breakers that disconnect both wires of the circuit when tripped. Because neither wire in an IPS circuit is connected to an earth ground, a person could stand in a pool of water while holding either of the wires of the circuit without macroshock occurring because no circuit is completed. Touching both wires simultaneously, of course, would deliver a macroshock.

Line isolation monitors

Any piece of equipment that has a power supply will have a small amount of current that reaches the earth ground without a physical connection to that ground. The phenomenon is termed capacitance coupling. With the amount of electrical equipment in a modern OR, it should not be a surprise that the net potential current of this capacitance coupling can be 2 to 5 mA or higher.

In an OR with an IPS, a line isolation monitor (LIM) must be used to continually monitor the leakage current from both lines or wires to ground to ensure that the system remains “isolated” from the earth ground. This device measures how much current could “leak” to earth ground (in milliamperes)—a quantitative assessment of how well the IPS is isolated. The LIM must be installed so that the green (status OK) lamp and the red (hazard leakage current) lamp are visible to personnel in the care areas where the IPS is used (Figure 2-1).

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Figure 2-1 Example of a line isolation monitor, the LIM2000plus. (Courtesy of Isotrol, Inc., Coatesville, PA.)

The LIM helps to protect against macroshock but not microshock. Currents up to 5 mA can pass to ground without triggering an LIM alarm. An activated LIM alarm does not indicate that the patient is receiving an electric shock; instead, the audible warning signal indicates that the impedance (to ground) within the circuit has decreased to the point that a current greater than 5 mA (macroshock threshold) could flow (leakage current). If no intervention is made and a circuit is completed (e.g., the patient is lying on a wet metal OR table), the patient may receive a macroshock. With higher currents (∼70-100 mA), fibrillation of the heart through intact skin may occur.

When an audible warning signal from an LIM is triggered, OR personnel should expediently search for the cause. Violations usually occur when wires become frayed and are exposed to the metal casing of the plug. The third prong in both isolated and nonisolated circuits is always grounded, as are the metal face plates and housings of equipment; these parts serve as the violation points where IPSs most frequently become grounded. If the LIM sounds, OR personnel should unplug the last device connected, then the one before that, and so on, until the offending device is identified.

Electrical requirements in the operating room

From 1948 to 1988, IPS and LIMs were required at all anesthetizing locations in the United States to reduce the risk of macroshock. As originally developed, LIMs alarmed if the current that could flow were greater than 2 mA; with the elimination of ether from ORs and the decreased concern about an electric spark triggering an explosion or fire, the Occupational Health and Safety Administration raised the threshold for the LIM to alarm at 5 mA.

In 1988, the national standards for electrical safety in health care institutions changed again; an IPS or a GPS with GFCI was required only if the OR was considered to be “wet.” Many hospitals and health care systems (e.g., the Department of Defense and the Veterans Administration) have objected to installing the more expensive IPS or GFCIs and have declared the majority of their ORs, including cystoscopy rooms, to be “dry.” A person who is in such a room and is standing or lying on a wet surface becomes grounded. If that person comes into contact with faulty electrical equipment, he or she will receive a macroshock, with the potential for serious harm occurring. In 2012, the National Fire Protection Association deemed all ORs to be wet procedure locations unless the health care governing body had conducted a risk assessment and determined otherwise. Therefore, all ORs should be provided with special protection against electric shock unless a risk assessment proves otherwise.

Electrical cords should not traverse the OR floor because they create a tripping hazard. Articulated booms or ceiling outlets should be used to bring the power supply close to the OR table. No multiple-plug outlet boxes that could become grounded if they come in contact with body fluids or liquids should be on the floor. Containers or liquids should not be placed on electrical equipment. Equipment operated by surgeons (e.g., lasers or electrosurgical [cautery] units) should be plugged into circuits separate from circuits used for anesthesia monitoring equipment; plugging these devices into the same circuit can increase the likelihood of 60-cycle electrical interference creating artifacts on the monitors. The use of lasers increases the risk of a fire occurring in a high-oxygen environment and, though electrosurgical units, especially modern bipolar units, do not cause macroshocks because they use very high frequency and low-voltage current, they can trigger cardiac defibrillators and interfere with cardiac pacemakers.

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