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Electrical contacts with the ground can cause injury when they complete a circuit that permits a large flow of current. One strategy for ensuring safety is to isolate
Traditionally, implementation of this strategy in the operating room has been accomplished by means of isolation transformers, which usually take the form of large wall panels having outlets and meters. The term isolation transformer comes from the fact that power outputs are isolated from the electrical ground. Figure 87-1A shows a schematic diagram of an isolation transformer. A device built into the transformer constantly checks that all is well with this setup. It monitors the extent to which the output power lines are isolated from the ground and is called the line isolation monitor (LIM). I review how LIM came into existence, why it is important, and precisely what it is.
Electrical power for an operating room comes from a primary hospital source that usually originates from a connection to an alternating electrical current (AC) station of the local power company. (Sometimes, an emergency, gasoline-powered electrical generator is the primary source.) After arriving at an operating room, electrical power is modulated, isolated, and dispensed to electrical outlets in the room by the secondary coils of one or more large isolation transformers. Connections in three-hole power outlets in operating rooms therefore are somewhat different from connections in standard outlets found elsewhere in the hospital.[19] In operating rooms, a circuit cannot be completed by connecting one of the two power contacts to the ground contact.
The panel of each isolation transformer is required to have an LIM, which is an electrical current meter that demonstrates the isolation of the transformer's output power from the ground. The large isolation transformers seen in operating rooms are somewhat anachronistic because the NFPA requires their presence only for environments in which inflammable anesthetics are used.[20] This does not mean that basic notions of safety have
Figure 87-1
Schematic diagrams of an isolation transformer (A)
and electrical grounding (B) for an electrocardiogram
in the operating room (i.e., outlet to equipment).
As shown in the schematic diagram of an isolation transformer in Figure 87-1A , for each power outlet, two hot-wire contacts come from the secondary coil of the transformer. The primary circuit of the transformer is attached to the ground contact, but the secondary circuit of the transformer is not. The third contact, which is at the end of the ground wire in the plug, is connected to the standard hospital ground and not to the isolation transformer.
When an ECG monitor is plugged into a power outlet in the operating room (see Fig. 87-1B ), neither of the two hot wires from the secondary coil of the transformer is connected to the ground by the ECG circuitry. This demonstrates the general principle that electrical circuits within an apparatus need not be grounded, although the metal case that houses the circuits is always grounded. For many circuits, proper functioning depends on good isolation. The statement "all operating room equipment must be grounded" is always true regarding connections between the ground contact and the external case but is not always true regarding powered circuits within the apparatus.
In the example given in Figure 87-1B , the ECG case is connected to the hospital's electrical ground, and the internal circuitry is connected to the output of the isolation transformer. This safe system is worthy of further discussion.
Let us suppose that mechanical or electrical damage causes us to worry about the electrical safety of an ECG monitor. Could a failure occur inside the ECG monitor that would place the patient or the anesthesiologist in contact with the internal circuitry? If so, would electric current travel through the person from the ECG circuitry to the ground, causing injury or distress? Thanks to isolation of the ECG circuitry, the answer is no.
Figure 87-2A shows how isolation provides safety. A grounded person is touching the internal circuitry at point B. The isolation transformer supplies current to pathways that connect the two hot leads in the outlet,
Figure 87-2
Diagrams show that no electric shock occurs if an isolated
power line is touched (A), an electric shock does
occur if a faulted secondary power line is touched (B),
and a line isolation monitor can watch for a fault (C).
Figure 87-2B , however, shows that two inadvertent ground contacts could produce a dangerous situation, especially if one of those contacts is a human being. Suppose that a fault occurs near point D, causing the internal circuitry to come in contact with the external metal case. A dangerous shock would result from touching the circuit at point B. Because of the fault, current could complete a pathway through the person and ground (i.e., from point A to D). It is useful to isolate power lines from ground and to know when isolation is compromised by a fault.
Every isolation transformer has an LIM that monitors the isolation of the transformer's two power output lines from the ground. Figure 87-2C shows how the LIM (an ammeter) replaces the smiling and frowning people in Figure 87-2A and B . When very low amperage is indicated, the LIM verifies that the power output lines of the transformer are isolated from the electrical ground, as in Figure 87-2A . The reality of the LIM connections is different because either of the hot wires could become grounded accidentally. The LIM is actually connected to both sides of the isolated power output ( Fig. 87-3 ) and is set to sound an alarm when either side has an impedance to ground that is less than 25,000 ohms or when the maximum current that a short circuit could cause exceeds 2 mA. The LIM is insensitive to currents below 2 mA. As discussed later, the LIM provides no protection against microamp currents and microshock.
Figure 87-3
Schematic diagram shows the complete connections of the
line isolation monitor (LIM). The alarm sounds a "first fault" if either isolated
power line has less than 25,000 ohms of impedance, corresponding to 2 mA or more
being drawn through the LIM.
One event that occurs commonly in operating rooms could produce a short circuit between the power and ground, causing the LIM to go off. This event is the dripping of saline, blood, or other conducting liquid into the receptacles of an electrical extension cord that is on the floor near the operating table. For this reason, electrical extension cords in operating rooms frequently have watertight covers that flip into place over unused outlets. If wet receptacles on an extension cord appear to set off an LIM, the extension cord should be changed. If the LIM rings suddenly and unexpectedly or immediately after a new piece of equipment is plugged in, that equipment should be unplugged immediately. If plugging in a piece of equipment repeatedly causes the LIM to sound an alarm, the equipment should not be used until it is checked. Similarly, if use of a particular power outlet repeatedly causes an alarm, it should not be used until it has been checked. When an LIM alarm indicates a first fault (i.e., one power line is grounded), large, potentially damaging currents may subsequently occur through ground connections if there is a second ground fault. The path of damage may include the anesthesiologist, the patient, or an essential piece of equipment. An alarming LIM warns that someone in the operating room may receive an electrical shock or burn when touching connections to electrical equipment.
Detection of a first fault in grounding is possible without the use of isolation transformers and LIMs. The direct-installation GFCI outlets provide protection against a first fault. However, they provide protection by suddenly shutting off all power from the outlet. An advantage of the LIM is that operating room personnel learn about a first fault without losing electrical power. A disadvantage of the LIM is its susceptibility to newly developed artifactual causes of alarm. Certain devices used in operating rooms emit electrical radiation, intentionally or incidentally, that falsely cause LIMs to sound an alarm. These include instruments that generate ultrasonic sound waves for tumor disintegration and aspiration and stereotaxic monitors that use radio waves to track sensors attached to patients.
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