ISOLATION of Electric Power from Ground

Electrical contacts with ground can cause injury when they complete a circuit that permits a large flow of current. One strategy for ensuring safety is to isolate all electrical power sources from ground, making it impossible for ground to be used as a path for injurious or damaging currents.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 ground.Electrical power for an operating room comes from a primary hospital source that usually originates from a connection to an alternating-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. ¹²  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 a line isolation monitor (LIM), which is simply an electrical current meter that demonstrates the isolation of the transformer‘s output power from 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. ¹⁴  This does not mean that basic notions of safety have changed with respect to the need to isolate circuits from ground. However, isolation of electrical power is now accomplished with advanced technology that was unavailable when large isolation transformers were initially required. In principle, manufacturers are capable of incorporating good electrical isolation into the design of each piece of electrical equipment used in the operating room. Indeed, it is possible to argue that the obsolescence of central isolation and LIMs has already been demonstrated by the absence of these items in the intensive care unit (ICU) and the postanesthesia care unit (PACU). All electrical instruments that are permitted in the operating room are also used in ICUs and PACUs, yet one does not see isolation transformers and LIMs in ICUs and PACUs. Should one then assume that isolation transformers and LIMs should be eliminated from operating rooms or not installed in new operating rooms? The answer is neither yes nor no, ^13,14  as suggested earlier. The issue of safety for surgical patients who are wet with fluids that conduct electricity is better understood after a review of some fundamentals of electrical isolation.Figure 83–1 A shows a schematic diagram of an isolation transformer. 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 ground, 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. 

FIGURE 83–1 Schematic diagrams of (A) an isolation transformer and (B) electrical grounding for an ECG in the operating room (outlet to equipment).

When an ECG monitor is plugged into a power outlet in the operating room ( Fig. 83–1 B), neither of the two “hot wires” from the secondary coil of the transformer is connected to 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. Indeed, for many circuits, proper functioning depends on good isolation. Thus the statement “all operating room equipment must be grounded” is always true regarding connections between ground and the external case but is not always true regarding powered circuits within the apparatus.In the example given in Figure 83–1 B, 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 ground, causing injury or distress? Thanks to isolation of the ECG circuitry, the answer is no.Figure 83–2 A shows how isolation provides safety. In Figure 83–2 A, 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, indicated by points A and D. However, the only way for electric current to get from point A to D is through impedance Z. Because each of the hot points in the wall outlet is not grounded, the person in Figure 83–2 A is safe from shock. 

FIGURE 83–2 Diagrams showing that (A) no electric shock occurs if an isolated power line is touched, (B) an electric shock does occur if a faulted secondary power line is touched, and (C) a line isolation monitor can watch for a fault.

Figure 83–2 B, 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). Thus, it is useful to isolate power lines from ground and to know when isolation is compromised by a fault.

As mentioned, every isolation transformer has a LIM that monitors the isolation of the transformer‘s two power output lines from ground. Figure 83–2 C shows how the LIM (an ammeter) replaces the smiling and frowning people in Figures 83–2 A and B. When very low amperage is indicated, the LIM verifies that the power output lines of the transformer are indeed isolated from ground, as in Figure 83–2 A. The reality of the LIM connections is actually different because either of the hot wires could become grounded accidentally. Therefore, the LIM is actually connected to both sides of the isolated power output (Fig. 83–3) 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. Note that the LIM is insensitive to currents below 2 mA. As is discussed later, the LIM provides no protection against microamp currents and microshock. 

FIGURE 83–3 Schematic showing complete connections of the line isolation monitor (LIM). The alarm sounds that there is a “first fault” if either isolated power line has less than 25,000 ohms impedence, corresponding to 2 mA or more being drawn through the LIM.

One event that occurs commonly in the operating room could produce a short circuit between 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, on the other hand, 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, too, should not be used until it has been checked. When an LIM alarm indicates a “first fault,” that is, that 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 might include the anesthesiologist, the patient, or an essential piece of equipment. Thus, an alarming LIM warns that someone in the operating room might receive an electrical shock or burn when touching connections to electrical equipment.

As mentioned earlier, 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, it provides protection, as described earlier, by suddenly shutting off all power from the outlet. An advantage of the LIM is that one learns about a first fault without losing electrical power. A disadvantage of the LIM is its susceptibility to newly developed artifactual causes of alarm. Certain new 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.