A basic principle of electricity is known as Ohm’s law, which is represented by the equation:

where E is electromotive force (in volts), I is current (in amperes), and R is resistance (in ohms). Ohm’s law forms the basis for the physiologic equation BP = CO × SVR; that is, blood pressure (BP) is equal to the cardiac output (CO) times the systemic vascular resistance (SVR). In this case, the blood pressure of the vascular system is analogous to voltage, the cardiac output to current, and the systemic vascular resistance to the forces opposing the flow of electrons. Electrical power (P) is measured in watts (W). Power is the product of the voltage (E) and the current (I), as defined by the formula:

The amount of electrical work done is measured in watts multiplied by a unit of time. The watt-second (a joule, J) is a common designation for electrical energy expended in doing work. The energy produced by a defibrillator is measured in watt-seconds (or joules). The kilowatt-hour is used by electrical utility companies to measure larger quantities of electrical energy.

Power can be thought of as a measure not only of work done but also of heat produced in any electrical circuit. Substituting Ohm’s law in the formula:

Thus, power is equal to the square of the current I² (amperage) times the resistance R. Using these formulas, it is possible to calculate the number of amperes and the resistance of a given device if the wattage and the voltage are known. For example, a 60-watt light bulb operating on a household 120-volt circuit would require 0.5 ampere of current for operation. Rearranging the formula so that:

We have:

Using this in Ohm’s law:

The resistance can be calculated to be 240 ohms:

It is obvious from the previous discussion that 1 volt of electromotive force (EMF) flowing through a 1-ohm resistance will generate 1 ampere of current. Similarly, 1 ampere of current induced by 1 volt of EMF will generate 1 watt of power.

Electrical utilities universally provide power that is grounded (by convention, the earth-ground potential is zero, and all voltages represent a difference between potentials). That is, one of the wires supplying the power to a home is intentionally connected to the earth. The utility companies do this as a safety measure to prevent electrical charges from building up in their wiring during electrical storms. This also prevents the very high voltages used in transmitting power by the utility from entering the home in the event of an equipment failure in their high-voltage system.

The power enters the typical home via two wires. These two wires are attached to the main fuse or the circuit breaker box at the service entrance. The “hot” wire supplies power to the “hot” distribution strip. The neutral wire is connected to the neutral distribution strip and to a service entrance ground (i.e., a pipe buried in the earth; Fig. 8-6). From the fuse box, three wires leave to supply the electrical outlets in the house. In the United States, the hot wire is color-coded black and carries a voltage 120 volts above ground potential. The second wire is the neutral wire color-coded white; the third wire is the ground wire, which is either color-coded green or is uninsulated (bare wire). The ground and the neutral wires are attached at the same point in the circuit breaker box and then further connected to a cold-water pipe (Figs. 8-7 and 8-8). Thus, this grounded power system is also referred to as a neutral grounded power system. The black wire is not connected to the ground, as this would create a short circuit. The black wire is attached to the hot (i.e., 120 volts above ground) distribution strip on which the circuit breakers or fuses are located. From here, numerous branch circuits supply electrical power to the outlets in the house. Each branch circuit is protected by a circuit breaker or fuse that limits current to a specific maximum amperage. Most electrical circuits in the house are 15- or 20-ampere circuits. These typically supply power to the electrical outlets and lights in the house. Several higher amperage circuits are also provided for devices such as an electric stove or an electric clothes dryer. These devices are powered by 240-volt circuits, which can draw from 30 to 50 amperes of current. The circuit breaker or fuse will interrupt
the flow of current on the hot side of the line in the event of a short circuit or if the demand placed on that circuit is too high. For example, a 15-ampere branch circuit will be capable of supporting 1,800 watts of power.

Therefore, if two 1,500-watt hair dryers were simultaneously plugged into one outlet, the load would be too great for a 15-ampere circuit, and the circuit breaker would open (trip) or the fuse would melt. This is done to prevent the supply wires in the circuit from melting and starting a fire. The amperage of the circuit breaker on the branch circuit is determined by the thickness of the wire that it supplies. If a 20-ampere breaker is used with wire rated for only 15 amperes, the wire could melt and start a fire before the circuit breaker would trip. It is important to note that a 15-ampere circuit breaker does not protect an individual from lethal shocks. The 15 amperes of current that would trip the circuit breaker far exceeds the 100 to 200 mA that will produce ventricular fibrillation.

The wires that leave the circuit breaker supply the electrical outlets and lighting for the rest of the house. In older homes, the electrical cable consists of two wires, a hot and a neutral, which supply power to the electrical outlets (Fig. 8-9). In newer homes, a third wire has been added to the electrical cable (Fig. 8-10). This third wire is either green or uninsulated (bare) and serves as a ground wire for the power receptacle (Fig. 8-11). On one end, the ground wire is attached to the electrical outlet (Fig. 8-12); on the other, it is connected to the neutral distribution strip in the circuit breaker box along with the neutral (white) wires (Fig. 8-13).

It should be realized that in both the old and new situations, the power is grounded. That is, a 120-volt potential exists between the hot (black) and the neutral (white) wire
and between the hot wire and ground. In this case, the ground is the earth (Fig. 8-14). In modern home construction, there is still a 120-volt potential difference between the hot (black) and the neutral (white) wire as well as a 120-volt difference between the equipment ground wire (which is the third wire), and between the hot wire and earth (Fig. 8-15).

A 60-watt light bulb can be used as an example to further illustrate this point. Normally, the hot and neutral wires are connected to the two wires of the light bulb socket, and throwing the switch will illuminate the bulb (Fig. 8-16). Similarly, if the hot wire is connected to one side of the bulb socket and the other wire from the light bulb is connected to the equipment ground wire, the bulb will still illuminate. If there is no equipment ground wire, the bulb will still light if the second wire is connected to any grounded metallic object such as a water pipe or a faucet. This illustrates the fact that the 120-volt potential difference exists not only between the hot and the neutral wires but also between the hot wire and any grounded object. Thus, in a grounded power system, the current will flow between the hot wire and any conductor with an earth ground.

As previously stated, current flow requires a closed loop with a source of voltage. For an individual to receive an electric shock, he or she must contact the loop at two points. Because we may be standing on the ground or be in contact with an object that is referenced to ground, only one additional contact point is necessary to complete the circuit and thus receive an electrical shock. This is an unfortunate and inherently dangerous consequence of grounded power systems. Modern wiring systems have added the third wire, the equipment ground wire, as a safety measure to reduce the severity of a potential electrical shock. This is accomplished by providing an alternate, low-resistance pathway through which the current can flow to ground.

Over time, the insulation covering wires may deteriorate. It is then possible for a bare, hot wire to contact the metal case or frame of an electrical device. The case would then become energized and constitute a shock hazard to someone coming in contact with it. Figure 8-17 illustrates a typical short circuit, where the individual has come in contact with the hot case of an instrument. This illustrates the type of wiring found in older homes. There is no ground wire in the electrical outlet, nor is the electrical apparatus equipped with a ground wire. Here, the individual completes the circuit and receives a severe shock. Figure 8-18 illustrates a similar
example, except that now the equipment ground wire is part of the electrical distribution system. In this example, the equipment ground wire provides a pathway of low impedance through which the current can travel; therefore, most of the current would travel through the ground wire. In this case, the person may get a shock, but it is unlikely to be fatal.

The electrical power supplied to homes is always grounded. A 120-volt potential always exists between the hot conductor and the ground or earth. The third or equipment ground wire used in modern electrical wiring systems does not normally have current flowing through it. In the event of a short circuit, an electrical device with a three-prong plug (i.e., a ground wire connected to its case) will conduct the majority of the short-circuited or “fault” current through the ground wire and away from the individual. This provides a significant safety benefit to someone accidentally contacting the defective device. If a large enough fault current exists, the ground wire also will provide a means to complete the short circuit back to the circuit breaker or fuse, and this will either melt the fuse or trip the circuit breaker. Thus, in a grounded power system, it is possible to have either grounded or ungrounded equipment, depending on when the wiring was installed and whether the electrical device is equipped with a three-prong plug containing a ground wire. Obviously, attempts to bypass the safety system of the equipment ground should be avoided. Devices such as a “cheater plug” (Fig. 8-19) should never be used because they defeat the safety feature of the equipment ground wire.

Numerous electronic devices, together with power cords and puddles of saline solutions on the floor, make the OR an electrically hazardous environment for both patients and personnel. Bruner et al.⁵ found that 40% of electrical accidents in hospitals occurred in the OR. The complexity of electrical equipment in the modern OR demands that electrical safety be a factor of paramount importance. To provide an extra measure of safety from macroshock, the power supplied to most ORs is ungrounded. In this ungrounded power system, the current is isolated from the ground potential. The 120-volt potential difference exists only between the two wires of the isolated power system (IPS), but no circuit exists between the ground and either of the isolated power lines.

Supplying ungrounded power to the OR requires the use of an isolation transformer (Fig. 8-20). This device uses electromagnetic induction to induce a current in the ungrounded or secondary winding of the transformer from energy supplied to the primary winding. There is no direct electrical connection between the power supplied by the utility company on the primary side and the power induced by the transformer on the ungrounded or secondary side. Thus, the power supplied to the OR is isolated from the ground (Fig. 8-21). Since the 120-volt potential exists only between the two wires of the isolated circuit, neither wire is hot nor neutral with reference to ground. In this case, they are simply

referred to as line 1 and line 2 (Fig. 8-22). Using the example of the light bulb, if one connects the two wires of the bulb socket to the two wires of the IPS, the light will illuminate. However, if one connects one of the wires to one side of the isolated power and the other wire to the ground, the light will not illuminate. If the wires of the IPS are connected, the short circuit will trip the circuit breaker. In comparing the two systems, the standard grounded power has a direct connection to ground, whereas the isolated system imposes a very high impedance to any current flow to ground. The added safety of this system can be seen in Figure 8-23. In this case, a person has come in contact with one side of the IPS (point A). Since standing on the ground (point B) does not constitute a part of the isolated circuit, the individual does not complete the loop and will not receive a shock. This is because the ground is part of the primary circuit (solid lines), and the person is contacting only one side of the isolated secondary circuit (cross-hatched lines). The person does not complete either circuit (i.e., have two contact points); therefore, this situation does not pose an electric shock hazard. Of course, if the person contacts both lines of the IPS (an unlikely event), he or she would receive a shock.

If a faulty electrical appliance with an intact equipment ground wire is plugged into a standard household outlet, and the home wiring has a properly connected ground wire, then the amount of electrical current that will flow through the individual is considerably less than what will flow through the low-resistance ground wire. Here, an individual would be fairly well protected from a serious shock. However, if that ground wire were broken, the individual might receive a lethal shock. No shock would occur if the same faulty piece of equipment were plugged into the IPS, even if the equipment ground wire were broken. Thus, the IPS provides a significant amount of protection from macroshock. Another feature of the IPS is that the faulty piece of equipment, even though it may be partially short-circuited, will not usually trip the circuit breaker. This is an important feature because the faulty piece of equipment may be part of a life-support system for a patient. It is important to note that even though the power is isolated from ground, the case or frame of all electrical equipment is still connected to an equipment ground. The third wire (equipment ground wire) is necessary for a total electrical safety program.

Figure 8-24 illustrates a scenario involving a faulty piece of equipment connected to the IPS. This does not represent a hazard; it merely converts the isolated power back to a grounded power system as exists outside the OR. In fact, a second fault is necessary to create a hazard.

The previous discussion assumes that the IPS is perfectly isolated from ground. Actually, perfect isolation is impossible to achieve. All AC-operated power systems and electrical devices manifest some degree of capacitance. As previously discussed, electrical power cords, wires, and electrical motors exhibit capacitive coupling to the ground wire and metal conduits and “leak” small amounts of current to the ground (Fig. 8-25). This so-called leakage current partially ungrounds the IPS. This does not usually amount to more than a few milliamperes in an OR. So an individual coming in contact with one side of the IPS would receive only a very small shock (1 to 2 mA). Although this amount of current would be perceptible, it would not be dangerous.

The line isolation monitor (LIM) is a device that continuously monitors the integrity of an IPS. If a faulty piece of equipment is connected to the IPS, this will, in effect, change the system back to
a conventional grounded system. Also, the faulty piece of equipment will continue to function normally. Therefore, it is essential that a warning system be in place to alert the personnel that the power is no longer ungrounded. The LIM continuously monitors the isolated power to ensure that it is indeed isolated from ground, and the device has a meter that displays a continuous indication of the integrity of the system (Fig. 8-26). The LIM is actually measuring the impedance to the ground of each side of the IPS. As previously discussed, with perfect isolation, impedance would be infinitely high and there would be no current flow in the event of a first fault situation (Z = E/I; if I = 0, then Z = ∞). Because all AC wiring and all AC-operated electrical devices have some capacitance, small leakage currents are present that partially degrade the isolation of the system. The meter of the LIM will indicate (in milliamperes) the total amount of leakage in the system resulting from capacitance, electrical wiring, and any devices plugged into the IPS.

The reading on the LIM meter does not mean that current is actually flowing; rather, it indicates how much current would flow in the event of a first fault. The LIM is set to alarm at 2 or 5 mA, depending on the age and brand of the system. Once this preset limit is exceeded, visual and audible alarms are triggered to indicate that the isolation from the ground has been degraded beyond a predetermined limit (Fig. 8-27). This does not necessarily mean that there is a hazardous situation, but rather that the system is no longer totally isolated from ground. It would require a second fault to create a dangerous situation.

For example, if the LIM were set to alarm at 2 mA, using Ohm’s law, the impedance for either side of the IPS would be 60,000 ohms:

Therefore, if either side of the IPS had less than 60,000 ohms impedance to the ground, the LIM would trigger an alarm. This might occur in two situations. In the first situation, a faulty piece of equipment is plugged into the IPS. In this case, a true fault to ground exists from one line to ground. Now the system would be converted to the equivalent of a grounded power system. This faulty piece of equipment should be removed and serviced as soon as possible. However, this piece of equipment could still be used safely if it were essential for the care of the patient. It should

be remembered, however, that continuing to use this faulty piece of equipment would create the potential for a serious electrical shock. This would occur if a second faulty piece of equipment were simultaneously connected to the IPS.

The second situation involves connecting many perfectly normal pieces of equipment to the IPS. Although each piece of equipment has only a small amount of leakage current, if the total leakage exceeds 2 mA, the LIM will trigger an alarm. Assume that in the same OR there are 30 electrical devices, each having 100 μA of leakage current. The total leakage current (30 × 100 μA) would be 3 mA. The impedance to ground would still be 40,000 ohms (120/0.003). The LIM alarm would sound because the 2 mA set point was violated. However, the system is still safe and represents a state significantly different from that in the first situation. For this reason, the newer LIMs are set to alarm at 5 mA instead of 2 mA.

The newest LIMs are referred to as third-generation monitors. The first-generation monitor, or static LIM, was unable to detect balanced faults (i.e., a situation in which there are equal faults to ground from both line 1 and line 2). The second-generation, or dynamic, LIM did not have this problem but could interfere with physiologic monitoring. Both of these monitors would trigger an alarm at 2 mA, which led to annoying “false” alarms. The third-generation LIM corrects the problems of its predecessors and has the alarm threshold set at 5 mA.⁶ Proper functioning of the LIM depends on having both intact equipment ground wires as well as its own connection to ground. First- and second-generation LIMs could not detect the loss of the LIM ground connection. The third-generation LIM can detect this loss of ground to the monitor. In this case the LIM alarm would sound and the red hazard light would illuminate, but the LIM meter would read zero. This condition will alert the staff that the LIM needs to be repaired. However, the LIM still cannot detect broken equipment ground wires. An example of the third-generation LIM is the Iso-Gard made by the Square D Company (Monroe, NC).

The equipment ground wire is again an important part of the safety system. If this wire is broken, a faulty piece of equipment that is plugged into an outlet would operate normally, but the LIM would not alarm. A second fault could therefore cause a shock, without any alarm from the LIM. Also, in the event of a second fault, the equipment ground wire provides a low-resistance path to ground for most of the fault current (see Fig. 8-24). The LIM will only be able to register leakage currents from pieces of equipment that are connected to the IPS and have intact ground wires.

If the LIM alarm is triggered, the first thing to do is to check the gauge to determine if it is a true fault. The other possibility is that too many pieces of electrical equipment have been plugged in and the 2 mA limit has been exceeded. If the gauge is between 2 and 5 mA, it is probable that too much electrical equipment has been plugged in. If the gauge reads >5 mA, most likely there is a faulty piece of equipment present in the OR. The next step is to identify the faulty equipment, which is done by unplugging each piece of equipment until the alarm ceases. If the faulty piece of equipment is not of a life-support nature, it should be removed from the OR. If it is a vital piece of life-support equipment, it can be safely used. (Note: If a critical piece of life support equipment—like the cardio-pulmonary bypass machine—is suspected as causing the alarm, do not disconnect it until it is no longer needed.) However, it must be remembered that the protection of the IPS and the LIM is no longer operative. Therefore, if possible, no other electrical equipment should be connected during the remainder of the case, or until the faulty piece of equipment can be safely removed.

The ground fault circuit interrupter (GFCI, or occasionally abbreviated as GFI) is another popular device used to prevent individuals from receiving an electrical shock in a grounded power system. Electrical codes for most new construction require that a GFCI circuit be present in potentially hazardous (e.g., wet) areas such as bathrooms, kitchens, or outdoor electrical outlets. The GFCI may be installed as an individual power outlet (Fig. 8-28) or may be a special circuit breaker to which all the individual protected outlets are connected at a single point. The special GFCI circuit breaker is located in the main fuse/circuit breaker box and can be distinguished by its red test button (Fig. 8-29). As Figure 8-5 demonstrates, the current flowing in both the hot and neutral wires is usually equal. The GFCI monitors both sides of the circuit for the equality of current flow; if a difference is detected, the power is immediately interrupted. If an individual should contact a faulty piece of equipment such that current flowed through the individual, an imbalance between the two sides of the circuit would be created, which would be detected by the GFCI. Since the GFCI can detect very small current differences (in the range of 5 mA), the GFCI will open the circuit in a few milliseconds, thereby interrupting the current flow before a significant shock occurs. Thus, the GFCI provides a high level of protection at a very modest cost. If the OR has a GFCI that tripped, then one should first attempt to reset it by pushing the reset button. This is because a surge may have caused the GFCI to trip. If it cannot be reset, then the equipment must be removed from service and checked by the biomedical engineering staff. It is essential that when GFCIs are used in an OR, only one outlet be protected by each GFCI. They should never be “daisy-chained,” so that one GFCI protects multiple outlets.

The disadvantage of using a GFCI in the OR is that it interrupts the power without warning. A defective piece of equipment could no longer be used, which might be a problem if it were of a life-support nature, whereas if the same faulty piece of equipment were plugged into an IPS, the LIM would alarm but the equipment could still be used.

There is one instance in which it is acceptable for a piece of equipment to have only a two-prong and not a three-prong plug. This is permitted when the instrument has what is termed double insulation. These instruments have two layers of insulation and usually have a plastic exterior. Double insulation is found in many home power tools and is seen in hospital equipment such as infusion pumps. Double-insulated equipment is permissible in the OR with IPSs. However, if water or saline should get inside the unit, there could be a hazard because the double insulation is bypassed. This is even more serious if the OR has no isolated power or GFCIs.⁷

As previously discussed, macroshock involves relatively large amounts of current applied to the surface of the body. The current is conducted through all the tissues in proportion to their conductivity and area in a plane perpendicular to the current. Consequently, the “density” of the current (amperes per meter squared) that reaches the heart is considerably less than what is applied to the body surface. However, an electrically susceptible patient (i.e., one who has a direct, external connection to the heart, such as through a central venous pressure catheter or transvenous cardiac pacing wires) may be at risk from very small currents; this is called microshock.⁸ The catheter orifice or electrical wire with a very small surface area in contact with the heart produces a relatively large current density at the heart.⁹ Stated another way, even very small amounts of current applied directly to the myocardium will cause ventricular fibrillation. Microshock is a particularly difficult problem because of the insidious nature of the hazard.