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Electrical and Fire Safety


Introduction

The myriad of electrical and electronic devices in the modern operating room (OR) greatly improve patient care and safety. However, these devices also subject both the patient and OR personnel to increased risks. To reduce the risk of electrical shock, most ORs have electrical systems that incorporate special safety features. It is incumbent upon the anesthesia caregiver to have a thorough understanding of the basic principles of electricity and an appreciation of the concepts of electrical safety applicable to the OR environment.

Principles of Electricity

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


E = I × R

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 BP of the vascular system is analogous to voltage, the CO to current, and the SVR 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:


P = E × I

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 discharged 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:


P = E × I

P = ( I × R ) × I

P = I 2 × R

Thus, power is equal to the square of the current I 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 both known. For example, a 60-W light bulb operating on a household 120-V circuit would require 0.5 amperes (A) of current for operation. Rearranging the formula so that:


I = P / E

We have:

  • I = 60 watts/120 volts

  • I = 0.5 ampere

Using this in Ohm’s law (R = E/I), the resistance can be calculated to be 240 ohms:

  • R = 120 volts/0.5 ampere

  • R = 240 ohms

It is obvious from the previous discussion that 1 V of electromotive force (EMF) applied across a 1-ohm resistance will generate 1 A of current flow. Similarly, 1 A of current induced by 1 V of EMF will generate 1 W of power.

Direct and Alternating Currents

Any substance that permits the flow of electrons is called a conductor. Current is characterized by electrons flowing through a conductor. If the electron flow is always in the same direction, it is referred to as direct current (DC). However, if the electron flow reverses direction at a regular interval, it is termed alternating current (AC). Either of these types of current can be pulsed or continuous in nature.

The previous discussion of Ohm’s law is accurate when applied to DC circuits. However, when dealing with AC circuits, the situation is more complex because the flow of the current is opposed by a more complicated form of resistance, known as impedance.

Impedance

Impedance, designated by the letter Z, is defined as the sum of the forces that oppose electron movement in an AC circuit. Impedance consists of resistance (ohms) but also takes capacitance and inductance into account. In actuality, when referring to AC circuits, Ohm’s law is defined as:


E = I × Z

An insulator is a substance that opposes the flow of electrons. Therefore, an insulator has a high impedance to electron flow, whereas a conductor has a low impedance to electron flow.

In AC circuits, the capacitance and inductance can be important factors in determining the total impedance. Both capacitance and inductance are influenced by the frequency (in cycles per second or hertz, Hz) at which the AC current reverses direction. The impedance is directly proportional to the frequency ( f ) times the inductance (IND):


Z ∝ ( f × IND )

and the impedance is inversely proportional to the product of the frequency ( f ) and the capacitance (CAP):


Z ∝ 1 / ( f × CAP )

As the AC current increases in frequency, the net effect of both capacitance and inductance increases. However, because impedance and capacitance are inversely related, total impedance decreases as the product of the frequency and the capacitance increases. Thus, as frequency increases, impedance falls and more current is allowed to flow.

Capacitance

A capacitor consists of any two parallel conductors that are separated by an insulator ( Fig. 24.1 ). A capacitor has the ability to store charge. Capacitance is the measure of that substance’s ability to store charge. In a DC circuit the capacitor plates are charged by a voltage source (i.e., a battery) and there is only a momentary current flow. The circuit is not completed and no further current can flow unless a resistance is connected between the two plates and the capacitor is discharged.

Fig. 24.1, A capacitor consists of two parallel conductors separated by an insulator. The capacitor is capable of storing charge supplied by a voltage source.

In contrast to DC circuits, a capacitor in an AC circuit permits current flow even when the circuit is not completed by a resistance. This is because of the nature of AC circuits, in which the direction of current flow is constantly being reversed. Because current flow results from the movement of electrons, the capacitor plates are alternately charged—first positive and then negative with every reversal of the AC current direction—resulting in an effective current flow as far as the remainder of the circuit is concerned, even though the circuit is not completed.

Since the effect of capacitance on impedance varies inversely with the AC frequency in hertz, the greater the AC frequency, the lower the impedance. Therefore, high-frequency currents (0.5 to 2 million Hz), such as those used by electrosurgical units (ESUs), will cause a marked decrease in impedance.

Electrical devices use capacitors for various beneficial purposes. There is, however, a phenomenon known as stray capacitance —capacitance that was not designed into the system but is incidental to the construction of the equipment. All AC-powered equipment produces stray capacitance. An ordinary power cord, for example, consisting of two insulated wires running next to each other will generate significant capacitance simply by being plugged into a 120-V AC circuit, even though the piece of equipment is not turned on. Another example of stray capacitance is found in electric motors. The circuit wiring in an electric motor generates stray capacitance to the metal housing of the motor. The clinical importance of capacitance will be emphasized later in the chapter.

Inductance

When electrons flow in a wire, a magnetic field is induced around that wire. If the wire is coiled repeatedly around an iron core, as in a transformer, the magnetic field can be very strong. Inductance is a property of AC circuits in which an opposing EMF can be electromagnetically generated in the circuit. The net effect of inductance is to increase impedance. Since the effect of inductance on impedance also depends on AC frequency, increases in frequency will increase the total impedance, therefore the total impedance of a coil will be much greater than its simple resistance.

Electrical Shock Hazards

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