ELECTRICAL GROUND

Discussions of electrical safety often center on whether circuits are grounded. To the clinician, an electrical ground is the wire connected to the third prong on plugs that are inserted into electrical wall outlets. With regard to a circuit, an electrical ground is any object connected

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When a circuit is intentionally grounded, the physical objects chosen to serve as the electrical ground can vary. In a remotely located Mobile Army Surgical Hospital unit or recreational camping van, equipment may be grounded by connections to a spike that goes into the earth. In a large urban medical center, the ground may consist of a subterranean network of pipes. For a very small, hand-held radio, a human being may serve as the connection to ground.

In hospital settings, the clinician does not need to understand the details regarding the choice of ground. However, clinicians must often know whether they, their patients, or their pieces of equipment are grounded. Living organisms tolerate only limited amounts of electric current. Unintended contacts with electrical ground, sometimes injurious when they enable very small electrical currents to reach the heart or neural tissue, are always dangerous when they permit large electric currents to travel through the body. Fortunately, it is possible to design alarms for operating rooms and surgical equipment that provide danger signals before errant currents are established. The line isolation monitor, which is explained later, gives a warning when faulty ground connections are one step away from establishing a large current in the patient. Human protection from large currents can be ensured if connections to ground always occur through a sizable resistance. The danger of being grounded through too small of an electrical resistance can be illustrated by an unrealistic but vivid hypothetical example. Imagine a person standing on top of a giant copper cylinder that extended down to the molten core of the earth. This low-resistance connection to the largest imaginable electrical ground would be quite unsafe because the person would essentially be a fuse, poised to suffer whatever large electrical current is applied. The safety that would be achieved by inserting very large electrical resistance between the person and the copper cylinder—but not so large a resistance as to cause perfect insulation—explains a requirement that was established long ago, when there was use of explosive anesthetic gases such as cyclopropane. Operating rooms were then required to have conductive floors made of specific resistive materials. Such substances had high carbon content, and they were neither as conductive as metal nor as insulating as rubber. The high carbon content imparted a dark black color, one that can commonly be found today in operating room tables and cushions made of resistive materials.

In the United States, electrical plugs have three prongs: two that provide voltage or, equivalently, power and a special third prong that serves only as a connection to ground (i.e., to an object that makes an electrical connection with the earth). Travelers to Europe and elsewhere may recall seeing electrical outlets for plugs that have only two prongs and no separate ground wire. A two-wire system becomes especially unsafe if three-pronged equipment is plugged into it, leaving the third prong unattached to anything.

A ground fault current interruption (GFCI) electrical outlet allows a person to test that the third connection in the outlet is connected to a functional ground. More significantly, GFCI outlets enhance electrical safety by serving as emergency circuit breakers that shut off power when one of the two power lines in the outlet is accidentally connected to ground. GFCI outlets, found in hospitals and homes, have two buttons: test and reset. Pushing the test button causes a special resistor to be connected across the power and ground terminals in the receptacle. If the ground connection (i.e., the third wire) is in place and connected to the system ground, as it should be, current flows through the resistor and causes a circuit breaker in the outlet to open. No electricity can then be delivered by the outlet. Pushing the reset button resets the circuit breaker and restores electrical power. Pushing the test button would not cause the circuit breaker to open if, for example, a contractor accidentally forgot to install the ground wire or if an earthquake or other mishap caused a break in an installed ground wire. Protection against accidental grounding of one power line in an outlet is required by the NFPA for wet environments, such as operating rooms. The installation of GFCI electrical outlets is one way to satisfy NFPA protection requirements. As is discussed later, a second way (the traditional but more expensive way) is to have power come from an isolation transformer equipped with a line isolation monitor. ^{[20] [21]}

Electrical engineers commonly choose the ground as the place where the electrical potential (voltage) is defined to be zero. Therefore, the voltage at any point in a circuit is the difference between the voltage at that point and the voltage at the electrical ground. Because of this, ground connections in commercial circuits help professional personnel check for proper functioning. A technician connects one voltmeter probe to ground while touching different circuit elements with the other voltmeter probe. The manufacturer's table can be used to compare readings and, in this way, identify problems in the circuit. Specifying the location of the electrical ground helps people communicate standards for circuit performance.