Aboard modern naval ships, most auxiliary machinery and equipment is run by electricity. Regardless of rate or rating, all personnel assigned to a ship will operate some electric devices in the performance of their duties. Electrical equipment is dangerous if handled incorrectly; therefore, you must observe all applicable safety precautions when working with or around electrical equipment.

In this chapter, we will discuss basic concepts of electricity, electrical terms, electrical equipment, and applicable safety precautions. You will find additional information on the basic principles of electricity in the Navy Electricity and Electronics Training Series (NEETS), modules 1 and 2, NAVEDTRA 172-01-00-79 and NAVEDTRA 172-02-00-79.

INTRODUCTION TO ELECTRICITY

Some materials will conduct electricity, and some offer more resistance than others. Metals such as silver, copper, aluminum, and iron offer little resistance and are called conductors. In contrast to conductors, some materials such as wood, paper, porcelain, rubber, mica, and plastics offer high resistance to an electric current and are known as insulators. Electric circuits throughout the ship are made of copper wires covered with rubber or some other insulator. The wire conductors offer little resistance to the current, while the insulation keeps the current from passing to the steel structure of the ship.

Definite units have been established so we can measure the electrical properties of conductors. Also, there are terms used to describe the characteristics of electric currents. A brief review of these fundamentals is given in the following sections.

ELECTRIC CURRENT

The flow of current through a wire can be compared to the flow of water through a pipe.

Current is the rate at which electricity flows through a conductor or circuit. The practical unit, called the ampere, specifies the rate at which the electric current is flowing. Ampere is a measure of the intensity or the number of electrons passing a point in a circuit each second.

ELECTROMOTIVE FORCE

Before water will flow through a pipe, there must be water pressure; before an electric current can flow through a circuit, there must be a source of electric pressure. The electric pressure is known as electromotive force (eml) or voltage (E). The source of this force may be a generator or a battery.

If you increase the pressure on the electrons in a conductor, a greater current will flow, just as an increased pressure on water in a pipe will increase the flow.

RESISTANCE

Electrical resistance (R) is that property of an electric circuit that opposes the flow of current. The unit of resistance is known as the ohm (4).

WATT

Power (P) is the rate of doing work. In a do circuit, power is equal to the product of the current times the voltage, or P = I x E. The practical unit of power is the watt (W) or kilowatt (kW) (1,000 watts). Power in an ac circuit is computed in a slightly different way. If you are interested in how ac power is computed, see chapter 4 of Introduction to Alternating Current and Transformers, NAVEDTRA 172-02-00-85.