Nuclear facilities rely on standardized wiring schemes to provide both singlephase and three-phase power distribution systems and protective grounds to insure safe operation.

EO 1.9DEFINE the following terms as they apply to wiring schemes used in power distribution systems:

a. Ampacity b. Bond c. Conductor d. Ground e. Ground voltage f. Leg

g. Neutral h. Phase voltage

EO 1.10 DESCRIBE the two methods of connecting singlephase loads to a three-phase power source.

EO 1.11 DESCRIBE the purpose of the following power distribution schemes.

a. 3-wire, single-phase Edison system

b. 3-wire, three-phase Delta system

c. 4-wire, three-phase Delta system

d. 4-wire, three-phase Wye system

Introduction

Many advisory boards exist to insure the standardization of electrical installations in accordance with accepted designs and safe practices. The Institute of Electrical and Electronics Engineers (IEEE) and the American National Standards Institute (ANSI) are two advisory boards that have published numerous standards. These standards are utilized by the Department of Energy and the nuclear industry. However, for a day-to-day practical guide for noncritical installations, the recognized guide is the National Electrical Code Handbook (NEC), published by the National Fire Protection Association and endorsed by ANSI. The NEC Handbook is the primary source of much of the material presented in this chapter and may serve as a ready reference for specific questions not covered in this fundamental discussion.

Terminology

To understand wiring schemes used in power distribution systems, you must be familiar with the following terms.

Ampacity - the current in amperes that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

Bond - the permanent joining of metallic parts or circuits assuring electrical continuity and capacity to safely conduct any current likely to be imposed.

Conductor - any wire, cable, or substance capable of carrying an electrical current.

Ground - a conducting connection, whether intentional or accidental, between a circuit or piece of equipment and the earth, or some body serving as earth; a place of zero electrical potential.

Ground voltage - the voltage between any given conductor and any point at ground potential.

Leg - a current-carrying conductor intended to deliver power to or from a load normally at an electrical potential other than ground.

Neutral - a current-carrying conductor normally tied to ground so that the electrical potential is zero.

Phase voltage - the greatest root mean square (effective) difference of potential between any two legs of the circuit.