the circuit. Figure 5-7 shows a two-pole knife-blade disconnect. As you can see, this switch has two fuses directly beneath the movable blades. Another type of disconnect is installed as a fuse block. The fuse block contains a fuse for each ungrounded conductor. Removal of the fuse block has the same effect as opening a switch to interrupt current flow. A third method of providing for service disconnect and overcurrent protection is the use of circuit breakers. These may be installed as a multiple assembly with a single-switch handle. The service disconnect must be permanently marked to identify it as a service disconnecting means. The grounded conductor is not normally attached to the disconnect switch, but when it is, the switch must be in the form of a circuit breaker, and all the ungrounded conductors must open simultaneously with the grounded conductor. Regardless of whether it is switched, the grounded conductor has to be fixed so it can be disconnected. A terminal or bus bar to which all grounded conductors can be attached by means of pressure connectors meets this requirement.
The service entrance must be grounded to a low- resistance ground (refer to section 250 of the NEC). Normally, a ground rod is driven into the ground for this
Figure 5-7. - Service entrance disconnect and overcurrent protection.
purpose, but a metal underground water pipe in direct contact with the earth for 10 feet or more and electrically continuous to the points of connection of the grounding electrode conductor and the bonding conductors may be used. Another way is to use the nearest available effectively grounded structural metal member of the building for grounding. Once a suitable grounding electrode is identified, the grounded or neutral conductor must be attached to it. The grounding electrode conductor is installed as a continuous conductor from the neutral bus bar to the grounding electrode. Small grounding conductors are enclosed in a protective metal covering that should be electrically continuous from the panelboard cabinet to the grounding electrode. Metal raceways, meter sockets, panelboard cabinets, and the grounding electrode conductor enclosure must all be electrically bonded together and to the grounding electrode conductor so as to be electrically continuous. This arrangement results in all metal parts and enclosures in the service entrance and the grounded conductor being at the same potential electrically.
Every interior wiring job you will be assigned will have its own particulars, depending upon what type of building or structure you are assigned to wire. Some of these particulars are whether to use rigid, flexible, or thin-wall conduits. The type of conductors you will use, whether single or a cable with multiconductors, such as Romex or flexible cord. These particulars, as mentioned above, depend on what type of job you are assigned, temporary or hard-wired, block or drywall construction. Use this information provided here to become familiar with installation requirements. Learn to use the proper cable, conductors, and conduit in the correct place. Make sure you are familiar with the various methods of bending, joining, and installing the various materials, and learn to select the proper fittings and accessories that you will need to install those materials. As you are referred to the NEC throughout the chapter, look up the articles indicated This action will help you get acquainted with the NEC. A good CE takes pride in doing a neat, safe, and proper job.
CONDUCTORS AND CABLE SYSTEMS Electrical conductors generally consist of drawn copper or aluminum formed into a wire. They provide paths for the flow of electric current and usually have insulating material encasing the metal. The insulation material is provided to minimize short circuits and to protect personnel. Atmospheric conditions, voltage 5-4Continue Reading