Meters
Consumption of electrical energy is measured by watt-hour meters. Utilities also include another charge, for demand, based on the maximum amount of power used in a specified time interval, usually about 15 to 30 mm.
Three-wire meters are generally used for residences, either 208 V or, in some areas, 230/240 V. The 208-V service is usually taken from a three-phase, four-wire street or pole main. The voltage therefore differs 120 in phase from the current.
There is a 120-V difference between the third, or neutral, wire and the phase leg.
For the three 120-V, single-phase circuits, the total power, W, is computed from P = 3EI cos@
where E voltage between phase legs and neutral
I current, A
cos @, power factor
Industries and commercial installations with large motors require three-phase, four-wire meters. Distribution can be over one of three different types of circuits:
208 V, three-phase (motors); 208 V, single-phase (motors, appliances); or 120-V, single-phase (lighting, motors, appliances).
Meters for services supplied by a utility are provided and installed by the utility.
The meter pans and current transformers must be provided by the customer in accordance with the utilitys requirements.
All the service to one building may be measured by one meter, usually called a master meter. Buildings with rented spaces may have one meter for the owners load and individual meters for each tenant.
Switches
These are disconnecting devices that interrupt electric current. Toggle switches (Fig. 15.8a) or snap switches are used for small currents like lighting circuits. They employ pressure contacts of copper to copper. Knife switches (Fig. 15.8b) are used for larger loads. A single-phase knife switch employs a movable copper blade hinged to one load terminal. To close a circuit, the blade is inserted between two fixed copper blades connected to the other terminal. The ground leg is usually continuous and unswitched, for safety reasons. For multiphase circuits, one hinged blade is used for each phase; thus, the switch may be double-pole (Fig. 15.9a) or three-pole (Fig. 15.9b), as the case may be.
A switch may be single-throw (Fig. 15.8b), as described, or double-throw (Fig. 15.9c). A double-throw switch permits the choice of connecting the load (always on the movable blade) to two different sources of power, each connected to opposite, fixed blades.
Once the blades of a switch are in solid contact, the heating effect at the contact surface is minimized. Opening and closing the switch, though, draws a hot arc, which burns the copper. This may cause an uneven surface of contact, with continuing small arcs across the separated points, and result in continual weakening of the contact switch and eventual breakdown.
Switches are carefully rated for load and classified for use by the National Electric Manufacturers Association (NEMA) and the Underwriters Laboratories (UL).
For example, a motor-circuit switch, which carries a heavy starting current, is rated in maximum horsepower allowed for connection.
Many types of service-entrance switches are available to meet the requirements of utility companies. They may be classified as fuse pull switch, externally operated safety switch, bolted pressure contact-type switch, or circuit breaker. In any case, the service switch must have a UL service-entrance label affixed.
An isolating switch may not be used to interrupt current. It should be opened only after the circuit has been interrupted by another general-use switch. Since isolating switches are very light, an arc will create high temperatures and can severely burn the operator.
For control of large, separate loads, the live copper blades of the various switches are concealed in steel enclosures, and the movable blades are operated by insulated levers on the front of the board. The equipment is called a dead-front switchboard.
