Electric services in a building may be provided for several different kinds of loads:
lighting, motors, communications equipment. These loads may vary in voltage and times of service, as for example, continuous lighting or intermittent elevator motors.
Motors have high instantaneous starting currents, which can be four to six times the running current, but which lasts only a brief time.
It is highly improbable that all of the intermittent loads will occur at once. To determine the probable maximum load, demand factors and coincidence factors (diversity factors) must be applied to the total connected load (see Art. 15.8).
Lighting Loads. The minimum, and often the maximum, watts per square foot of floor area to be used in design are specified by building codes for various uses of the floor area. Maximum wattages are set to conserve energy and should be followed wherever possible. Electrical engineers, however, may exceed the minimum wattages if the proposed use requires more. For example, lighting may be designed to give a high intensity of illumination, which will require more watts per square foot than the code minimum. (Recommended lighting levels are given in the Illuminating Engineering Society Lighting Handbook.)
Power Loads. In industrial buildings, the process equipment is normally the largest electrical power load. In residential, commercial, and institutional buildings, the power loads are mainly air-conditioning equipment and elevators. Some commercial and institutional buildings, though, contain significant computer and communication equipment loads, and special attention is required to properly serve these electronic equipment loads.
Electronic Equipment Loads. The electric power from the utility company is contaminated with electrical noise and spikes and is subject to sags, surges, and other power-line disturbances. The sensitivity of electronic equipment requires that the electrical system include equipment that will reduce the effect of these disturbances.
Selection of this protection equipment should be based on the functions to be performed by the electronic equipment and a consideration of the consequences disturbance might cause, such as disruption of service lost data equipment damage and attendant costs. Most manufacturers specify the power quality needed for satisfactory operation of their electronic equipment. In fact, manufacturers of many computer systems furnish specific site-preparation instructions that address not only electrical power, but also lighting, air conditioning, grounding, and room finishes.
For protection purposes, for a personal computer or workstation, it may be only necessary to provide a good-quality plug-in strip with a transient-voltage surge suppressor (TVSS). A medical imaging system, such as a CAT-scan machine, may require a power conditioner that combines a voltage regulator, to eliminate sags and surges, with a shielded isolation transformer, to block spikes and noise. In critical installations, though, where equipment failure or an outage can have serious effects, more extensive steps must be taken. For example, loss of service to a satellite communication facility, a banking computer center, or an air-traffic control tower can have severe adverse consequences. To prevent this, such facilities should be provided with an uninterruptible power supply, backed up by either an alternate utility service or a stand-by generator.
A commonly used uninterruptible power supply (UPS) has a rectifier that is fed from the utility power line and delivers dc power to a large bank of batteries, keeping them fully charged, and to an inverter, which converts the dc back into high-quality ac power (Fig. 15.4). This arrangement isolates the electronic equipment from the utility power line, and, if the utility power fails, the batteries will instantly deliver power to the inverter, which will serve the load for 15 to 30 min.
The stand-by generators will start and assume the load until the utility power is restored. To allow for the possibility that the generator may not start, the batteries should be designed to provide a protection time long enough to allow an orderly shutdown of the equipment. The UPS includes a fast-acting, internal, static bypass switch, used in case the UPS itself fails.
Most electronic equipment will draw large amounts of harmonic current, principally odd-number harmonics (180 Hz, 300 Hz, . . .), which can overload and damage electrical equipment. Also, triplen harmonics (3rd, 6th, . . .) add arithmetically and can overload a neutral conductor without tripping a breaker or blowing a fuse. To compensate for this, system neutral capacities must be increased. Empirical data indicate that, in systems with heavy electronic equipment loads, the neutral current will be about 1.8 times the phase current. So, use of a doublecapacity neutral will usually suffice. Other system components, such as transformers (k-rated) and circuit breakers, also must be selected to operate satisfactorily for high harmonic loads.
Grounding. The National Electrical Code (NEC) requires an equipment ground system for every electrical installation to ensure personal safety. In sensitive electronic installations, a facility grounding system has the additional requirement of preventing damage to extremely sensitive computer equipment. Soil-resistivity measurements should be obtained at the site for use in design of a low-impedance ground system that will safely conduct lightning discharge currents to earth and allow sufficient ground current to flow to enable circuit-protective equipment to trip under fault conditions.
In addition, a low impedance (0.1 ohm or less) signal-reference ground system (often referred to as an equipotential plane) should be connected to the building ground. All electronic equipment and peripherals, as well as electrical equipment, HVAC equipment and ductwork, piping, raised floor system, and structural steel in proximity with the computer equipment should be connected to this system to ensure that all these items are at the same ground potential. This will ensure that ground current will not flow through the equipment and will also lessen potential damage from electrostatic discharge (ESD). For this purpose, a manufactured copper grid with 2-ft by 2-ft spacing may be used. It should be located under the entire raised floor area.