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CHARACTERISTICS OF BUILDING ELECTRICAL DISTRIBUTION Typical Building AC Distribution Circuits Typical branch circuits in offices or factories are the same as home electrical circuits. They carry 100-V to 120-V AC power from a building source (or subsource), and distribute it safely throughout the building. The three primary parts of a branch circuit are the receptacle (wall outlet), the current carrying conductors (the wires), and the overcurrent protection device located in the main panel or in a subpanel (typically a resectable circuit breaker). These circuits are sized to carry either 12 A or 15 A continuous and are protected from overload by either a 15-A or 20-A circuit breaker. When electricians refer to a circuit as being either a 15-A or 20-A circuit, they are referring to the maximum current capacity of the circuit. Circuit capacity is determined by the gauge of the current carrying conductors and the current rating of the receptacle. Conductors or receptacles rated to carry 12 A continuous would become hot and thus pose a fire hazard if not protected by a circuit breaker. A short circuit anywhere in a branch circuit will immediately cause the circuit breaker to open. Loading a circuit to near, or just beyond, its capacity can cause the breaker to trip from time to time in response to random overload conditions resulting from a combination of load interaction and low line voltage. Midrange and mainframe computers, high-capacity printers and duplicators, and large PBXs require more power than can be delivered by a 15-A or a 20-A branch circuit. These high-capacity circuits have special receptacles so that loads that need more power cannot be inadvertently connected to standard circuits. There are three wires in a branch circuit. One wire (commonly referred to as the live, hot, or line) is energized at the source. It is always live, ready to supply current the moment it is connected to a return path that completes the circuit. The second wire (neutral) serves as the proper return path. The neutral wire is connected to an earth ground at the building power source. The neutral-earth connection is typically made at the main transformer but is sometimes allowed at the main panel. Safety Ground The connection of the electrical power system to earth ground is a safety measure, intended to reduce the risk of shock to people using electrical devices and to reduce the risk of fires caused by a breakdown in the electrical system. The National Electrical Code prescribes standards which govern the diameter of wire, type and thickness of insulation, circuit breaker characteristics, receptacle construction, maximum distance of the circuit from the breaker panel, and specific connection practices designed to assure safe grounding of the building power distribution system. Classes and Causes of Power Quality Defects Power loss is a visible defect. Electrical transients are not. Yet as has already been shown, even relatively low-level electrical transients can reset a file server running an operating system in protected mode. Experts are still developing definitive labels for different types of power quality defects, but it is probably sufficient for network managers to be familiar with the following two key concepts:
Typically, electronic systems are more sensitive to fast transient events than slow ones and are more susceptible to interference from common mode impulses than from normal mode impulses. Fast defects typically occur much more often than slow ones. Exhibit 1-6-7 summarizes the characteristics of these problems, which are described in more detail in the following paragraphs.
Normal Mode and Common Mode These terms describe specific pathways that conduct electrical energy in branch circuits. Normal Mode. In normal mode, defects occur between the current carrying conductors in a branch circuit (i.e., the wires connected to the two flat slots on a typical wall socket). Normal mode impulses must pass through the systems power supply to contaminate the low-voltage DC that computers use internally. Common Mode. In common mode, defects occur between either conductor and the ground connection (measured between neutral and ground). Common mode defects bypass the systems power supply and more easily contaminate the computers internal electric environment. System designers usually consider only normal mode power problems. Common mode events are not even acknowledged as real in some circles. But both modes are real and should be considered when planning power conditioning implementations. Power quality defects occur over a frequency continuum. This continuum extends from power transmission frequencies (60 or 50 Hz, depending on country) to approximately 5 MHz. Beyond 5 MHz the energy content of conducted transients is quite low; also, noise above 5 MHz is as likely to be radiated into the computer system as it is to enter through the power line. To simplify the analysis of power quality defects that affect electronic systems, they are classified as either low frequency or high frequency. Low-frequency events are in the range from 50 Hz to 20 kHz. High-frequency events occur above 20 kHz.
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