Switchgear and Metal-Enclosed Bus

Transcription

1 Section 18 Switchgear and Metal-Enclosed Bus 180. SWITCHGEAR ASSEMBLIES 180A. General Requirements for All Switchgear. This rule covers general requirements for all switchgear. Examples of switchgear found in electric supply stations are shown in Fig The general requirements for all switchgear are outlined below: Secure to minimize movement. Support cables to minimize force on terminals. Locate away from liquid piping unless protected. Locate away from flammable gases or liquids. Install after general construction or provide temporary protection. Protect when doing maintenance in the area. Do not use as a physical support unless so designed. Interiors shall not be used for storage unless so designed. Metal instrument cases are to be grounded, enclosed in grounded metal covers, or made of insulating materials. 180B. Metal-Enclosed Power Switchgear. In addition to the general requirements for all switchgear, requirements exist for metal-enclosed power switchgear. The rules for metal-enclosed power switchgear are outlined in Fig C. Dead-Front Power Switchboards. Dead-front power switchboards with uninsulated rear connections must be installed in rooms capable of being locked 107

2 108 ELECTRIC SUPPLY STATIONS Rule 180D Fig Examples of switchgear assemblies (Rule 180A). with access limited to qualified personnel only. The uninsulated rear connections typically will not meet the vertical and horizontal clearances of Rule 124; therefore, a separate locked room is required for these energized parts. 180D. Motor Control Centers. The fault current within the electric supply station can be very high due to low source impedance and little or no distribution-line impedance. The bus withstand ratings of motor control centers must therefore be given proper consideration. Bus bracing in low-voltage (e.g., 480- V) motor control centers commonly come in 50,000-, 100,000-, and 200,000-A values. This is a withstand rating, not an interrupt rating as the bus must withstand the force of a momentary fault but not interrupt it. A fuse or circuit breaker in the motor control center will interrupt the fault. Current-limiting fuses can reduce the available fault current to which the motor control center bus and motor control center circuit breakers are exposed. Peak let-through currents for current-limiting fuses can be obtained from fuse manufacturer s literature. This rule also requires a safety sign for a motor control center cubicle having more than one voltage source. The rules regarding motor control

3 Rule 180E SWITCHGEAR AND METAL-ENCLOSED BUS 109 Fig Metal-enclosed power switchgear (Rule 180B). center short-circuit ratings are outlined in Fig E. Control Switchboards. The requirements for control switchboards are outlined below: Control switchboards include cabinets with meters, relays, annunciators, computers, etc., for substation and generating station controls. Carpeting in control switchboard rooms must be antistatic and minimize toxic gas emissions under any conditions (e.g., water damage, fire damage, etc.). Adequate clearance in front and rear to read meters without stools. Personnel openings must be covered when not in use and must not be used for cable routing. Reading control switchboard meters from stools produces a tripping or falling hazard. The rule related to adequate clearance for reading meters without stools is outlined in Fig METAL-ENCLOSED BUS 181A. General Requirements for All Types of Bus. The rules for metal enclosed bus are outlined in Fig

4 110 ELECTRIC SUPPLY STATIONS Rule 181A Fig Motor control center short-circuit ratings (Rule 180D). See Photo Fig Adequate clearance for reading meters on control switchboards (Rule 180E3).

5 Rule 181B SWITCHGEAR AND METAL-ENCLOSED BUS B. Isolated-Phase Bus. Metal-enclosed bus is available in three common designs, segregated (isolated) phase bus bar, nonsegregated phase bus bar, and cable bus. Examples of metal-enclosed bus are shown in Fig The use of isolated-phase bus requires the following special conditions: Clearance to magnetic material per manufacturer s recommendations Nonmagnetic conduit for alarm circuits Piping if enclosure drains are used Nonmagnetic wall plates Nonmagnetic conduit for grounding conductorsfig General requirements Fig General requirements for metal-enclosed bus (Rule 181A).

6 112 ELECTRIC SUPPLY STATIONS Rule 181B Fig Examples of metal-enclosed bus (Rule 181).