Electrical Distribution Systems SYSTEM COMPONENTS AND PROTECTION DEVICESFailure-FreePowerFailure-free power is accomplished by providing vital equipment with automatic switchingbetween two or more power supplies so that interruption of power is minimized.NeutralGroundingNeutral grounding in electrical distribution systems helps prevent accidents to personnel anddamage to property caused by: fire in case of lightning; a breakdown between primary andsecondary windings of transformers; or accidental contact of high-voltage wires and low- voltagewires. If some point on the circuit is grounded (in this case neutral ground), lightning strikingthe wires will be conducted into the ground, and breakdown between the primary and secondarywindings of a transformer will cause the primary transformer fuses to blow. Another advantageof neutral grounding is that it reduces the amount of insulation required for high-voltagetransmission lines.VoltageClassVoltage in distribution systems is classified into three groups: high voltage, intermediate voltage,and low voltage. High voltage is voltage that is above 15,000 volts, intermediate voltage isvoltage between 15,000 volts and 600 volts, and low voltage is voltage at 600 volts or less.ProtectiveRelaysProtective relays are designed to cause the prompt removal of any part of a power system thatmight cause damage or interfere with the effective and continuous operation of the rest of thesystem. Protective relays are aided in this task by circuit breakers that are capable ofdisconnecting faulty components or subsystems.Protective relays can be used for types of protection other than short circuit or overcurrent. Therelays can be designed to protect generating equipment and electrical circuits from anyundesirable condition, such as undervoltage, underfrequency, or interlocking system lineups.There are only two operating principles for protective relays: (1) electromagnetic attraction and(2) electromagnetic induction. Electromagnetic attraction relays operate by a plunger beingdrawn up into a solenoid or an armature that is attracted to the poles of an electromagnet. Thistype of relay can be actuated by either DC or AC systems. Electromagnetic induction relaysoperate on the induction motor principle whereby torque is developed by induction in a rotor.This type of relay can be used only in AC circuits.Rev. 0 Page 3 ES-15
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