Electrical Distribution Systems
SYSTEM COMPONENTS AND PROTECTION DEVICES
Failure-free power is accomplished by providing vital equipment with automatic switching
between two or more power supplies so that interruption of power is minimized.
Neutral grounding in electrical distribution systems helps prevent accidents to personnel and
damage to property caused by: fire in case of lightning; a breakdown between primary and
secondary windings of transformers; or accidental contact of high-voltage wires and low- voltage
wires. If some point on the circuit is grounded (in this case neutral ground), lightning striking
the wires will be conducted into the ground, and breakdown between the primary and secondary
windings of a transformer will cause the primary transformer fuses to blow. Another advantage
of neutral grounding is that it reduces the amount of insulation required for high-voltage
Voltage 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 is
voltage between 15,000 volts and 600 volts, and low voltage is voltage at 600 volts or less.
Protective relays are designed to cause the prompt removal of any part of a power system that
might cause damage or interfere with the effective and continuous operation of the rest of the
system. Protective relays are aided in this task by circuit breakers that are capable of
disconnecting faulty components or subsystems.
Protective relays can be used for types of protection other than short circuit or overcurrent. The
relays can be designed to protect generating equipment and electrical circuits from any
undesirable 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 being
drawn up into a solenoid or an armature that is attracted to the poles of an electromagnet. This
type of relay can be actuated by either DC or AC systems. Electromagnetic induction relays
operate 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.