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 4.4.4.2 Backup Protection. Remote backup protection is inherently provided by the
primary relaying at the remote ends of the supply lines.
4.4.5 Power Cable Protection. Most faults in a power system occur on the transmission and
distribution lines. A cable must be protected from overheating due to excessive short-circuit
current flowing in its conductor. The fault point may be on a section of the protected cable or on
any other part of the power system. Devices to protect cables against short-circuit damage
should have high reliability and fast tripping.
4.4.5.1 Protective Techniques. Seven protective techniques are commonly used for
isolating faults on power lines:
(a)
Instantaneous overcurrent.
(b)
Time overcurrent.
(c)
Directional instantaneous and/or time overcurrent.
(d)
Step-time overcurrent.
(e)
Inverse-time distance.
(f)
Zone distance.
(g)
Pilot relaying.
All seven relay systems are used both for phase-fault and ground-fault protection.
4.4.6 Motor Protection. Motors must be protected against one or more of the following
hazards:
(a) Faults in the windings or associated circuits.
(b) Excessive overloads.
(c) Reduction or loss of supply voltage.
(d) Phase reversal.
(e) Phase unbalance.
(f) Out-of-step operation (synchronous motors).
(g) Loss of excitation (synchronous motors).
While protective relays may be applied to a motor of any size and rating, in practice, they are
usually applied only to the larger or higher voltage motors. The relays generally used for motor
protection are indicated in Table 4-1.
4-20
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