GE AKD-10 Application Manual page 25

at high short-circuit currents. This combination of trip char-
acteristics permits application of short-time delay trips to
override inrush currents to downstream loads and coordinate
with downstream current devices for lower fault current val-
ues. It also permits the use of instantaneous trips to provide
maximum system protection for high values of fault current.
This is called the zone-selective arrangement and is often
desirable when the load-center feeder serves a motor control
center or other large load.
Long-time and instantaneous trip characteristics (LI) are
often used on feeder breakers when short-time delay is not
required to coordinate with downstream devices. Depending
on the magnitude of fault current and the circuit impedance
between breakers, a feeder breaker with LI trips may also
be able to coordinate (at least partially) with a similar
downstream breaker also having LI trip characteristics.
Breakers with LI trips are sometimes referred to as fully rated
since they may have higher interrupting capabilities when
provided with instantaneous trips (LI or LSI characteristics).
The majority of breakers manufactured today, however,
have the same interrupting rating regardless of the trip
characteristic. Long-time and instantaneous trips (LI) could
also be used on main breakers when minimum breaker
interrupting time is required for the rare occurrence of a fault
on the switchgear main bus, or when the system design does
not require selective coordination.
Application tables
Application tables are based on the following factors:
• A three-phase bolted fault at the low voltage terminals
of the substation
• Transformer impedance listed in table (only source of
power to the bus is the substation transformer)
• Total connected motor kVA does not exceed 50% of
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transformer rating at 208Y/120 volts and 100% of
transformer rating at 240, 480, and 600 volts
• The motor contribution is taken as 2.0 times the rated
current of the transformer at 208Y/120 volts and 4.0 times
the rated current at 240, 480 and 600 volts
• Tabulated values of short-circuit current are in terms of
RMS symmetrical current per NEMA Standard SG-3
• Tables estimate short circuit duty (Reference GET-3550 for
short circuit calculation)
Example
The application tables make it easy to select the proper
General Electric breakers for use with distribution systems
using various trip devices. For example, a 1500kVA transformer,
with 750MVA maximum primary short-circuit available and a
480V secondary, requires at least a WPS-20 main secondary
breaker shown in column 7 of the 480V application table
(page 27).
Full utilization of a load center transformer with dual temper-
ature rise (such as 55/65°C) capability or forced (fan) cooling
capability would require a larger frame size breaker — or a
WPS-32 in this example.
The appropriate feeder circuit breaker is found in columns
8, 9 or 10, depending on the combination of long-time,
short-time and instantaneous trips required by the system
design. In this example, the same type breakers (WPH-08)
are required regardless of the type of trip device utilized,
although a larger frame breaker may be required in order to
meet the continuous load requirement of a particular feeder.