Technical Data
Effective: May 1998
Page
13
TD.44A.01.T.E
Cutler-Hammer
Magnum DS
Metal-Enclosed
Low-Voltage Switchgear
Application
System Application
Most Magnum DS
Switchgear is fed
from power transformers. To facilitate
minimum breaker sizing, Tables A1
through A4 Iists the calculated second-
ary short circuit currents and applica-
ble main secondary and feeder
breakers for various transformer sizes
and voltages.
The short circuit currents are calculated
by dividing the transformer basic
(100%) rated amperes by the sum of the
transformer and primary system imped-
ances, expressed in “per unit.” The
transformer impedance percentages are
standard for most secondary unit sub-
station transformers. The primary
impedance is obtained by dividing the
transformer base (100%) kVA by the pri-
mary short circuit kVA. The motor con-
tributions to the short circuit currents
are estimated as approximately 4 times
the motor load amperes, which in turn
are based upon 50% of the total load for
208 system voltages and 100% for all
other voltages.
High transformer impedances and/or
lower percentages of motor loads will
reduce the short circuit currents corre-
spondingly. Supplementary trans-
former cooling and temperature
ratings will not increase the short cir-
cuit currents, provided the motor loads
are not increased.
The tables do not apply for 3-phase
banks of single-phase distribution
transformers, which usually have
impedances of 2% to 3% or even lower.
The short circuit currents must be
recalculated for all such applications,
and the breakers selected accordingly.
Transformer Main Secondary Breakers
Transformer secondary breakers are
required or recommended for one or
more of the following purposes:
1. To provide a one-step means of
removing all load from the trans-
former.
2. To provide transformer overload
protection in the absence of an indi-
vidual primary breaker, and/or when
primary fuses are used.
3. To provide the fastest clearing of
a short circuit in the secondary
main bus.
4. To provide a local disconnecting
means, in the absence of a local pri-
mary switch or breaker, for mainte-
nance purposes.
5. For automatic or manual transfer
of loads to alternate sources, as in
double-ended secondary selective
unit substations.
6. For simplifying key interlocking with
primary interrupter switches.
7. To satisfy NEC service entrance
requirements when more than six
feeder breakers are required.
Main secondary breakers, as selected
in Tables A1 through A4, have ade-
quate interrupting ratings, but not nec-
essarily adequate continuous current
ratings. They should be able to carry
continuously not only the anticipated
maximum continuous output of the
transformer but also any temporary
overloads.
For a fully selective system, instanta-
neous protection on main breakers
should be defeated, as they typically
cannot be coordinated with down-
stream devices.
Maximum capabilities of transformers
of various types, in terms of kVA and
secondary current, are given in Tables
A1 through A4. It will be noted that the
maximum ratings will often require the
substitution of larger frame main
breakers than those listed in the tables.
Even if a self-cooled transformer only
is considered, it should be remem-
bered that with ratings of 750 kVA and
higher, provision for the future addi-
tion of cooling fans is automatically
included. It is recommended that the
main breaker have sufficient capacity
for the future fan-cooled rating, plus an
allowance for overloads, if possible,
particularly since load growth cannot
always be predicted.
The same considerations should be
given to the main bus capacities and
main current transformer ratios.
Bus Sectionalizing (Tie) Breakers
The minimum recommended continu-
ous current rating of bus sectionalizing
or tie breakers, as used in double-
ended secondary selective unit
substations, or for connecting two sin-
gle-ended substations, is one-half that
of the associated main breakers. The
interrupting rating should be at least
equal to that of the feeder breakers. It
is common practice to select the tie
breaker of the next frame size below
that of the main breakers. However,
many users and engineers prefer that
the tie breaker be identical to and inter-
changeable with the main breakers, so
that under normal conditions it will be
available as a spare main breaker.
In general, the tie breaker, like the main
breaker, trip unit should have its
instantaneous tripping defeated.
Generator Breakers
In most applications where generators
are connected through breakers to the
secondary bus, they are used as emer-
gency standby sources only, and are
not synchronized or paralleled with the
unit substation transformers. Under
these conditions, the interrupting rat-
ing of the generator breaker will be
based solely on the generator kVA and
sub-transient reactance. This reactance
varies with the generator type and
rpm, from a minimum of approxi-
mately 9% for a 2-pole 3600 rpm tur-
bine driven generator to 15% or 20% or
more for a medium or slow speed
engine type generator. Thus the feeder
breakers selected for the unit substa-
tion will usually be adequate for a
standby generator of the same kVA as
the transformer.
Most generators have a 2-hour 25%
overload rating, and the generator
breaker must be adequate for this
overload current. Selective type long
and short delay trip protection only is
usually recommended for coordination
with the feeder breakers, with the long
delay elements set at 125% to 150% of
the maximum generator current rating
for generator protection.
In the case of two or more paralleled
generators, anti-motoring reverse
power relays (device 32) are recom-
mended for protection of the prime
movers, particularly piston type
engines. For larger generators requir-
ing a Magnum MDS-632 or larger, volt-
age-restraint type overcurrent relays
(device 51V) are recommended.
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