55
Magnum PXR applications
MAGNUM PXR PRODUCT GUIDE
PA013006EN September 2022 www.eaton.com
5
Magnum PXR applications
Transformer breakers
Transformer breakers are required or recommended for
one or more of the following purposes:
•
To provide a one-step means of removing all load from
the transformer.
•
To provide transformer short-circuit and overload protec-
tion per NEC table 450.3(A) and (B).
•
To provide the fastest clearing of a short circuit in the
secondary equipment.
•
To provide a local disconnecting means for maintenance
purposes.
•
For automatic or manual transfer of loads to alternate
sources, as in double-ended secondary selective unit
substations.
•
For simplifying key interlocking with primary interrupter
switches.
•
To satisfy NEC service entrance requirements when
more than six feeder breakers are required.
Transformer breakers must have the appropriate interrupt-
ing and continuous current ratings. They should be able
to carry continuously not only the anticipated maximum
continuous output of the transformer, but also any tempo-
rary overloads. When sizing transformer breakers to allow
for temporary overload conditions, consideration should
be given to transformer capacity at various nameplate
temperature rise ratings and with any provided transformer
fan-cooling.
For a fully selective system, instantaneous protection on
transformer secondary main breakers can be turned OFF
so they can be fully selectively coordinated with down-
stream devices.
Bus sectionalizing (tie) breakers
The minimum recommended continuous current rating of
bus sectionalizing or tie breakers, as used in double-ended
secondary selective unit sub-stations, or for connecting
two single-ended substations, is one half that of the asso-
ciated main 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 interchangeable with the
main breakers, so that under normal conditions it will be
available as a spare main breaker.
In general, the tie breaker trip unit, like the main breaker
trip unit should have its instantaneous tripping set to
OFF to allow full selective coordination with downstream
devices.
Generator breakers
In most applications where generators are connected
through breakers to the secondary bus, they are used as
emergency standby sources only, and are not synchronized
or paralleled with the unit substation transformers. Under
these conditions, the interrupting rating of the generator
breaker will be based solely on the generator kVA and sub-
transient reactance. This reactance varies with the genera-
tor type and rpm, from a minimum of approximately 9%
for a two-pole 3600 rpm turbine 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
substation 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 protec-
tion only (instantaneous tripping set to OFF) 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 protec-
tion. Additionally, generators typically have very low short
circuit current availabilities. For this reason, generator
circuit breakers need to have wide ranges of adjustment
with low available short time pick-up and instantaneous
values to ensure proper 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 requiring a
3200A breaker or larger, voltage-restraint type overcurrent
relays (device 51V) are recommended.
Resistance welding
The application of Eaton’s Magnum PXR circuit breakers
to resistance welding circuits is shown in
Figure 74
of the
Continuous Current (I
n
) Selection Guide. I
n
ratings only are
given; the breaker frame must be selected as required for
interrupting ratings.
The Magnum PXR microprocessor-based true rms sensing
devices have a thermal memory and are well suited
for this service. The thermal memory functions prevent
exceeding the breaker and cable maximum permissible
thermal energy level. The circuit also replicates time dissi-
pation of thermal energy.
The size of the thermal memory is
30
T (I
r
/ I
n
)
2
unit
amperes 2 seconds. It fills at a rate of (
I
w
/ I
n
)
2
unit
amperes 2 seconds/second, trips at
3
0T seconds, and
empties at the rate of (I
r
/ I
n
)
2
unit amperes 2 seconds/
second, where:
T = Long time delay setting in seconds (range is 2–24
seconds)
I
w
= rms value of the welding current in amperes
I
n
= Rating current value in amperes
The memory is filled during the weld and empties during
the non-welding period of the duty cycle. These welding
applications are based on long delay and instantaneous
trip devices with the following settings. The long time
delay setting is based on the weld amperes and duty
cycle. Instantaneous trip setting is two times the average
weld amperes (weld amperes times percent duty cycle) or
higher.
N
ote:
Making Current Release (MCR) may prevent closing
the breaker during a welding cycle
