Rockwell Automation 140U Q, M frame CB Electronic RMS Trip Unit installation and operation - Manual (Page 7)

Page 7
For ambient conditions above 40°C and where the
maximum ampere rating plug has been installed, derating
of the circuit breaker frame should be considered to avoid
exceeding a safe terminal temperature operating range.
Consult Allen-Bradley for recommendations.
4.1 General
The Electronic RMS Trip Unit provides a tripping signal
to the flux transfer shunt trip when current and time delay
settings are exceeded. This is accomplished by employing
the Allen-Bradley custom designed integrated circuit
which includes a microcomputer to perform
its numeric and logic functions.
In the Electronic RMS Trip Unit, all required sensing and
tripping power to operate its protection function is derived
from the current sensors in the circuit breaker. The
secondary currents from these sensors provide the
correct input information for the protection functions, as
well as tripping power, whenever the circuit breaker is
carrying current. These current signals develop analog
voltages across the appropriate calibrating resistors.
The microcomputer, in cyclic fashion, repeatedly scans
the voltage values across each calibrating resistor and
enters these values into memory. These data are used to
calculate true RMS current values, which are then repeat-
edly compared with the protection function settings and
other operating data stored in the memory. The software
program then determines whether to initiate protection
functions, including tripping the breaker through the flux
transfer shunt trip device in the circuit breaker.
4.2 Overload Trip: In accordance with standards require-
ments, the trip unit initiates a trip of the circuit breaker
within two hours for an overload of 135 percent, and will
trip in less time for higher overload currents.
A “Thermal Memory” effect prevents the breaker from
being re-energized immediately after an overload. A
“cooling off” period of up to 5 minutes is required, which
allows time for cabling to cool off.
4.3 Short Trip: For short circuit
conditions that exceed the short delay pick-up settings,
the trip unit initiates a trip after a delay prescribed by the
ramp function for trip units with catalog number suffixes
LS, LSG. A flat response time delay action
is provided by trip units with catalog number suffixes LSI,
and LSIG unless the instantaneous
(I) setting
is selected.
4.4 Ground Fault Protection: When selected, ground
fault pick-up and time delay settings shown in Table 1-2
allow selective ground fault coordination with other circuit
protection devices.
5.0 PROTECTION SETTINGS
5.1 General
Prior to placing any circuit breaker in operation, each trip
unit protection setting must be set to the values specified
by the engineer responsible for the installation. The
available settings along with the effect of changing the
settings are illustrated in Figures 12-1 to 12-3.
The installed rating plug establishes the maximum
continuous current rating
(I,) of the circuit breaker. Short
delay current settings are defined in multiples of l
n
.
, .
One to four time and pick-up adjustment settings are
available depending on the particular trip unit purchased.
A rotary switch is provided for each setting. The rotary
switch is adjusted using a small flatblade screwdriver
(Figure 11).
5.2 Short Delay Setting
Seven settings are available that range from 2 to 8 (I,) as
shown in Figure 12-1. This feature is included on all
Electronic RMS Trip Units.
5.3 Short Settings
For catalog number LS and LSG the short time delay is an
ramp configuration with the actual time delay a
function of the trip current involved.
For catalog number LSI and LSIG the short time delay
gives a trip response with no intentional delay
5.4 Instantaneous Pickup Setting
For catalog number LSI and LSIG Instantaneous Pickup
(Instantaneous.) Short Delay Pickup (see paragraph 5.2)
then becomes Instantaneous Pickup.
5.5 Ground Fault Setting
Five settings ranging from 1 through 5 (xlG) are available
(see Figure 12-3) and correspond to the fixed ampere
values listed on the trip unit nameplate and in Table 1.2.
Note: These ampere values are always the same no
matter what rating plug is installed in the circuit
breaker.
40752-072(2) Effective 6/02
is a flat response. Four settings (I,.1, .2, .3 second)
are available (see Figure 12-2). The "I" setting
(Instantaneous).
is achieved by setting Short Delay Time to "I"