The Digitrip
RMS 31
0
Trip Unit is completely self-con-
tained and when the circuit breaker is closed, requires no
external control power to operate its protection systems.
It operates from current signal levels and control power
derived through current sensors integrally mounted in the
ci rcu i t b rea ke
r.
Digitrip RMS 310 Trip Units are suitable for
Hz AC
applications only.
The Digitrip
RMS 31
0
Trip Unit is available in 4 different
types. Each trip unit contains a fixed long delay time func-
tion (adjusted by changing the rating plug), and may be
equipped with a maximum of two phase and two ground
(time-current) adjustments to meet specific application
requirements. The types of adjustments available for
each model include the following, which are illustrated in
Figures 3.1 through 3.4.
Adjustment
Refer to
Figure
1)
Short Delay Pick-up
3.1
2)
Short Delay (Inst.)
3.2
Delay
Time
3)
Short Delay Pick-up/
3.3
Ground Fault Pick-up/
Ground Fault Time
4)
Short Delay (Inst.)
3.4
Delay
Fault
Time
1.2 Testing
Test points (Figs. 3.1 through 3.4) are provided for func-
tional field testing of the trip unit when connected to a test
kit (Catalog
No.
2.0 UL LISTED DEVICES
The Digitrip
RMS 31
0
Trip Unit is listed in accordance
with Underwriters Laboratories, Inc. Standard UL 489,
under file
and satisfies the applicable require-
ments of the International Electrotechnical Commission
(IEC) recommendations for molded case circuit breakers.
3.0 PRINCIPLE
OF
OPERATION
In open air at
an R-Frame circuit breaker with a
Digitrip RMS 310 Trip Unit installed will carry continu-
ously up to 1600,2000, or 2500 amperes without
exceeding a 50°C rise at the terminals. The calibration of
the trip unit is insensitive to ambient temperatures over a
range of
-20”
to
However, the trip unit contains
thermal temperature protective circuitry that initiates a
trip operation for self-protection
if the internal ambient
temperature at the printed circuit board (PCB) reaches
approximately 100°C. This may occur for open-air tem-
peratures above 40°C with circuit breaker currents near
full load.
For ambient conditions above 40°C and where the maxi-
mum 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 Cutler-Hammer for recommendations.
3.1
General
The Digitrip
RMS 31
0
Trip Unit provides a tripping signal
to the flux transfer shunt trip when current and time delay
settings are exceeded. This is accomplished by employ-
ing the Cutler-Hammer custom designed integrated cir-
cuit
chip, which includes a microcomputer to
perform its numeric and logic functions.
In the Digitrip
RMS 31
0
Trip Unit, all required sensing and
tripping power to operate its protection function is derived
from the current sensors in the circuit breaker. The sec-
ondary 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 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
repeatedly compared with the protection function settings
and other operating data stored in 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.
A green status light indicates the operational status of
the trip unit. If the load current through the circuit break-
of a properly functioning trip unit. If the status light is not
blinking, the current through the breaker may be less
than 20% of the maximum current rating of the trip unit
If the current exceeds 20% and the status light is not
blinking, use the STK2 test kit to investigate. IF THE
STATUS LIGHT IS ON STEADY, IT INDICATES A
Effective February 2006
I.L. 29C883E
Page 2
TRIP IS PENDING.
er exceeds approximately 20% of the maximum current
rating of the tirp unit, the status light will blink on and off
once each second. A blinking status light is an indication
