I.B. 17555
Page 18
Effective 03/97
SECTION 3: OPERATION
3-1 INTRODUCTION
This section specifically describes the operation and
functional use of the Digitrip 3000 Protective Relay. It
does not address in detail rear power connections and
DIP switch settings. These topics are covered in SEC-
TION 5 entitled “INSTALLATION/TESTING/STARTUP.”
It would be helpful, however, to become familiar with the
relay’s wiring diagram before proceeding with the rest of
this section (Figure 3-1).
3-2 POWER-UP AND SELF TESTING
When the proper AC or DC control voltage is applied to
terminals 5 and 6 of TB1, the unit will initiate a “Power
On Reset” to its chip circuitry. This causes the unit’s
firmware to perform some self-testing and initialization
of its ROM, RAM and E
2
(non-volatile) memory. If any
problem exists, a diagnostic message will be displayed
in the Settings/Test Time/Trip Cause Window. A com-
plete list of messages and their meanings are given in
Table 3.2. Additionally, if a problem does exist, the
“Operational LED” will light red and the “Protection Off
Alarm” relay will not energize. When all self checks are
good, the “Protection Off Alarm” relay will energize, and
the “Operational LED” will blink green.
3-3 PANEL OPERATIONS
Begin by reviewing the material presented in SECTION
2 entitled “FUNCTIONAL DESCRIPTION.” Since basic
definitions and explanations were given in SECTION 2,
no further explanation as to function will be offered in
this section. It is assumed that the operator is now famil-
iar with Digitrip 3000 terms, available settings and over-
all capabilities.
3-3.1 CHARACTERISTIC CURVE
Digitrip 3000 Protective Relays provide circuit breakers
with an extensive degree of selective coordination poten-
tial and permit curve shaping over a wide range. Available
pickup settings, inverse time overcurrent time multiplier
settings and inverse time overcurrent (phase and ground)
curve selections are addressed here with respect to their
effect on the resultant characteristic curve. In general,
there are three different families of curves to choose from
as shown in Table 3.1. These curves were discussed
briefly in Sections 1 and 2. The operating characteristics
of the relay are graphically represented by time-current
characteristic curves shown in Figure 3-2.
As shown in Figure 3-2, the ANSI and IEC “Curve
Shapes” are in terms of multiples of I
pu
(Pickup
Current of the CT Primary), whereas “short delay”
and “instantaneous” are in terms of multiples of I
n
(5A secondary of CT primary current). The thermal
curve is represented in terms of multiples of I
n
for its
curve shape, short delay, and instantaneous set-
tings. This must be considered in the coordination
study and in the programming of the Digitrip 3000
Protective Relay.
The ANSI curves are defined by ANSI C37.112 and IEC
curves are defined by IEC 255-3. These curve shapes
combine with the customized capability of the short
delay and instantaneous functions to allow for very ver-
satile coordinated protection schemes. The thermal
curve shape is also customized by the user to any
desired type of coordinated protection scheme.
These curves show how and when a particular relay will
act for given values of time and current. The more ver-
satile the relay, the easier it is to accomplish close coor-
dination and achieve optimum protection. Since the
Digitrip 3000 Protective Relay is very versatile, the
makeup of a typical curve is presented for clarification
purposes.
For the sake of simplification, the curve discussion will
center around a single line curve. Keep in mind, however,
that a characteristic curve in reality is represented by a
band of minimum and maximum values, not a line (Figure
3-3). Minimum and maximum values are generally the
result of tolerances introduced by the manufacturing
process for components and the relay’s accuracy. Any
expected value of tripping current or time could be the
nominal value anticipated within the plus or minus toler-
ance. The tolerances just mentioned are usually stated in
terms of the relay’s accuracy and frequently highlighted
on the actual working curves. Accuracy is stated in terms
Thermal
ANSI Curves
IEC Curves
Curves
(Per ANSI C37.112)
(Per IEC 255-3)
It
Moderately Inverse
IEC-A
I
2
t
Very Inverse
IEC-B
I
4
t
Extremely Inverse
IEC-C
FLAT
IEC-D
Table 3.1 Digitrip 3000 Curve Shapes
NOTICE
