IB02602004E
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Instructional Literature
Effective: May 2008
Page
149
Instructions for the FP-6000 Protective Relay
8.3 Overcurrent Functions – Instantaneous and Time
8.3.1 Instantaneous
The Instantaneous Trip occurs when any of the phase currents
exceeds the pickup current defined by the user. That is:
I
phase
> I
pick up
Instantaneous Trip may be set for a delay from zero (0) to 9999
cycles after the pickup current is exceeded. Since the current is
evaluated twice per cycle, the initiation of the trip action begins
within a cycle for a setting of 0. Completion of the trip depends
upon the usual electromechanical contact output delays in the
FP-6000 (included in specification of trip time accuracy) and in
the time for the breaker to open. The FP-6000 provides two timed
or instantaneous phase trip functions: 50P1 and 50P2. One of the
trip functions can be used as a truly instantaneous with zero
delay while the other can be used as a short delay function.
8.3.2 Inverse Time-Overcurrent Protection (51)
Three families of curves provide inverse time overcurrent
protection. The three families are: Thermal, ANSI and IEC curves.
The FP-6000 provides 10 standard Time overcurrent curves: Flat,
It, I
2
t, and I
4
t for Thermal; Extremely Inverse, Very Inverse, and
Moderately Inverse for ANSI; and IEC-C, IEC-B and IEC-A for IEC.
Thermal
Thermal curves include It, I
2
t, I
4
t and FLAT, and are defined by the
following equation:
T
= Trip Time in Seconds
D
= Time Multiplier Setting
M
= Slope (0=FLAT, 1= It, 2=I
2
t, 4=I
4
t)
K
= 3 for Phase, 1 for Ground
I
nom
= Nominal Current either 5 or 1 A
I
= Measured Current
Note:
The 5X multiplier in the FP-6000 Thermal Curve equation means
the phase trip time at 3 per unit current is 5 * D, where D is the Time Mul-
tiplier setting. The ground trip time at 1 per unit current is 5 * D.
ANSI & IEC
The ANSI Moderately Inverse, Very Inverse and Extremely
Inverse curves are defined by ANSI C37.112. The IEC curves are
defined by IEC 255-3. The ANSI and IEC families of curves are
defined by the same equation and differ only in the selection of
constants A, B and P as contained in Table 28. The trip time, T, in
seconds is defined by the following equation:
T
= Trip Time in seconds
D
= Time Multiplier Setting
I
pu
= Pickup Current Setting
I
= Measured Current
A, B, P = Constants
Reset
The reset function describes what occurs to the accumulated
tally of the TOC function if the current goes below pickup. The
accumulated tally is similar to how far an induction disk has
traveled if an electromechanical relay was being used. The time
delayed reset function provides improved coordination with
electromechanical and other relays. The reset function is very
important if the breaker re-closes quickly after tripping. The
FP-6000 allows a choice of three different reset functions:
Instantaneous Reset – If the current is less than the programmed
pickup current, the accumulated memory is erased.
Time Delay – This is exactly like the instantaneous reset except
the current must be below pickup for the set number of cycles
before the accumulated memory is removed.
Calculated – For the thermal curves, the following reset time is
given to zero the accumulated memory:
t
= Time to Reset Accumulated Memory
D = Time Multiplier Setting
K = 1 for Ground, 3 for Phase
Notice that the reset time is independent of the magnitude of the
current as long as the current magnitude is below pickup.
The reset function for the ANSI and IEC curves are given in the
following equations defined in ANSI C37.112. The constant t
r
is
defined in Table 39.
Gr = See Table 39
D = Time Multiplier Setting
I
= Measured Current
I
pu
= Pickup Current Setting
The constants for the indicated curve equations are contained in
Table 39.