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Directional Overcurrent Ground Relays
Types IRP, IRC and IRD
3.0 CHARACTERISTICS
The time characteristics of the directional overcurrent
relays are designated by specific numbers as indi-
cated below (e.g., IRD-8).
Time Characteristics
Short Time . . . . . . . . . . . . . . . . . . . . 2
Long Time . . . . . . . . . . . . . . . . . . . . 5
Definite Time . . . . . . . . . . . . . . . . . . . 6
Moderately Inverse Time . . . . . . . . . . . 7
Inverse Time . . . . . . . . . . . . . . . . . . . 8
Very Inverse Time . . . . . . . . . . . . . . . 9
Extremely Inverse Time . . . . . . . . . . . 11
The relays are available in the following current
ranges:
Instantaneous Overcurrent Unit
Range
0.5-2 Amps
0.5
0.75
1-4
1.0
1.5
2-8
2
3
4-16
4
6
10-40
10
15
20-80
20
30
Time Overcurrent Unit
Range
.5-2.5
0.5
0.6
2-6
2
2.5
4-12
4
5
The tap value is the minimum current required to just
close relay contacts.
The time vs. current characteristics for the time-over-
current unit are shown in Figures 16 to 22 (page 31
to page 37). These characteristics give the contact
closing time for the various time dial settings when
the indicated multiples of tap value current are
applied the relay.
Designation
Taps
1.0
1.25
1.5
2
2.0
2.5
3.0
4.0
4
5
6
8
8
9
12
16
20
24
30
40
40
48
60
80
Taps
0.8
1.0
1.5
2.0
2.5
3
3.5
4
5
6
6
7
8
10
12
4.0 TIME CURVES
The times curves for the IRD relay are shown in Fig-
ures 23 (page 38) and 24 (page 38). Figure 23 con-
sists of three curves which are:
1. Directional Unit opening times for current
and voltage polarized.
2. Directional Unit closing time for current and
voltage polarized.
3. Directional Unit closing time for 1 volt, volt-
age polarized.
Figure 24 shows the instantaneous overcurrent unit
closing time.
The voltage polarized curve B begins to deviate from
curve A for less than 5 volts.
Both the directional unit and the overcurrent unit
must operate before the trip circuit can be completed.
Hence, the unit which takes the longer time to oper-
ate determines when the breaker will be tripped. The
overcurrent unit contacts cannot operate until the
back contacts of directional unit open; therefore, the
total time for overcurrent unit to operate is its closing
time given in Figure 24 (page 38) plus the directional
unit opening time given in Figure 23. The total clos-
ing time for the directional unit is given in Figure 23.
The two examples below will serve to illustrate the
use of the curves.
Example 1: Using the formulas and definition of sym-
bols on Figure 23,
Let:
Ipol = 2 amps.
Iop = 2.31 amps.
Tap Value (T) = 0.5 amp.
φ - 40° = 0°
(For timing unit, assume CO-9 with 1/2 time
dial setting)
For current polarized relay:
(
φ 40°
lop lpol
cos
–
MPP
=
--------------------------------------------------
0.25
(
) 2 ( )
2.31
MPP
=
---------------------- -
=
18.5
0.25
Referring to Figure 23 at multiples of product pickup
of 18.5, the directional unit operating time is about 11
ms, and the closing time for this unit is 56 ms.
41-133S
)
3
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