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Type IAC77 and IAC78 relays are single- � h � s e, current operated, � -c d : vices � ith extremely inverse
time-current characteristics.
distribution systems against either multi-phase or phase to ground faults.
Each relay consists of a basic induction-disk time overcurrent unit, a dual-rated target seal-in
unit, and may include a hinge-type instantaneous overcurrent unit.
instantaneous unit are adjustable over a range of about 8 to 1.
units as well as information on their continuous rating, contact rating, and tap ratings of the target
seal-in unit are given in the s ection on RATINGS.
A standard Sl case is used to mount each relay.
are shown in Figures 11 and
The basic difference� between the relays covered by these instructions are noted in the following
T able I.
c:
Tne extremely inverse time current characteristics make the IAC77 and IAC78 relays particularly
well suited for the protection of primary distribution circuits.
relay characteristics closely parallel those of the power fuses normally employed in these circuits,
it is possible to obtain selective fault protection with a minimum time delay.
The unique time-current characteristics of these relays also make them well suited for picking up
cold loads . For any given cold load pick up capability, the resulting setting will provide faster protection
at high fault currents with the extremely inverse relay than with less inverse devices.
The general practice is to use a set of three relays for interphase faults.
residually connected, can be utilized for single-phase-to-ground faults.
is advantageous because it can be adjusted to provide more sensitive protection on
connections for applying these relays are shown in Figures 14 and 15.
In the application of IAC77 and 78 relays with automatic reclosing devices , the reset time should be
considered.
The reset time of all relays covered by these instructions is approximately 60 seconds from
the fully closed to the fully open position when set at the number 10 time dial.
When setting these relays to coordinate with "downstream" relays, a coordination time of from 0. 25
to 0. 40 seconds is generally allowed.
time, 0. 10 seconds for relay overtravel and 0. 17 seconds for safety factor.
clearing time is 0. 13 seconds (8 cycles) the coordination time would be 0.40 seconds.
is set by test at the current level in question, the safety factor may be reduced to 0. 07 seconds.
if the "downstream" breaker time is 5 cycles (0. 08 seconds) a minimum of 0. 2 5 seconds could be allowe d
for coordination.
clearing time of the fuse should be used and the time allowed for breaker clearing time should be omitted.
These instructions do not purport to cover all details or variations in equipment nor to provide for
every possible contingency to be met in connection with installation,
further information be desired or shoul · d p articular problems arise which are not covered sufficiently for
the purchaser's purposes,
To the extent required the products described herein meet applicable
but no such assurance is given with respect to local codes and ordinances because they vary greatly.
TIME OVERCURRENT RELAYS
TYPES IAC77A, 778, 78A , 788 FORM 800 and U P
They can be utlllZed to protect commerc1al, 1ndustnal, and utility pov1er
12.
INSTANTANEOUS
TY PE
IAC77A
IAC77B
IAC78A
IAC78B
These coordination times include, in addition to breaker clearing
If the downstream devices are fuses rather than circuit breakers , then the maximum
the matter should be referred to the General Electric Company.
GEK-34055
DES CR I PTI ON
The ava1lable ranges 1n amperes of both
The outline and mounting dimensions of this case
TABLE
I
CONTACT
UNIT
CIRCUITS
1
NO
YES
1
2
NO
2
YES
APPLICATION
In such applications, because the
3
Both
the time over � urrent unit and the
.
INTERNAL
CONNECTIONS .
FIG. 4
FIG. 5
FIG. 6
FIG. 7
A separate relay,
Use of a
separate ground relay
ground faults.
For example, if the breaker
If the relay time
operation or maintenance.
NE��
ANSI,
IEEE and
Typical
Then
Should
standards;
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