GE STD15C Instructions Manual page 23

GEK-45307
This curve represents the 510 relay characteristic.
Measurement of a voltage across
studs 8
and 9 which
is 75% or less of the value given on the curve,
does not
necessarily indicate that the relay will operate at higher through-current values.
This is especially true where very high through-faults may cause saturation.
Small
rectifier-type AC voltmeters are suitable for the measurement of unbalance.
The voltmeter should not be left permanently connected, since the shunt current it
draws reduces the relay sensitivity.
PERCENT SLOPE SETTING
Scribe marks for 15, 25 and 40% slope settings are provided in both the STD15 and
STD16 relays. It
is
common
practice to use
the
25%
setting
unless special
connections
make it advisable to
use one of the others.
See the corresponding
heading under CALCULATIONS for further details.
OPERAT ION
TARGETS
Targets are provided for both
the seal-in unit and the instantaneous overcurrent
unit. In the event of an internal
fault, one or both of these units will operate,
depending upon the fault magnitude.
This will produce a target indication of the
particular unit which operates. After a
fault is cleared, the
target should be
reset by the reset slide located at the lower left—hand corner of the relay.
DISABLING OF TYPE STD RELAY
CAUTION
When bypassing a breaker for maintenance, it will be necessary to disable
[_
the relay to prevent false tripping.
If the disabling of the relay is done by a remote switch, rather than by removing
the relay connection plug, the following precautions should be taken:
1.
The relay must be disabled by short-circuiting studs 8 and 9 of the relay, or
by opening the trip circuit at stud 1.
2.
If the CT secondaries are short-circuited as part of the disabling procedure,
the trip circuit should be opened at stud 1, and studs 8 and 9 should be short-
circuited
first.
It is not sufficient to rely on short circuiting the CT
secondaries alone, because any difference in time of shorting them
may
cause
false tripping.
MAT NTENANCE
CONTACT CLEANING
For cleaning fine silver contacts, a flexible burnishing tool should be used. This
consists of a flexible strip of metal with an etched-roughened surface, resembling
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