GEH-1753
Time Overcurrent Relays Type lAC
ADJUSTMENTS
TARGET AND SEAL-IN UNIT
For trip coils operating on currents ranging
from 0.2 up to 2.0 amperes at the minimum control
voltage, set the target and seal-in tap screw in the
0.2-ampere tap.
For trip coils operating on currents ranging
from 2 to 30 amperes at the minimum control volt
age, place the tap plug in the 2-ampere tap.
The tap screw is the screw hol.ding the right
hand stationary contact of the seal-in unit. To
change the tap setting, first remove the connecting
plug. Then, take a screw from the left-hand station
ary contact and place it in the desired tap. Next,
remove the screw from the other tap, and place it
in the left-hand contact. This procedure is neces
sary to prevent the right-hand stationary contact
from getting out of adjustment. Screws should not
be in both taps at the same time.
INDUCTION UNIT
CURRENT SETTING
The tap plug must not be removed without first
withdrawing the connecting plug.
The current at which the contacts operate may
be changed by changing the position of the tap plug
in the tap block at the top of the relay. Screw the
tap plug firmly into the tap marked for the desired
current (below which the unit is not to operate).
When changing the current setting of the unit,
remove the connecting plug to short circuit the cur
rent transformer secondary circuit. Next, screw
the tap plug into tap marked for the desired current
and then replace the connecting plug.
The pickup of the unit for any current tap is
adjusted by means of a spring-adjusting ring. The
ring may be turned by inserting a screw driver in
the notches around the edge. By turning the ring,
the operating current of the unit may be brought
into agreement with the tap setting employed, if for
some reason, this adjustment has been disturbed.
This adjustment also permits any desired setting
intermediate between the various tap settings to be
obtained. The unit is adjusted at the factory to close
its contacts from any time-dial position at a mini
mum current within five percent of the tap-plug
setting. The unit resets at 90 per cent of the mini
mum closing value.
TIME SETTING
The setting of the time dial determines the
length of time the unit requires to close its contacts
when the current reaches a predetermined value.
The contacts are just closed when the dial is set on
0. When the dial is set on 10, the disk must travel
the maximum amount to close the contacts and there
fore thi.s setting gives the maximum time setting.
12
The primary adjustment for the time of opera
tion
of
the unit is made by means of the time dial.
However, further adjustment is obtained by moving
the permanent magnet along its supporting shelf;
moving the magnet toward the disk shaft decreases
the time, while moving it away increases the time.
If
selective action of two or more relays is
required, determine the maximum possible short
circuit current of the line and then choose a time
value for each relay that differs sufficiently to in
sure the proper sequence in the operatio� of the
several circuit breakers. Allowance must 1\le made
for the time involved in opening each break
�
r after
the relay contacts close.
For this reason, unless
the circuit time of operation is known with accuracy,
there should be a difference of about
o;s
second (at
the maximum current) between relays whose opera
tion is to be selective.
EXAMPLE OF SETTING
The time and current settings of the over current
unit can be made easily and quickly. Each time value
shown in Fig. 2 indicates the time required for the
contacts to close with a particular time-dial setting
when the current is a prescribed number of times
the current-tap setting.
In
order to secure any of
the particular time-current settings shown in Fig. 2,
insert the removable plug in the proper
tap
re
ceptacle and adjust the time-dial to the proper
position.
The following example illustrates the
procedure
in
making a relay setting.
Assume a Type IAC relay is used in a circuit
where the circuit breaker should trip on a sustained
current of approximately 450 amperes; also, the
breaker should trip in 1.9 secon«;Js on a short-circuit
current of 3750 amperes.
Assume further that
current transformers of 60/1 ratio are used.
The current tap setting is found by dividing
the
minimum primary tripping current by the current
transformer ratio.
In
this case, 450 divided by 60
equals 7.5 amps. Since there is no 7 .5-amp tap,
the
8-amp. tap is used. To find the proper time-dial
setting to give 1.9 seconds time delay at 3750 am
peres, divide 3750 by the transformer ratio. This
gives 62.5 amperes secondary current which is 7.8
times the 8-ampere setting.
By referring to the
time current curves (Fig. 2), it will be seen that 7.8
times the minimum operating current gives 1.9
seconds time delay when the relay is set sllghtly
above the No.
6
time-dial setting.
The above results should be checked by pteans
of an accurate timing device. Slight readjustment
of the dial can be made until the de�ired time is
obtained.
Aid in making the proper selection of relay
settings may be obtained on application to the nearest
Sales Office of the General Electric Company.
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