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 c ircuit. Next, screw
the tap plug into tap marked for the desired current
and then r eplace 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 oper ating current of the unit may be brought
into agreement with the tap setting
if for
some reason, this adjustment has been
This adjustment also permits any desired setting
intermediate between the v arious tap settings t o
be obtained. The unit is adjusted at the factory to
close its contacts from any time-dial position at
a
minimum current within five percent of the tap
plug setting. The unit resets at 90 per cent of the
minimum closing value.
TIME SETTING
The s e t t i n g
of
the time dial determines the
length oftime 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 this setting gives the maximum time setting.
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;
movingthe magnet in toward the back of the unit de
creases the time, while moving it out increases the
time.
If
selective action of two or more relays is re
q u i r e d, determine the maximum possible short
circuit current of the line and then choose a time
value for each relay that differs sufficiently to insure
the proper sequence in the operation of the several
circuit breakers. Allowance must be made for the
time involved in opening each breaker after the relay
contacts close . For this reason, unless the circuit
time of operation is k n o w n with accuracy� there
should be a difference of about 0.5 second (at the
maximum current) between relays whose operation
is to be selective .
EXAMPLE OF SETTING
The time and current settings of the overcurrent
unit can be made easily and quickly. Each time value
shown in Fig. 1_ 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 recepta
cle and adjust the time -dial to the proper position.
The following example illustrates the procedure in
making a relay setting.
Assume a Type lAC relay is used in a circuit
where the circuit breaker should trip on a sustained
c u r r e n t of approximately 450 ampere s ; also, the
breaker should trip in 1.9 seconds on a short -circuit
current of 3750 amperes. Assume further that cur
rent transformers of 60/1 ratio are used.
Time Overcurrent Relays Type lAC
The current tap setting is found by dividing the
minimum primary tripping current by the current
transformer ratio. In this case, 4 50 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 3
J
50 am
peres, divide 3750 by the transformer ra�10 . . This
gives 62.5 amperes secondary current wh1ch 1s 7.8
times t h e 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 sec
conds time delay when the relay is set slightly above
the No. 6 time .. dial setting.
The above results should be checked by means
of an accurate timing device. Slight readjustment
of the dial can be made until the desired time is
obtained.
Aid in making the p r o p e r selection of relay
settings may be obtained on application to the near
est Sales Office of the General E lectric Company.
CONTACT ADJUSTMENT
The contacts should have approximately 1/32
inch wipe. That
the stationary contact should be
deflected about
32 inch when the disk completes
its travel. The contact wipe is adjusted by turning
the screws in the contact brush which r egulates the
position of the brush, in relation to the brush stop.
For r elays with two circuit -closing contacts, the
tips should be in the same vertical plane.
When the t i m e dial is m o v e d to a position
where the contacts just close, the time-dial scale
should indicate zero. If this is found incorrect, and
the brushes a r e correctly adjusted, r egulate the
dial to read z ero. This is done by changing the
position of the arm attached to the shaft whi«lt is
located b e l ow the t i m e dial. Loosen the screw
which clamps the arm to the shaft and turn the arm,
relative to the shaft, until the contacts just make at
the z ero time-dial setting.
INSTANTANEOUS ELEMENT
Select the current above which is desired to have
the instantaneous element operate and set the ad
justable pole piece so that its hexagon head is even
with the desired calibration on the scale. To raise
or lower the pole piece loosen the locknut and turn
it
up or down a�d then tighten in position.
The contacts should be adjusted to make at about
the same time and to have approximately 1/8" wipe.
This adjustment
can
be made by loosening the screws
holding the stationary contacts and moving the con
tacts up or down as required.
A·C TRIPPING ELEM ENT
The a-c tripping element should not require any
attention other than occasional cleaning of the con
tacts. However, if the adjustment should
be
lost,
it may be restored as follows :
1 . CONTACT .t\.DJUSTMENT
With t h e element de -energized, t h e movable
contact should lie against the stationary contact with
enough tension to always insure a good closed cir
cuit. The movable contact brush should be free of
any kinks. Also this contact brush should not toli�h
the compound bushing supported from the top of
armature . The
i
brass backing strip should be
1 3
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