Advertisement
Type JAC Time Overcurrent Relay
GEH- 1788
ADJUSTMENTS
TARGET
AND
SEAL-IN
UNIT
For trip coils operating on currents ranging
from 0.2 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
voltage place the tap screw in the 2-ampere tap.
The tap screw is the screw holding 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
stationary 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
necessary
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 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
current-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 tool 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 be
tween 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 minimum current
within five per cent of the tap plug setting. The
unit resets at 90 per cent of the minimum closing
value.
TIME SETTING
The setting of the time dial determines the
length of time the unit requires to close its con
tacts when the current reaches the 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 therefore this setting gives the max
Imum time setting.
The primary adjustment for the time of oper
ation 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 main shaft of
the unit 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
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 known with
accuracy, there should be a dhfference 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 over-
current unit can be made easily and quickly. Each
time value
shown
in Fig. 3 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. 3, insert the removable plug in the
proper tap receptacle and adjust the time-dial to
the proper position.
The following example Il
lustrates the procedure in making a relay setting:
Assume a Type [AC 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 1.0 second 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 amperes.
Since there is no 7.5 ampere
tap, the 8-ampere tap is used.
To find the proper
time-dial setting to give 1.0-second time delay at
3750 amperes,
divide
3750 by the transformer
ratio.
This gives 62.5 amperes secondary current
which is 7.8 times the 8-ampere petting.
By re
ferring to the time current curves (Fig. 3), it
will be seen that 7.8 times the minimum operating
current gives 1.0-second time delay when the relay
is set on 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.
13
Advertisement
