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41-347.12C
CHARACTERISTICS
Taps are incorporated in the HU and HU-1 relays to
compensate for main current transformer mismatch.
These taps are as follows: 2.9, 3.2, 3.5, 3.8, 4.2, 4.6,
5.0, 8.7.
To measure the effective unbalance, a sensitive low-
reading voltmeter (5000 ohms per volts) can tempo-
rarily be connected across the operating coil resistor
(at top of case). With a perfect balance the voltmeter
reading will be zero. The reading should not exceed
the values indicated by the 15% mismatch curve in
figure 10 when the relay pickup is 0.30 times tap. If
the amount of mismatch is measured or calculated,
the measured voltage can be checked against the
interpolated value from the curve. For example,
assume that the larger restraint current is measured
at 1.5 tap multiple and the calculated mismatch is
7%. Then, from figure 10 the measured voltage
should be approximately 1.0 volts. Use figure 11 if
the pickup is 0.35 times tap.
Pickup of the harmonic-restraint unit and the differen-
tial unit is either 30% or 35% of tap value current.
Pickup of the indicating instantaneous trip unit is ten
times tap value current.
Components of the harmonic-restraint unit are
selected such that 15% second harmonic will prevent
operation of the unit. This factor is adequate to pre-
vent false operation on inrushes.
The frequency response of the HU and HU-1 relays
is shown in figures 13 and 14.
The indicating instantaneous trip contacts will close
30 amperes at 250 volts dc and will carry this current
long enough to trip a breaker.
T
C
RIP
IRCUIT
The main contacts will safely close 30 amperes at
250 volts dc, and the seal-in contacts of the indicat-
ing contactor switch will safely carry this current long
enough to trip a circuit breaker.
Trip Circuit Constants
Indicating Contactor Switch Coil
Ampere
Pickup
0.2
1.0
2.0
10
Ohms dc
Resistance
8.5
0.37
0.10
SETTING
CAUTION
!
Since the tap block screw carries operating cur-
rent, be sure that the screws are turned tight.
In order to avoid opening current transformer cir-
cuits when changing taps under load, the relay
must first be removed from the case. Chassis
operating shorting switches on the case will
short the secondary of the current transformer.
The taps should then be changed with the relay
outside of the case and then reinserted into the
case.
To set the relay, calculations must be performed as
shown under "Setting Calculations". After the correct
tap is determined, connections can be made to the
relay transformers by placing the connector screws
in the various terminal-plate holes in front of the
relay. Only one tap screw should be inserted in any
horizontal row of taps.
I
C
NDICATING
ONTACTOR
There are no settings to make on the indicating con-
tactor switch (ICS).
I
I
NDICATING
NSTANTANEOUS
No settings is required on the indicating instanta-
neous trip unit. This unit is set at the factory to pickup
at 10 times tap value current.
SETTING CALCULATIONS
Select the ratio matching taps. There are no other
settings. In order to calculate the required tap set-
tings and check current transformer performance the
following information is required.
Required Information:
1. Maximum transformer power rating (KVA)
2. Maximum external fault currents
3. Line-to-Line voltage ratings of power transformer
(V
, V
, V
)
H
I
L
4. Current transformer ratios, full tap (N
5. Current transformer "C" accuracy class voltage,
S
(ICS)
WITCH
T
(IIT)
RIP
M
)
T
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