GE Digital Energy Multilin 239 Instruction Manual page 34

CHAPTER 2: INSTALLATION
programmed correctly so that each RTD input matches the installed type. The factory
default is 100 Ω platinum. RTDs are placed in the stator slots and/or motor bearings to
provide the required sensing signals to the 239 relay.
Up to 3 resistance temperature detectors (RTDs) may be used for motor stator and bearing
temperature monitoring. Since an RTD indicates temperature by the value of its resistance,
it is necessary to compensate for the resistance of the connecting wires, which is
dependent on lead length and ambient temperature. The 239 uses a lead compensation
circuit to cancel this lead resistance and read only the actual RTD resistance. Correct
operation will occur providing all three wires are of the same length and the resistance of
each lead is not greater than 25% of the RTD 0°C resistance (see Section 1.2: Specifications
on page –8). This can be accomplished by using identical lengths of the same type of wire.
Each RTD COM terminal is internally connected to the safety ground, terminal 13.
Consequently, where code permits, the 3 RTD terminals should not be grounded at the
motor end for the lead resistance compensation to work correctly. If 10 Ω copper RTDs are
used, special care should be taken to keep the lead resistance as low as possible. If no RTD
sensor is installed, the corresponding terminals may be left unconnected and the RTD
programmed as OFF.
Shielded, three wire cable must be used in industrial environments to prevent noise pickup.
Wherever possible, the RTD leads should be kept close to grounded metal casings and
avoid areas of high electromagnetic or radio frequency fields. RTD leads should not run
adjacent to, or in the same conduit as high current carrying wires. Use either
multiconductor shielded cable for all 3 RTDs or separate three wire shielded cable of #18
AWG copper conductors for each RTD. The 239 shield terminal (48) and each RTD COM
(51,54,57) are internally connected to safety ground, terminal 13. The shield connection of
the RTD cable should not be grounded at both ends. This arrangement prevents noise
pickup that could otherwise occur from circulating currents due to differences in ground
potentials on a doubly grounded shield.
FIGURE 2–7: RTD Wiring
ANALOG OUTPUT (OPTION) (18/19/20)
Terminals 18-20 of the 239 are available for a single analog current output of one
parameter. Verify that the Analog Output option is installed by noting that the product
identification label on back of the relay includes -AN in the order code. The choice of
output and current range is selected in
S1:239 SETUP\ANALOG OUTPUT\ANALOG
. Use the TYPE message to select one of the following for
OUTPUT TYPE & RANGE
2–28 239 MOTOR PROTECTION RELAY – INSTRUCTION MANUAL
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