GE 239 Instruction Manual page 26

2 INSTALLATION
g) RTDS (OPTION) (48-57)
Up to 3 resistance temperature detectors (RTDs) must be supplied with the motor to use this option.
Verify that the RTD option is installed by noting that the product identification label on back of the
relay includes -RTD in the order code. When ordering a motor with RTDs, the 100 Ω platinum DIN
43730 type is the preferred choice for optimum sensitivity and linearity. Other RTDs that can be
selected and used with the 239 are 100 Ω nickel, 120 Ω nickel and 10 Ω copper. RTDs do not have
2
to be the same type, however the 239 must be 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 tempera-
ture 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 ambi-
ent 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 Sec-
tion 1.4: SPECIFICATIONS on page 1–7). 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 cor-
responding 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. Wher-
ever 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 ter-
minal (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 pre-
vents noise pickup that could otherwise occur from circulating currents due to differences in ground
potentials on a doubly grounded shield.
239
RELAY
SAFETY
GROUND
SHIELD
HOT
COMP
RET
2-10
Courtesy of NationalSwitchgear.com
MOTOR
STARTER
13
48
49
50
51
OPTIONAL GROUND
Shield and RTD com are
internally connected to
safety ground terminal 13
Figure 2–6: RTD WIRING
239 Motor Protection Relay
3 WIRE SHIELDED CABLE
Route cable in separate conduit from
current carrying conductors
RTD
TERMINALS
Maximum total lead resistance
IN MOTOR 25 ohms (Platinum & Nickel RTDs)
3 ohms (Copper RTDs)
STARTER
2 INSTALLATION
MOTOR
RTD TERMINALS
AT MOTOR
RTD IN
MOTOR
STATOR
OR
BEARING
819781AA.CDR
GE Multilin