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CHAPTER 10: MISCELLANEOUS
10.4.1 Overview
10.4.2 MM3 Motor
Manager 3
Grounding
10.4.3 Grounding of
Phase and
Ground CTs
NOTE
10.4.4 RS485 Com-
munications
Port
MM3 MOTOR MANAGER 3 – INSTRUCTION MANUAL
10.4 Installation checklist
For proper and reliable operation of the GE Multilin MM3 Motor Manager 3 Motor Manager
3, it is imperative that the steps, recommendations and practices listed below be adhered
to at all times. This installation checklist has been compiled as a result of years of trouble
free operation by a variety of GE Multilin products.
Users are requested to ground terminals 21, 48 (safety ground) and 22 (filter ground)
directly to the ground bus using a heavy gauge wire or braided cable. Terminals 21, 22, and
48 will accept up to #12 AWG wire. These terminals must be grounded for proper filtering
of noise, and protection against transient conditions.
All external phase CT and ground CT secondary windings must be grounded to the ground
bus to keep the potential difference to a minimum. If the CT secondary windings are not
grounded, capacitive coupling could allow the CT secondary voltage to float up to the
voltage of the mains. This is a serious safety hazard.
Terminal 20 of the external ground CT is internally grounded therefore do not ground
terminal 19 since the ground CT signal would then be shunted.
It is also recommended that, in addition to the solid grounding of the ground CT described
above, a shielded twisted pair cable be employed when using the GE Multilin 50:0.025
ground CT. The reasoning behind this recommendation is that the 50:0.025 ground CTs are
typically used on high resistance grounded systems where the fault currents are limited to
less than 200 A. The alarm and trip levels on these systems are usually between 0.5 A and
15.0 A. This equates to a secondary current of 0.25 to 7.5 mA. Due to the very low levels
that must be monitored by the MM3 Motor Manager 3, any noise picked up by these
secondary wires must be kept to a minimum.
The MM3 Motor Manager 3 interfaces with PCs, PLCs, and DCSs using the Modicon Modbus
RTU protocol. The MM3 Motor Manager 3 supports Modbus function codes 01, 03, 04, 05,
06, 07, 08, and 16. The communications port is a very important part of the MM3 Motor
Manager 3s process and control applications. The port allows reading and writing of data
as well as full control to start and stop the motor from a remote location. For these
reasons, proper wiring practices are critical.
A shielded, twisted pair cable, such as 24 gauge Belden 9841 (120 Ω characteristic
•
impedance) or equivalent, MUST be used for the communications link. The cable
should be routed away from all power carrying cables, such as the motor mains,
power supply wiring, CT wiring and noisy contactors or breakers.
When using the GE Multilin 232/485 converter box at the master, GE Multilin
recommends placing no more than 32 GE devices on the same data link which
should be of no greater length than 4000 ft. The devices on the data link should be
daisy chained for reliable operation. Star or stub connections are not recommended.
If more than 32 devices are required to go onto the data link, or the distance must be
INSTALLATION CHECKLIST
10–9
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