Page 16 of 27 MCM-Inst-1.10
MeterConfig registers have not been configured yet.
This can be corrected after installation using
the
CtDirections
register.
Install the CTs around the phase conductors to be measured as shown in section
above. Connect the CT leads to the MCM.
For revenue accuracy, use revenue-grade current transformers; other CTs are less accurate and may
not provide revenue accuracy. Contact sales for more information on appropriate CTs.
3.6.2 CT Wiring
The current transformers connect to the six-position black screw terminal blocks for each submeter
block. Connect the white and black CT wires to the terminals marked
CT1
,
CT2
, and
CT3
for an overview. Excess length may be trimmed from the wires if desired.
The white and black lead wires mark the CT polarity. The polarity for the MCM is marked with
WHT
and
BLK
on the MCM board and with white and black circles on the connection diagram label on the
inside of the front cover.
Connect each CT with the white wire aligned with the white dot on the connection diagram, and the
black wire aligned with the black dot. Be sure to fill in the meter and CT schedule for each CT; see
section
above. You will need this information for correct power measurement!
Torque the screws to 3.5 lbf·in (0.4 N·m).
3.7 Modbus RTU (RS-485) Communication
This section only applies for MCM-MB models (MCMxx-y-
MB
-zzzz) with Modbus RTU communica-
tion using the RS-485 physical link. The MCM-MB can be connected to PCs with RS-485 interfaces,
gateways, data loggers, and other devices that accept RS-485 Modbus RTU.
3.7.1 RS-485 Cables
There are many choices for RS-485 cabling. Ideally, use cable specifically rated for RS-485 applica-
tions as it will have the best impedance for long distances to minimize interference and reflections.
However, for distances up to 100 meters, especially at baud rates of 38,400 or lower, a special
RS-485 cable is normally not required. See
There are some standard conductor configurations for RS-485 cables:
●
Two conductors without shield: we do not recommend using this type
of cable because the MCM design expects the common terminal to be
connected.
●
Two conductors with a shield: use the two conductors for the
A-
and
B+
terminals, use the shield for
C
(common). If you connect the shield
to earth, only connect it at the Modbus master device.
●
Three conductors without a shield: use the three conductors for the
A-
,
B+
, and
C
(common). If two of the conductors are twisted together
as a pair, use those for the
A-
and
B+
terminals.
●
Three conductors with a shield: use the three conductors for the
A-
,
B+
, and
C
(common). If two of the conductors are twisted together as
a pair
, use those for the
A-
and
B+
terminals. The shield may be
connected to earth at the Modbus master device for improved noise
immunity.
●
If there are additional conductors, they may be left disconnected.
●
Use a twisted-pair cable to prevent interference.
●
For longer distances (>30 meters) or electrically noisy environments,
use shielded cable.
A
-
B
+
C
RS-485
MCM #1
RS
-485
A-
B+
C
MCM #2
RS
-485
A-
B+
C
Figure 9: RS-485 Wiring