As with the power circuits, MorningStar MeterBus (RJ-11) and MODBUS Serial (RS-232 and EIA-485)
ports all share a common negative with the negative power circuits of the system. With certain
configurations involving these types of communications, single-pole battery breakers, switches or fuses
cannot be used on the negative power conductors of the system. This is because the disconnect
switches will be bypassed through the communications and damage can occur due to the unintentional
current path through the smaller communications wiring and circuits.
There are a few scenarios where using a single pole disconnect and/or overcurrent protection on the
ungrounded conductor could present a problem.
● EIA-485 MODBUS networks (Fig. 4)
● Ethernet MeterBus Converter (EMC-1) systems (Fig. 5)
● Systems which include Relay Driver(s) with an RJ-11 MeterBus connection (Fig. 6)
● RS-232 MODBUS connections to 3rd party devices
Note that the MeterHub provides an isolated connection between the MeterBus ports when creating a
MeterBus network. Therefore, MeterBus networks will not be problematic except where the direct
connection between a controller and a Relay Driver can cause harmful current through the MeterBus
connection.
The systems below are examples of where the use of a single-pole breaker would not be an option.
When in doubt it is best to use a double-pole breaker to avoid any common negative communications
issues.
Fig. 4 – Positive grounded TriStar MPPT/ TriStar system with EIA-485 network: The EIA-485 network
wiring creates a common negative connection between the controllers. The single-pole battery breakers
on the negative, ungrounded conductors as shown on the left will not break the negative connection to
the controller when the other controller is connected and could cause damage to the communications
circuits. Double-pole breakers which disconnect both the positive and negative conductors shown in the
right can be used in order to disconnect each controller independently without harm to the serial
communications.
