Fig. 7 – TriStar MPPT with an RS-232 connection to a 3rd party device: The 3rd party device includes a
common negative between the power and RS-232 port and will create a common negative connection
between the controller and device. The single-pole battery breakers as shown on the left will not break
the negative connections and it could cause damage to the communications circuits. A double-pole
breaker which disconnects both the positive and negative conductors as shown in the right can be used
in order to disconnect the TriStar MPPT controller without harm to the MeterBus communications. Using
an isolated RS-232 device is another option.
Ground fault Protection and Positive Grounded Systems
One last consideration is Ground Fault Protection. Ground Fault Protection protects the system from
ground faults that may be created in the PV circuit. Morningstar’s Ground Fault Protection products,
GFPD-150V and GFPD-600V can be used with either negative or positive grounded systems to detect
and interrupt ground fault currents. If the system is grounded at the load and the load is disconnected the
PV array and battery would not be grounded and the ground fault protection would not work. Therefore,
in order to operate effectively under all circumstances positive grounded systems should be grounded on
the battery circuit or PV array circuit and not on the load circuit when using a load controller which can
break the positive leg.
