Morningstar TRISTAR TS-MPPT-60 Instruction Manual

Morningstar Positive Ground Technical Note
Overview
Morningstar offers many solar charge controllers that are ideal for telecom applications. This can range
from low power requirements with the ProStar and ProStar MPPT controllers for 24V systems to higher
power 24V or 48V systems with TriStar and TriStar MPPT controllers. For very high power systems
controllers can easily be configured to charge the battery bank in parallel. Digital data output is standard
with these controllers for monitoring so they are ideally suited for telecom sites that need to measure
performance and available power.
A common system characteristic of these telecom systems is that they often operate on a positive
ground (PG) circuit. This is required because many DC telecom systems use PG equipment. Morningstar
controllers are oriented towards use in a negative ground system since these are most typical. The
negative terminals of Morningstar controllers are electrically common and charge control switching is
done in the positive leg of the circuit.
The common negative makes it possible to establish a common negative ground for the entire system
including the solar array, battery and load. The ground bond is almost always made with a single
connection to the negative conductor. One exception would be to provide a separate ground bond at the
PV array and another ground bond at the controller which can help keep voltage surges induced from
nearby lightning from affecting the power electronics equipment and loads. This is not an option for
systems that include ground fault protection as it will be tripped if there is more than one ground bond.
A Single Point to Earth
In addition to negative grounded systems, all Morningstar controllers can be used in positive grounded
systems if they are wired in the proper way. In addition, the positive ground of the system will not result
with a sustained ground of all of the circuits in the system like it would with a negative grounded system.
Positive grounded systems are basically wired the same way as a negative grounded system. The main
issue to keep in mind is that you cannot connect a positive ground bond to two separate circuits in the
system. This means if you ground the + leg of the battery, do not do so on the + leg of the PV array. The
same goes for the load. If you ground the + terminal of the load, do not ground the battery + or PV array
+ as well. Doing so will create a positive ground connection which will bypass the positive switching
circuit. This will result with a direct connection from the battery to the solar array (disabling charging
control) or a direct connection from the battery to the load (disabling load control).
The only exception to this rule would be to use a load control relay switch on the negative conductor of
the load circuit so that there would be a permanent common positive connection between the battery and
load. Morningstar controllers which include load control have a common negative with positive switching.

Positive Grounding Graphic Illustrations
Fig. 1 – This system is grounded properly on the battery circuit only. Charging and Load Control will
function properly.

Fig. 2 – This is another example of a PG system that will function properly
Fig. 3 – This system will not charge properly. It has been grounded in two locations and a ground loop
will be created around the TriStar charge controller. The controller will now no longer be able to properly
control charging current since its circuit has been bypassed.

Fig. 4 – Example with positive grounding for a ProStar MPPT system.
Positive Ground for the Telecom Load
In order to keep the telecom load grounded during operation the positive ground has to be connected to
the battery circuit or the load circuit as shown in Fig. 1 or Fig. 2. If it is grounded on the PV array circuit
the battery and load will not have a positive ground. Morningstar controllers that include load control
have a common negative so when it disconnects the load the load will no longer be grounded. However,
the telecom equipment should not require a positive ground when disconnected. If for some reason this
is considered a problem grounding the load circuit is an option.
Chassis/Enclosure Ground
For Morningstar controllers the chassis is isolated from the DC circuits of the controller. Therefore, the
chassis can be grounded the same for a positive ground system as it is for a negative grounded system.
Grounding the chassis is required by code in many jurisdictions. Surge protection devices include a
connection to the equipment ground in order to function properly to limit voltage surges from nearby
lightning strikes. Many Morningstar controllers include a chassis ground terminal. If it doesn't it is
possible to wire a equipment grounding conductor directly to the chassis. Please see the product manual
for more information about equipment grounding and follow local electrical code equipment ground
requirements.
Disconnect and overcurrent protection with Communications Wiring
The NEC and many jurisdictions allow for using a disconnect and overcurrent protection on the
ungrounded conductor. Therefore, single-pole battery breakers, switches or fuses may be installed on
the negative power conductors. However, this may not be an option with some systems depending on
the communications configuration.

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.

Fig. 5 – ProStar MPPT system with Ethernet MeterBus Converter (EMC-1): The EMC-1 power has a
common negative with the RJ-11 port. Therefore, using a single-pole battery breaker as shown on the
left will not break the negative connection to the controller 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 ProStar MPPT controller without
harm to the serial communications. Also note that the fuse for the power circuit to the EMC-1 should be
installed on positive wire.
Fig. 6 – TriStar MPPT and Relay Driver system with an RJ-11 MeterBus connection: The MeterBus
RJ-11 creates a common negative connection between the controller and relay driver. The single-pole
battery breakers as shown on the left will not break the negative connection to the controller 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.

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.