Page 127 - Applied Photovoltaics

6.8 BALANCE OF SYSTEM COMPONENTS
As the above discussion has shown, there are many other costs associated with the
establishment of a photovoltaic power supply, other than those directly attributed to
the solar modules. Batteries, regulators, inverters and other system components are
collectively referred to as the balance of system, or BOS. There are also such items as
transportation, installation, land (if necessary), site preparation, drainage, electrical
wiring, mounting structures and housing. These non-PV costs are expected to
dominate, as PV prices fall. Photovoltaic system field downtime is dominated by
BOS component problems and failures (Durand, 1994). The necessary increased
emphasis on BOS reliability in the future may add to their relative cost, although that
cost may be offset by economies of scale as PV systems gain more market acceptance
with improved reliability.
Frequently used BOS components to be considered here include constituents of the
electrical wiring system, mounting and housing. The emphasis for these components
is on reliability and long life, with the aim of providing trouble-free operation for at
least 20 years at reasonable initial cost.
Many BOS aspects are now covered by standards in Australia and elsewhere and
these should always guide component selection and use (Standards Australia 1999–
2000a, 2005). Local standards should always be observed or, if no local standard
exists, reference may be made to others (Appendix E).

6.8.1 Wiring
Wire costs can become quite substantial, particularly for low voltage, high current
applications, or where current must be conducted over long distances. Ideally, copper
wire should always be used in PV power systems, although it is more expensive than
aluminium wire. However, given the cost difference, aluminium wire may be
acceptable for very long runs, provided there is no interconnection with copper wire,
and connectors specifically designed for aluminium wire are used.
When designing a system, wire cross-sections should be selected to limit resistive
losses to less than 5% between the PV array and the battery and between the DC
control board and DC loads, and less than 2% between the battery and the DC control
board (Standards Australia, 2002). Exceeding current ratings of wires can lead to
overheating, insulation breakdown and potentially to fires. The draft Australian
Standard for array installation (Standards Australia, 2005) and an appendix to the
System Design Guidelines (Standards Australia, 2002) specify how cable sizes and
insulation quality should be chosen, according to system voltage, voltage drop,
current-carrying capacity and the trip levels of protection devices. Issues related to
low voltage wiring and other aspects of small systems are discussed by Roberts
(1991).
2
Cable size is specified according to its size (in mm ) and its stranding (number of
individual wires and their diameter), with larger cable used as current increases.
Wiring should be protected from vermin attack.

6.8.2 Over-current protection
As in all electrical systems, over-current protection devices, such as circuit breakers
or fuses, are needed to protect equipment and personnel. They are available in many

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