Page 128 - Applied Photovoltaics

shapes and sizes. Care must be taken to ensure that devices used within the DC circuit
are rated for DC operation and for the potentially high DC currents possible in PV
systems. The detailed recommendations are addressed by Standards Australia (2005).
The recommendations differ greatly depending on system arrangement and ratings
and are not repeated here except as the general rule: “wiring shall be protected from
short circuit and overload by high rupturing capacity (HRC) fuses or appropriate type
circuit-breakers (AC or DC) sized to limit the current below the maximum current-
carrying capacity of any part of the connected circuit” (Standards Australia, 1999–
2000a). Battery over-current protection devices should be mounted as close as
practicable to the battery while avoiding any chance of spark ignition of any evolved
hydrogen (Standards Australia, 2002). Each string of cells must be separately
protected. Standards Australia (2005) also recommends earth fault protection.

6.8.3 Switches
Switches, which may sometimes take the form of circuit breakers or fuses with
removable elements, are installed to isolate the array, battery, controller and load.
They should be installed in suitable enclosures for protection against the environment.
DC switches are heavy duty and more expensive, and care must be taken that AC
switches are not used for DC applications. Switches should be rated for 1.2 times the
array open circuit voltage and should interrupt all poles. Specific recommendations,
which depend on system arrangement, are given by Standards Australia (2005) and
requirements and satisfactory circuit configurations are specified by the design
standard (Standards Australia, 2002). Safe isolation must be provided between extra
low voltage (up to 50 V ac or 120 V dc ) circuits and higher voltages, and provision must
be made to de-energise the system in an emergency (Standards Australia, 1999–
2000a, 2005).

6.8.4 Connectors
Poorly made connections are by far the greatest source of reliability problems in
photovoltaic systems. To minimise problems:
1. Ensure the connector and wire sizes are compatible.
2. A ring type connector should always be used in preference to a spade-type
connector (it cannot fall off the terminal).
3. Strip about 1 cm of insulation from the wire, clean with solvent if necessary
and use a crimp tool to attach the connector to the wire.
4. Make connections between subsystems using terminal strips in weather-
resistant boxes. Terminals and connectors must be of the same type of metal.

5. Look carefully for places where bare wire might touch metal at a different
potential.
6. Check for nicks or cuts in the wire insulation.

7. At completion, inspect all connections again.
8. Battery cable terminations should be crimped, and bolted battery connections
should use stainless steel bolts, nuts, washers and spring washers (Standards
Australia, 1999–2000b).

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