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Optimum system tolerance is achieved with single-string source circuits and large
numbers of bypass diodes. Field studies indicate that the best approach in large
systems is not to replace modules containing failed cells, but to design the system to
be tolerant to such failures.
10.5 SAFETY
Many safety issues are common for building-integrated, building-mounted or central
station grid-connected PV systems.
Safety aspects that need to be considered (Florida Solar Energy Centre, 1987; Abella
& Chenlo, 2004) include fire resistance, correct wiring, placement, grounding, and
security against local weather conditions, particularly wind. Modules can be graded
according to their effective resistance against severe, moderate or light fires (Florida
Solar Energy Centre, 1987).
Simple disconnection of large (high voltage) arrays from loads or inverters does not
make them safe since they are live whenever illuminated. Protection of the DC side is
controversial and regulations vary between countries. Floating (unearthed) arrays and
inverters are commonly used in Europe but earthing is mandatory in the USA.
Commercial systems are only now available to meet the various specifications that
differ from place to place. Some of these are listed in Appendix E.
The Australian Standard (Standards Australia, 2005, 2002a, 2002b) defines the safety
requirements in Australia. The inverter system must be able to be isolated from live
conductors by a labelled and lockable switch, which is lockable in the off position to
interrupt all live conductors. The inverter must be connected on the utility side of any
residual current devices (RCDs) rather than the provision of an RCD on the output of
the inverter. An isolation device is necessary on each inverter input from the array
and various labelling requirements are specified (Standards Australia, 2005).
Protective features are described in an Australian Standard (Standards Australia,
2005):
1. Blocking diodes and over-current devices—As with any generating system,
protection against large current flows must be in-built. Blocking diodes and
over-current devices (e.g. circuit breakers or fuses) are used for protection in
photovoltaic arrays. Blocking diodes protect against large current flows into
shorts to ground, while over-current devices provide fuse protection in the
event of failure of the blocking diode. Blocking diodes are not substitutes for
over-current devices and are optional (Ibid.). Refer to Standards Australia
(2005) or local standards for detailed recommendations and/or requirements
for various array arrangements. The discussion in Chapter 6 and by Standards
Australia (2005) also applies for grid-connected systems.
2. Array arcing—An open circuit in a high voltage branch of a solar array can
produce voltages higher than the 70 V required to maintain an arc, as
illustrated in Fig. 10.7. These can burn for hours, but can be prevented by
introducing redundant connections, to prevent open circuiting and parallel cell
connections.
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