iSustain Technical Section
Cable Sizing
Because the voltages used in renewable energy genenerating
systems are quite low, usually from 12 to 110 volts, the currents
(measured in Amps) drawn by various appliances are proportionally
higher compared with 240 volt systems (Power = Volts x Amps).
As a result, the cables used for 240 volt systems are usually
unsuitable for use at lower voltages.
The cables used for low-voltage (or more correctly, extra-low
voltage) systems are often quite thick, especially if they
are to be quite long. This is because while losing 6 volts
from 240 volts is a mere 2.5% drop, losing the same in a 12
volt system is a full 50% voltage (and therefore power) loss
in the cable! So, the cables must be appreciably thicker to
reduce voltage losses, even though the cable's maximum current
rating may be far in excess of what will be drawn by the appliance.
A common example is lighting circuits. The cable often used
is 6mm², which has a current rating in excess of 50 amps
continuous. However, most extra-low voltage lamps will only
draw 2 to 3 amps, and the whole circuit might only draw 10
amps. However, for a 20 metre long circuit (ie, 10 metres
to the lights, 10 metres back to the battery), the voltage
loss at 10 amps is around 0.6 volts, or 5% of the voltage
at 12 volts.
This is considered the maximum allowable voltage drop in
a system. Another problem with switching extra-low voltage
DC is that the considerable greater degree of arcing means
thatswitches and circuit breakers must be designed to cope
with this. AC designed switches may not last very long when
switching DC currents.
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Lightning Protection
Lightning is a major cause of damage to inverters and sometimes
other equipment in RAPS installations and the results are
usually catastrophic. Seldom is the damage caused by a direct
strike, rather it is due to the surge, lasting only a few
microseconds, induced in electrical cabling in the vicinity
of the strike.
It is possible to protect against lightning damage by using
suitably rated and installed surge diverters.
The most common cause of failure of installed inverters is
due to surges caused directly or indirectly by lightning strike.
Experience has shown that the inverter is really the only
piece of equipment susceptible this way. Lightning protection
is covered by Australian Standard AS1768.
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