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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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iSustain Technical Section
Remote Power Supplies - iSustain Australia