Principles and practice of lightning protection in decoder based irrigation
control systems By Tony Ware B. Sc., Tonick Watering Ltd.
This technical article describes the two kinds of protection against lightning damage in decoder based two or three wire irrigation control systems.
When properly installed, lightning protection in decoder systems will generally be stronger than that found in smaller multi-wire systems. The exception is pedestal-mounted multi- wire, where extensive extra lightning protection components can be housed within the pedestal.
In decoder based systems, there are only two (or three) wires to protect, thus there is more space to fit tough protection components within the controller and in each decoder.
Although the eye only sees one flash, there are typically 5-10 such strikes in quick succession. No affordable lightning protection scheme will withstand such a direct hit.
The more common event is a voltage surge caused by a nearby strike. In this instance the GROUND goes up to a potential of between 6 thousand to 10 thousand volts. Flashover from the ground onto the wiring usually occurs at a wiring joint where the insulation is at its weakest. If this is a solenoid joint, the currents, typically 10,000A-30,000A, flow through the decoder, destroying it, and travel around the main cables seeking another weak point to go back to a ground which will be at a different potential.
Figure 1 illustrates three decoders interconnected using a 2 wire main cable. Normally the distance between each decoder would be tens or hundreds of metres.
The wires on some decoders do not come out of the other end, but this diagrammatic representation helps understand the principles better.
‘Main cable’ is the common 2 wire path around the site to all the decoders.
The black dots represent waterproof connectors, such as 3M DBY or Connector King equivalents.
When solenoids and decoders coexist in the same valve box, just one set of joints on the solenoid end is needed.
Where lightning strikes, the ground goes up to around 2 million to 4 million volts, with currents in the region of 25 thousand to 100 thousand amps.
Figure 2 illustrates these current paths.
Damage may also occur when lightning discharges between two oppositely charged storm clouds which are further apart than they are above the ground (Sheet Lightning). In this case, the cables pick up the surge directly, through a phenomenon called induction..
Many manufacturers of decoders rely on the ‘bypass’ principle, using earth stakes to shunt destructive currents around their decoders. In some form, they use Gas Discharge Tubes (GDTs) which conduct at around 90V - 120V. (Think of them as neon bulbs on steroids!). This voltage is above that used in normal irrigation systems’ operation, so they remain ‘out of circuit’ until a strike. These tubes may be incorporated inside the decoder, or be external. In all cases
PROTECTION 89
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