IRRIGATION ISSUES: Satellites, decoders and disaster

Decoder irrigation control systems are being used increasingly around the world and there are idiosyncrasies that should be addressed. While decoder systems offer advantages in many areas, primarily cost, they fall short in lightning protection and survivability. When any decoder system is used for larger systems in hot, dry climates with the possibility of lightning, it’s a concern.

Here’s why. With decoder systems, you have no effective back-up watering in the event of a power surge or lightning strike, other than manually keying each sprinkle. If you lose a decoder, the communication cable or the central PC control system, you are done until the damage is repaired.

With multiple communication cables and other decoder challenges, repair could take days. This isn’t a problem in areas where 72 hours can go by without watering. But where conditions require a quarter-inch-plus of water every day, your facility could face disaster.

Decoder applications work best when designed around 500 sprinklers. If need be, the course’s critical areas can be hand watered on a 24-hour cycle. It’s inconvenient, but at least turf wouldn’t be lost. However, larger systems with 800 or even 1,500+ sprinklers don’t have enough people to "cover your grass."

Consider Catastrophic Failure
The greatest threats to an irrigation system are power surges, lightning damage, central-computer failure and communication cable interception. Each can put you into manual irrigation mode.

A stand-alone satellite system, though, does not require the computer to be online or even powered up for each satellite to perform. Each field satellite has a back-up program to coordinate and activate irrigation system cycles. Each satellite also retains the times to activate every zone, operating as a complete, integrated system.

This ensures all desired irrigation is performed through a perfectly managed hydraulic system maintaining pump output and efficiencies that protect the piping network’s integrity. In short, the satellite system could be unaffected by any of these threats and perform as problems are repaired.

What about Lightning Damage?
Let’s examine the differences between satellites and decoders when the skies are electric. Because each and every satellite is grounded at its exact location, about a third of the cost can be in surge protection and survivability components that provide lightning an immediate shunt to the ground. Every wire going in or out of a satellite is protected for a very high level of lightning resistance.

Even with the tangle of 24 VAC cabling out to sprinklers, each circuit can be highly protected through grounding at each satellite. For example, some field satellites provide no surge-protection options, while others offer lightning protection that is rated up to 5,000 volts (5 Kv). With advanced surge protection, every wire going in or out of the satellite is rated at more than 20,000 volts survivability (20 Kv). But not all satellites are the same, so do your homework.

Consider installation first when examining decoders. It is extremely expensive to establish a ground grid at each decoder location, so manufacturers specify that grounding and surge protection be added every 500-1,000 feet along the power/communications path. Think of this at the beginning and end of every hole, and on many; a third set halfway down the hole.

When lightning enters the wire path, it travels both directions until it finds the grounds and is dumped. This means that it will travel past and "touch" all decoders within a length of 1,000-2,000 feet. Since decoders are not individually grounded, they’re susceptible to these high-voltage surges.

Even more susceptible is the decoder’s output side, especially with multi-station decoders that can have 500 to 600 extra meters of wiring connecting them with sprinklers.

In lightning applications, most major manufacturers suggest installing single-station decoders at each sprinkler, therefore eliminating or minimizing any "output" side wire. While an improvement, this still leaves the decoder on its own should there be any localized lightning strike. Remember, the decoder is not grounded, so it has to absorb in its entirety all of the high voltage it receives.

Clearly satellite systems will be more reliable in high-lightning areas with back-up potential for watering in the event of damage. Every wire going in or out of a field satellite with 20Kv protection will be four to 20 times better protected than a decoder system, mostly because of grounding location and surge devices.

With decoder systems, a lightning strike can destroy any or all decoders within a 1,000-foot run. The result? Once decoders are damaged, part or all of your irrigation system is down – bring your shovels and test equipment. However, if a direct strike takes out a satellite, the rest of the system irrigates properly.

There are many considerations when selecting your irrigation control system, but certainly daily irrigation demand, lightning protection and budget balancing are at the top. As you dig in to evaluate your site, step back and assess your needs and risks before making that final decision.

Erik Christiansen is a licensed irrigator and president of EC Design Group, an irrigation consulting and water management firm based in West Des Moines, Iowa. A board member for the American Society of Irrigation Consultants, Christiansen can be contacted at erik@ecdesigngroup.com.