High-voltage advice on irrigation wiring
Ask anyone involved with irrigation-system maintenance and they'll probably agree: Troubleshooting an irrigation electrical system is one of the most frustrating and time-consuming aspects of the job. As existing systems show their age, and as more sites install systems, the need to troubleshoot systems continues to rise.
A wide selection of specialized equipment is available for tracing wire, locating wire nicks and breaks, and finding "misplaced" solenoid valves. Nevertheless, you truly need a fundamental understanding of how electricity behaves in an irrigation system to most easily diagnose system ills. For example, being able to track wire with a specialized piece of equipment assumes that you've already established precisely why you need to track the wire. It's kind of like following a lion; it's important to understand why you're following it, and you'd better be equipped to deal with it once you've found it. Therefore, the best piece of specialized equipment for troubleshooting is - and always will be - the ability to logically think through a problem.
"Equip yourself with some mind tools," says Bill Derryberry, the author of Troubleshooting Irrigation Control Systems. According to Derryberry, "mind tools" are the most important tools an irrigation professional has. "We can't learn just a few quick tricks and then fake it....We must force our minds back to the fundamentals for a systematic, logical way of reasoning. When we can understand, we can remember, we can diagnose, we can see solutions," he says.
Think back to the first topic you ever tackled in an irrigation class. Chances are it was an introduction to basic hydraulics. The notion that you could proceed further in studying irrigation without this basic fluid-hydraulics knowledge was unthinkable. After all, sizing and routing pipe meant little until you mastered concepts such as operating pressures and friction loss. Today, even if you are an expert at holding a pitot tube and pressure gauge in a sprinkler's spray stream - no small task - evaluating an irrigation system's hydraulics would be difficult without a basic understanding of hydraulic principles.
Electricity basics A basic understanding of electricity, therefore, is an important prerequisite to efficient troubleshooting. Admittedly, the concept of electrons flowing through a conductor is a bit more difficult to grasp than water flowing through a pipe. This is in part due to the fact that few irrigation professionals have any difficulty locating 50 gpm surging out of a broken 2-inch mainline. Yet, while water escaping from a pipe is easier to spot than current trickling from a nicked wire, each behaves in a predictable and logical manner.
You measure the flow of electrons through a wire in amps and call it current. It is analogous to the gpm that flow through a pipe. For comparison: * Voltage moves current along the conductor in much the same way water pressure affects water flow. * Hydraulic systems experience pressure loss from the friction water experiences as it flows against pipe walls. So, too, "electrical pressure" - voltage - is lost due to resistance as current travels through a conductor. Understanding what happens to these three key measures of electricity - amps, voltage and resistance - under various circumstances goes a long way toward solving some perplexing irrigation riddles.
Let's continue our analogy to water moving through a pipe. Troubleshooting basic hydraulic problems is fairly straightforward. A break in a pipe results in lost gpm, a new - unwelcome - pond and insufficient sprinkler-head flow. In addition, undersized piping systems with corresponding high flow velocities may result in excessive pressure loss and, perhaps, too low an operating pressure at sprinkler heads.
Electrical systems behave in a similar fashion. To make this even more clear, let's consider another analogy. Imagine a newly paved double-lane bridge. Cars represent amps, gasoline energy represents voltage and the bridge's condition reflects changes in resistance. Many cars can pass across the new bridge quickly and efficiently, although it requires lots of fuel to move the cars across. They experience low resistance to passage across the bridge. With time, as potholes develop in the pavement, drivers exercise more caution and slow down due to the poor condition of the bridge "conductor." Fewer cars (amps) cross per minute due to the increased "resistance" of the roadway. Passage may, in fact, be reduced to a single lane each way while repair crews work to fix the bridge. Less gasoline energy, or voltage, is used because fewer vehicles travel the road as drivers search for shorter, alternate routes. If the old bridge finally just falls down, no cars (amps) can pass, and thus they require no gasoline energy (voltage) to move them across the bridge. Resistance to traffic flow across the bridge is total, or "infinite." If a new bridge is put into place that actually reduces the distance of travel, more drivers will follow this shorter route, which offers lower resistance. Therefore, the number of cars (amps) will increase, requiring more gasoline fuel (voltage).
With irrigation electrical systems, broken and severed wires result in open circuits, with corresponding resistance at high levels (infinity). No current flows across the break, and thus no voltage is necessary to push the current along. Small nicks in a wire or bad wire splices increase the level of resistance across the conductor, thereby reducing both amperage and voltage. You'll often hear these called partials; they represent one of the most perplexing problems to diagnose. Nicks in a common wire and a valve's station wire that touch can allow current to flow along a shorter circuit than if it traveled through the solenoid coil. This results in a short circuit. Current that follows this shorter path experiences lower levels of resistance due to the decreased length of wire path. As a result, both current and voltage increase. This is often the case in the valve solenoid's coil of fine copper wire.
In summary, open circuits result in infinite levels of resistance with no current or voltage passing across the break. Partials increase the level of resistance across the conductor. They decrease the normal level of amps and volts passing across the "bad bridge." Short circuits reduce the level of resistance through a circuit. They increase amps and voltage through the shortened circuit. Understanding these basic principles (mind tools, again) is a must before working on equipment.
Multimeter applications Perhaps the most common and indispensable tool for electrical-system troubleshooting is the multimeter. Pocket-size, digital multimeters (sometimes called digital voltmeters, DVMs or digital multitesters) are available for as little as $25. Every irrigation technician should have one. Simpler to use than the standard analog style, the units clearly express all measured values on a digital display. Not only are they less confusing to use than analog models, they are more accurate. Multimeters permit the user to measure amperage, voltage and resistance through a circuit.
Of course, while it's fine to be able to make the measurement, you still must understand what it means. With solenoids, for example, you must have a reference point to which you can compare the resistance readings. Levels of resistance through solenoids differ based on size, age and manufacturer. Generally, most solenoids measure between 20 and 60 ohms of resistance. If an irrigation system has 10 solenoid valves - each the same size and make - they probably will measure about the same resistance.
Keeping accurate records of solenoid resistance readings for valves on different systems is important. This knowledge lets you recognize when a change occurs. Perhaps, for example, you can manually activate a valve by bleeding off water at the top of the diaphragm. Nevertheless, the valve fails to operate when the controller supplies power. Determining whether the problem is in the controller, the field wiring or the valve can be tricky. A good place to begin is at the clock with a handy piece of equipment called a portable solenoid actuator. Using a portable battery, these devices provide 25 to 28 volts. Therefore, you can test suspect stations by disconnecting the common and station wires in question and reconnecting them to the actuator. This provides surrogate power to the valve. With the actuator hooked up, if the station works properly, then the problem probably is in the clock. If the station doesn't irrigate, check the clock to verify that the proper 25 to 28 volts is coming off the terminal strip for the station in question. If it is, then it's time to take a look at the valve and field wiring.
While you're at the clock, check the resistance of the problem circuit. If this system has valves that typically measure about 30 ohms of resistance, and the one in question measures only 8, suspect a short circuit. After all, lower resistance is due to shorter circuits. To verify this, go to the valve box and disconnect the field wires from the valve. Check the resistance directly at the valve. Confirm whether the short is in the solenoid. If so, replace the solenoid. If not, it's time to look closer at the field wiring.
If, however, you measured 100 ohms of resistance, the wire may have a bad wire splice or nick that is increasing the circuit's resistance. Again, go to the valve box. Inspect the wire splices from valve to field wiring. Bad splices probably account for most irrigation problems. Don't just look at the splices, however; remove the wires from their waterproof connectors and make sure the splices are sound. Of course, you may find the problem right there. Were waterproof connectors used? You might find someone previously used wire nuts, small, plastic "capsules" with a threaded metallic insert into which you insert two or more wires and twist to ensure a connection. Wire nuts alone, however - even with a foot of electrical tape - will not cut it; the splices will not be waterproof. (King Technology of St. Louis makes direct-bury, gel-filled wire nuts commonly called King Connectors.) Neglecting to use waterproof connectors for the sake of saving $1.50 per valve simply does not make sense. One callback to honor a warranty as a result of bad splicing techniques will not make anybody any money.
If direct-bury splice tubes were used, remove the wires from the tube and remove the wire nut. (You did find a wire nut, didn't you?) You may have to remove some electrical tape from the wiring that helped secure the wire union to the nut. (Using electrical tape here isn't a bad idea because it helps protect wires from pulling free from the nut as you push and shove splices within the valve box.)
Check the condition of the twisted wire union. It's possible someone used just the nut to twist them together - not lineman's pliers. It's also possible someone carelessly stripped the insulation from the copper and scored the wires. This could result in the wire snapping during twisting. (Waterproof connectors are available for irrigation that are similar to those used with low-voltage lighting. These connectors don't require you to strip off the insulation. The wires are inserted into the connector and the insulation pierced. (3M also makes a gel-filled irrigation connector.) With the wires still free from the splices, twist the common and station wires together to get a good contact. Return to the controller once again. Check the resistance through the circuit. With the solenoid removed from the loop, the resistance will be only that through the field wiring - probably just a couple of ohms. If the resistance measures more than this, it's time to reach for some of the high-tech tools that now are available to locate such problems.
High-tech troubleshooting tools Virtually all equipment for locating wire paths or finding nicks or breaks works by transmitting a signal from one end of the wire and receiving it at the other end. While manufacturers design some of these tools to perform one task - such as finding tiny nicks in buried wire - others are all-purpose troubleshooting tools.
Progressive Electronics (Mesa, Ariz.) produces a variety of specialized tools for irrigation-system troubleshooting. Their Model 521 can follow a wire's path as well as determine its depth. The unit also can find wire breaks, large nicks and solenoid valves. Using a battery-operated transmitter connected to the wire you want to trace, and an earth ground, the receiver meter/headset registers a pulsing signal or tone. When located directly over the path of a buried wire, the unit registers an absence of tone or "null." Any deviation from the wire path results in an increase in tone. You locate broken wires by listening for the null to disappear; this indicates the transmitted signal no longer can continue down the wire.
The Model 521 also determines wire depth. It first locates the wire path, then locates the wire perpendicular to the original path at a 45-degree angle. Doing so forms the hypotenuse of a right triangle having legs of equal length.
Other devices can track wire up to 5,000 feet long and up to 7 feet deep. They track a signal rather than a null. An inductive antenna is available that allows you to apply a tone to a wire from the ground rather than connecting directly to the wire. This is particularly useful for locating objects you want to avoid during an irrigation installation.
Still another potentially time-saving piece of specialized equipment is the Model 2003, or Pulser. Designed specifically to locate shorts to ground in buried wire, this device locates even the smallest of leaks in wire coating.
Of course, while an increasing array of impressive equipment now is available to assist in troubleshooting systems, our "mind tools" remain our greatest specialized tool. Knowing when and why to use a particular tool is as important as knowing how to use it. According to Ed Santalone Jr. of Atlantic Irrigation (Thornwood, N.Y.), "Time is a contractor's most valuable resource....Sometimes efficient troubleshooting means being able to take a step backwards away from a problem to thoroughly evaluate a situation before stepping forward to use a particular piece of equipment." Keep these mind tools in good working order by taking advantage of quality educational programming.
George Crosby is an associate professor of plant science at the State University of New York at Cobleskill, N.Y. He teaches courses in irrigation-system design, installation and maintenance in the college's Turfgrass Management and Landscape Development program.
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