A guide to fixing common problems with irrigation systems

Alistair Dunsmuir
By Alistair Dunsmuir October 30, 2011 14:06

A guide to fixing common problems with irrigation systems

Almost all modern irrigation systems rely on many pre-numbered decoders connected along a common two-wire path, each connected to a solenoid valve. The controller feeds typically 24V to 40V AC down the path, together with a digital signal commanding a decoder to turn on or off. The decoder, which has a number that matches the signal, obeys the command, all the others ignore it.

This scheme saves copper cables and with the right equipment is easy to repair, being only two wires rather than a huge bundle. Expansion of the network is easy, with further decoders and cable being spliced anywhere along the existing path.

The problem with any shared path is that a fault somewhere along the cable can sometimes bring down the whole system. However, with some low-cost measuring equipment and the following simple techniques, the fault can be quickly located.

Types of Faults:

A measuring meter that reads AC volts and resistance is a valuable tool in diagnosing faults. If a sensitive current measuring capability without breaking the wire is available too, the Leakage Clamp Meter becomes almost indispensable.

The meter’s AC volts (V~) are used to detect the location of high resistance joints and open circuits. Its resistance (Ohms, Ω) allows testing of solenoid coils. It can also be used for the measurement of end-to-end resistance of the cable.

Its Clamp Current Measuring (Ã) is used with great effect to detect the point of short circuits, abnormal currents in decoders and the whereabouts of earth leakage from the cable.

When a current flows, it produces a magnetic field. This is how the solenoid can lift its plunger. If a ring of magnetic material is placed around a wire carrying a current, it can be used to detect and measure that current. If the ring can open like the jaws of a crab’s claw, be placed around the wire, then closed, there is no need to break the wire to measure the current. Such a device is called a Current Clamp Meter. However, most clamp meters have been designed to measure hundreds of amps and are not sensitive enough to measure the current taken by an individual decoder. However a  Leakage Clamp Meter can easily measure to less than zero point one of a milliamp (0.1mA) and can be used to check a decoder’s standby current, which is often a reliable indication of its goodness. Also knowing the standby currents of decoders allows an estimate of the number on a branch of the cable! Most modern decoders take between 0.5mA to 5mA when not operating a solenoid.

Both voltage and resistance are measured using the red and black test leads. When measuring resistance (Ω), there must be no voltage on the circuit being measured.

Currents are measured by opening the red jaws by pressing the red trigger with the thumb and clamping the jaws around the wire. Make sure the jaws shut fully. Keep the open ends of the jaws clean and free from grit and water. A build up of rust or deposits will cause false readings. It is important to understand that if both flow and return wires carry the same current and are placed inside the jaws, the multimeter will read zero.

It is important to note that:

• Having a clamp meter means it is hardly ever necessary to cut wires to pinpoint faults.

• Making a general sketch of the wiring on a clipboard then writing down decoder numbers found and any line currents / voltages measured will sometimes help diagnosis later over a cup of coffee! The sketch, if filed, can help in subsequent visits.

• The Halving Procedure can minimise the number of measurements to pinpoint the fault. The procedure is:

Go to roughly the halfway point. Make a measurement. Decide which half of the run the fault is in. Walk to the point roughly halfway between the beginning / end of the faulty half. Make another measurement. Decide whether the half is between you and the end, or you and the place were you were last. Repeat the procedure. For example in a run with 20 boxes, the box containing the fault can be located in a maximum of five.

This and any other tests must be done with loop(s) broken as the ‘halving procedure’ doesn’t work on loops.

Measuring the standby currents down a run allows comparison between the number of decoders expected ‘downstream’ and the actual current. Most solenoids when energised take 150mA to 250mA.

Earth leakage faults should be cleared first (more on this later). The presence of this type of fault will impede the diagnosis of other faults. Also it may happen that a ‘short circuit’ and earth leakage are both from the same severed cable end!

With regards fault tracing short circuits, most controllers will refuse to power up a two-wire path that has more than a certain amount of load or leakage on it. Fuses may blow, software may shut the cable down, or even worse, a drive transistor in the controller may overheat. If at any time, faults are suspected, or the controller behaves erratically, it is best to test the wiring to the decoders using a power transformer and a current clamp meter.

A big power transformer, such as the one illustrated above, plugs into the mains and produces 30V AC at up to six amps to power up the two-wire path. The field wiring is removed from the controller and the transformer’s output is connected to it instead. Because of its size, the transformer can still produce a powerful voltage in the presence of quite serious field wiring faults. Because it lacks signaling circuitry, the transformer itself cannot turn decoders on or off. However, this is not a disadvantage for the sort of faults that would shut down or damage a controller.

When used with a current clamp meter, digital voltmeter and the good old Mk. 1 hand, the transformer allows fault finding with the minimum of effort and confusion.

Be careful not to exceed the transformer’s maximum current for more than a minute or so. Check the initial current with a clamp meter as soon as the transformer is connected to the wire path.

The transformer illustrated will produce around 30V AC at currents of at least 6A. This is enough to locate all but a dead short circuit on the wiring. The 30V is produced between the red terminal (live) and the black terminal (neutral). The 30V AC is divided in half at the yellow terminal (centre tap). That is, 15V AC from yellow to black and likewise 15V AC from yellow to red. This yellow terminal is useful in three-wire paths as used with some older decoder systems. The two green sockets are connected to the earth pin on the 13A mains plug.

The output of the transformer is isolated from earth unless the green wire is plugged into either the black, yellow or red terminal. Do not under any circumstances connect more than one of these terminals at a time to the green earth.

Ascertain the whereabouts of major branches of the cable run. Go to the junctions and determine which branch the short is in.

Having identified the branch, do a ‘halving procedure’ along the run to pinpoint the short. Be aware that all decoders do take standby currents, so a rough knowledge of the number ‘downstream’ will prevent chasing a phantom current drain.

Do remember to make a rough sketch of the cables and decoder locations. Write down the current readings on it as you go.

In the next figure, the thick connecting lines indicate higher-than-normal currents measured. Once you are past the short (somewhere in the first branch), the currents will either fall to near zero (if the voltage is cut off downstream) or go back to near normal.

To measure the short circuit currents, place the current clamp over just one of the power wires.

If the joints are made so that the individual wires in the main cable are not accessible individually, the main cable will have to be split open to reveal the individual conductors. Remember it is the currents in the main cable you are trying to measure, not those in the decoders attached.

Once the area of the short is found, the exact cause can be identified. Check for warm decoders or joints, especially if the short is several amps. Place the current clamp over individual wires to see which ones are carrying the current. Do not forget that a current flowing ‘out’ from the transformer must ‘return’ down the other conductor.

With fault tracing high resistance joints, the high resistance joints can be identified by connecting the transformer then measuring the voltage down the cable at each joint with the load probe and multimeter. If you do not have a load probe, get a helper to touch the wires of a spare solenoid to the multimeter probes while measuring the volts. Set the meter to Volts AC (V~). When the ‘apply load’ button is pressed on the probe or the solenoid is attached, the voltage will drop. A voltage drop exceeding about three or four volts indicates a high resistance in one of the joints. As you travel back towards the transformer, you will eventually pass the bad joint and the voltage drops under load will go back to normal.

In a two-wire system, just measure between the two joints in the box. An excessive voltage drop will tell you that one or the other side is high resistance, but not which side.

As before, the ‘halving procedure’ search technique can be used to reduce the number of measurements made.

With fault tracing open circuits, the open circuits are just an extreme case of high resistance joints! The techniques in the previous section apply. As before, the ‘halving procedure’ technique can be used to reduce the number of measurements made.

With fault tracing leakage to earth, when a cable or joint is not well insulated, some electricity can leak to earth. This causes problems for some controllers, either refusing to control at all, or sometimes giving erratic operation, leading to the controller being suspect.

Earth leakage must be repaired first as it can interfere with the diagnosis of other faults.

The transformer and the clamp meter can be used to easily find earth leakage. With one side of the transformer earthed, leakage currents can flow back through the ground causing unequal currents in the main two-wire path.

In the diagram below, point X represents a leakage point to earth through some value of resistance R2. R1 is representative of a quantity of decoders. Current flows ‘out’ of the transformer through C and splits at X to flow ‘back’ through A and B. The resistors R1 and R2 are effectively in parallel and see almost all the transformers voltage. The clamp meter will read the difference between the currents in A and C which is equal to that flowing in B.

To locate the leakage point(s), place the clamp meter over whole main two-wire cable. Connect the green earth link to the terminal on the transformer that produces the greatest reading on AC mA. Go out on the course placing the clamp meter round the whole main cable. When the leakage reading drops significantly you are past the leakage fault, that is it is between you and the controller.

The ‘halving procedure’ can be used to minimise the number of measurements made to pinpoint the fault.

In the diagram below, the clamp meter will read much lower when past the greyed box.

Phantom earth leakage (broken loops): When placed over the whole field cable, the current clamp will measure the current imbalance among the conductors. This is caused by some current flowing through the ground back to the transformer (one side of which will be deliberately earthed). However, another reason is cable loops.

Field cables are sometimes looped and connected back to themselves to lower their resistance, which means less voltage drop when solenoids are on. The currents for the decoder / solenoid can flow in both sides of the loop. If, however, one wire in one side of the loop is broken or has a high resistance joint, the current in it will favour the good side of the loop. We then have a situation where the total currents when measured in a cable are not equal and opposite. This will show up as a phantom leakage current which can be quite large.

The symptoms are as follows:

The ‘leakage current’ stays substantially the same if the earth connection is removed from the transformer.

Resolving the problem:

Break the loop (or loops). After breaking, the good half will have nearly full volts on it, the bad substantially less. If in doubt use the load probe.

Any other tests are best done with loop(s) broken as the ‘halving procedure’ doesn’t work on loops. When finally rejoining the loop(s), check the resistance and loaded voltage with the load probe.

Fault tracing decoders: Failed decoders that are taking a higher than normal standby (quiescent) current can be isolated using the current clamp multimeter, or detecting warmth with the fingers or lips. The leakage clamp meter can easily measure to zero point one of a milliamp (0.1mA) and can be used to check a decoder’s standby current which is often a reliable indication of its goodness.

Other than a high standby current, the best way to detect station failure is with the controller or a decoder programmer / tester. The suspect decoder can then be cut out and tested as shown below. This illustrates one such decoder programmer / tester made by the author’s company.

If  one like this is not available, the controller itself can be used with the decoder wired directly into the two-wire path terminals and a solenoid temporarily attached to give the decoder a load. Use manual-single station, set the required decoder number and then command the controller to run.

With the decoder disconnected, the resistance of the solenoid coil can be checked using the multimeter on resistance (Ω). Most solenoids have a resistance of between 25Ω and 55Ω, depending on the make.

Fault tracing solenoids: A failed solenoid is usually indistinguishable from a failed decoder as far as the controller is concerned.

With the controller set to energise the solenoid, (manual – single station), it is sometimes possible to probe the voltage output connections of the decoder with a voltmeter. If the 24-30V AC is measured, it is most likely an open circuit coil. A short circuit coil, although rare, will cause an excessive current to flow only when the decoder is activated. The best way to isolate this problem is to place the current clamp meter over one of the field wiring power conductors at the controller and observe the current as each decoder is turned on.

With these low cost test equipments and simple procedures it is usually possible to clear a fault in less than half a day, sometimes just half an hour.

Alistair Dunsmuir
By Alistair Dunsmuir October 30, 2011 14:06
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1 Comment

  1. Dominic February 24, 10:18

    There are recent technologies to bypass the use of wires coming from the IoT market ! The STREGA Time-Controlled Emitter is able to control any sprinkler valve up to 15km (10 mi.) from the gateway (the receiver): no more wires, no more decoder and…no more irrigation controllers required !

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