13 Feb Cable fault finding in irrigation wiring
Manually fault finding an irrigation wiring system can be a tedious and frustrating exercise. A good place to start is to ensure that there’s power at the control box and that the box is sending power to the irrigation system. Then, by working back from the irrigation system solenoids, you can establish if they are receiving voltage, and if not, why not. In most cases all that is needed is a good multimeter, an idea of how the system works, and a bit of patience.
Although this approach works well, it’s not much use when there’s an underground cable fault and often all you can do is randomly excavate the cable until you find the fault. Fortunately, there’s a better way and that is to use a low-cost cable fault locator.
Typical Underground Cable Faults
There are three types of faults that you’re likely to find underground. These are:
- Open circuit: This is common and usually happens when something breaks the underground conduit and severs one or more conductors. This fault appears as an open circuit when measured with a multimeter.
- Short circuit: A short circuit occurs if the insulation on the underground wires is damaged and the wires touch. This will most likely trip the circuit break or blow a fuse.
- Poor connection: Occasionally wires are spliced and then pulled into the conduit. If the splice is poorly made or damaged, the resistance between the ends of the wire increases and although there’s voltage at the end of the cable, the high resistance prevents solenoid operation. This fault can also occur if several strands of the cable are severed.
Fault Finding Problems
Once you know what sort of fault you’re looking for, you need to find the fault. There are two problems. Firstly, you need to find out where the cable goes. Although it may run in a straight line between two points, that’s not always the case, especially if there’s a good reason for a circuitous route. Fortunately, it’s possible to locate conductors in the control box using a tone generator connected at the other end of the cable, and a small portable detector that picks up its signal as it gets closer to the conductor with the tone on it.
Secondly, having traced the cable, it’s now necessary to find the fault. This can be done usinga cable fault locator that sends a signal down the cable and utilizes electrical reflection techniques; this provides an indication of how far the fault is from the start of the cable. This technique is known as Time Domain Reflectometry (TDR) which can be very accurate.
Snap Shot™ Fault Locator Simplifies Fault Finding
The Snap Shot Cable Fault Finder uses TDR and supplements it with a more advanced technique known as Spread Spectrum Time Domain Reflectometry (SSTDR). The reason for this is that TDR is susceptible to interference that can affect the accuracy of the measurement whereas SSTDR uses multiple frequencies to counter noise and gives a more accurate result.
The first task is to identify the Nominal Velocity of Propagation (NVP) of the signal through the cable. This differs from one type of cable to the next. Fortunately, Snap Shot has a table of common cable types on the back of the device. It’s also possible to measure the cable’s NVP using a short piece of the cable of known length.
If you need to repair an underground cable fault, the Snap Shot Cable Fault Finder helps you find the fault quickly and easily. Snap Shot can identify shorts and breaks to an accuracy up to 1 percent and it includes a convenient tone generator.