Rain Bird MDC2 - Page 90

Troubleshooting, continued
84
Problem Using a systematic method to find a break in the two-wire communications cable.
Solution There is a systematic method to find a cable break, but it can be time-consuming. Measure the
voltage midway between the field decoder that is working and the one that is not.
If the voltage is out of the acceptable range (33 – 40 VAC), you can locate the break between the
field decoder that works and the point where you measured the line voltage. Measure the line
voltage again at the middle of this section of cable. If the voltage is still out of the acceptable
range, you have narrowed the area of the break even further. Continue to narrow the sections
this way until you find the break.
Problem Overloading the two-wire communications cable caused a short circuit.
Solution If the two-wire cable is damaged or a field decoder develops a defect, a short circuit can occur
on the cable. A short circuit can mean that nothing in the system works or only parts of the
system work.
If the MDC Controller senses a short circuit on the lines, it will automatically limit the current
to a safe value by changing the line frequency from 1 to 50 Hz. and display a Short Circuit
message. Please see Sensing a Short Circuit on page 55. When this happens, you must check
the line voltage.
If the line voltage reading is between 24 and 34 V, a field decoder, using too much power, is
probably causing the fault. If the system is still operational, you should test all the decoders.
If all the decoders pass the test, please see the problem, A two-wire communications cable
shows signs of corrosion on page 85.
If the line voltage reading is less than 24 V, you will not be able to use the field decoders to
locate the fault. This type of short circuit affects all parts of the cable by dropping the voltage
below the controller’s minimum operating voltage. Here, you will need to use a Clamp
Ampmeter to locate the short circuit.
A Clamp Ampmeter allows you to measure the current running in a cable without cutting the
cable. Attach the clamp to one of the two communications wires and measure the resistance. If
you clamp both wires, the reading will be zero (0).