Rain Bird MDC2, Инструкция по эксплуатации, Страница: 91 / 124

Troubleshooting,
continued
85
Clamp Ampmeters normally measure only rapidly alternating currents. Navigate the MDC
Controller to the 50 Hz mode, causing the Controller to send 50 Hz voltage and current through
the two-wire cable. Please see
Finding a short in the system on page 40. You can measure this
current directly on the Clamp Ampmeter.
If the two-wire communications cable is installed as a ring, you must disconnect one end of the
ring from the controller before measuring the current.
If the controller is using more than one two-wire communications cable, you can use the
Clamp Ampmeter to find the two-wire cable that is using too much current. When measuring
the cables, pay close attention to the magnitude of the current. If you learn that none of
the communications wires are using too much current, then the short circuit may be in the
controllers lighting protection. To check this, disconnect all the communications cables and
test the terminals. If the condition remains after you disconnect the lines, the short is in the
lighting protection and you will need to replace the controller.
To locate the short, use the Clamp Ampmeter to “follow” the current from the controller to the
decoders in the field. The large current will run out from the controller on one wire to the short
circuit and back again to the controller on the other. The current running beyond the short
circuit in the cable will read zero (0) or have a very low reading.
If the cable splits into two or more branches, the current will only continue in the branch where
you will locate the short circuit.
If the fault is in a field decoder, you can measure the current in the decoder’s connecting
(blue) input wires.
Problem A two-wire communications cable shows signs of corrosion.
Solution To safeguard the two-wire cables against corrosion, their mean voltage relative to the
surrounding ground must be slightly negative. The controller initially determines this voltage. A
ground fault means that a current is running from the system’s two-wires through the ground
and back to the ground rod. If the mean cable-to-ground voltage were positive, a defect in the
cable’s insulation would result in a very destructive corrosion of the cable.
This ground current will not affect the mean voltage of the two-wires but it may overload the
controller if it gets too high.