NMC-64 Irrigation
Short Circuit Control
General Description:
The NMC-64 uses a fuse to prevent damage due to high current short circuits; if a
high current short circuit occurs the fuse either burns (in the plastic box version) or
pops (in the metal box version).
In order to enable the NMC-64 to continue operating functioning devices the NMC-64
incorporates a short circuit detection mechanism. When a short circuit is detected the
NMC-64 performs a test procedure to detect the faulty outputs, an alarm indicating
that a short circuit was detected is given and an events indicating which outputs are
faulty are registered.
Functionality:
When a short circuit is detected the system starts searching for the specific device
which is causing it. The relay cards are shut down one after the other starting from
card # 8 down to card # 1. When detecting a card that has a short circuit the relays are
tested one after the other and those that have a short circuit are disabled.
Once a short circuit occurs an alarm is generated (one for all devices) and an event is
given (
4.12 SYSTEM LOG) for each device that is detected with a short circuit.
Reset:
Outputs that are short-circuited (market “AL” in table 5.1 TEST > RELAYS ) are not
turned on automatically and can not be manually turned on. After fixing the short-
circuit reset the alarm (table
3.1 ALARM RESET), the outputs are now functional.
Short-Circuit Threshold Level (A/D):
The NMC-64 CPU uses the short circuit detection mechanism to measure the 24VAC
current load and translates it to an A/D reading of 0-999. The short-circuit threshold is
user defined and depends on various parameters; 24VAC transformer size, number of
devices to be operated simultaneously, power consumption per device, distance
between controller to devices, cables dimension and resistance, etc’.
Setting Short-Circuit Threshold Level:
• Operate the maximum amount of devices to be used
simultaneously (maximum
load possible).
• View measured A/D load on lower side of table
5.1 RELAYS and write it down.
• Calculating desired threshold level:
• If the measured A/D value is up to 60 add 30.
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