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PO. Box 726 n TULSA, OK 74101 n TEL 918-627-5530 n FAX 918-641-7336 n www.nelsonheaters.com
GA-2318, R1
Sheet 7 of 15
May 2008
OPERATION
The SMMC-3 is a programmable controller, capable of controlling three separate snow/ice melting zones. A control relay
for each zone is included in the SMMC-3 to operate a contactor for each zone to energize the snow/ice melting
equipment. There are two operating mode selections possible with the SMMC-3:
MODE 1 – INDEPENDENT
In this mode each snow/ice melting zone is controlled independently. Mode 1 allows all 3 zones to be energized
simultaneously. This mode is best used where circuit loading is not a concern (e.g. adequate circuit ampacity is
available to operate the entire snow/ice melting load simultaneously).
MODE 2 – PRIORITY
In this mode each zone is controlled on a priority basis, with the most critical zone (always Zone 1) being melted first,
followed then by the less critical zones. Mode 2 allows only 1 of the zones to be energized at a time. This mode is best
used where circuit loading is a concern. A slight delay is provided when switching power between zones to ensure
circuits are not overloaded. Set-up in Mode 2 must be done either with Zones 1 & 2, (with Zone 3 not being used), or
Zones 1, 2 & 3. Operation is sequential, beginning with Zone 1. When Zone 1 is melted, the SMMC-3 de-energizes it
and then energizes Zone 2. However if snow/ice is detected on Zone 1, Zone 2 is de-energized and Zone 1 re-energized.
Similar logic applies for Zone 2 & 3; i.e. the lower numbered zone always takes priority.
The Priority mode available in the SMMC-3 can reduce circuit loading by splitting up a large snow melting area into
separate, smaller zones. For example, if a large area would require 90 Amps of current, this could be split into two
separate zones of, say, 50 Amps in one zone and 40 Amps in the other. Then, by programming the SMMC-3 in the
Priority Mode, only one zone will be enabled at any on time, resulting in a maximum circuit loading of 50 Amps.
Similarly, the area could be split into three zones of, say 25, 35 and 30 Amps; in this case circuit loading would be 35
Amps maximum. It should be further noted that when in Priority mode, the SMMC-3 always gives priority to Zone 1;
when Zone 1 is completely melted, then Zone 2 is enabled until melted, and then, finally Zone 3; i.e. Zone 1 always has
higher priority over Zone 2, which has higher priority over Zone 3. Further, if snow/ice is detected in a zone with higher
priority, then operation reverts to the zone with higher priority. For example, if melting has been completed in Zone 1,
and Zone 2 has been enabled, then if snow/ice is detected in Zone 1, operation in Zone 2 will be suspended, and Zone 1
will be re-enabled until melting is again complete, at which time melting in Zone 2 will recommence.
It is also important to give consideration to the assignment of zones; usually, high traffic areas will be given a high
priority, with lower traffic areas given lower priority. Zones can be easily reassigned at the terminals of the SMMC-3.