WarmlyYours SCP-120 User Manual

Automatic Snow/Ice Melting System Control Panel

MODEL SCP-120 (APS-3C) SNOW SWITCH

Installation and Operation Manual

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Safety

Make all electrical connections in compliance with the National Electric Code (NFPA 70)
and local electrical code. If you have questions concerning the installation or application, contact
Customer Service.

General

Introduction

The SCP-120 Series Snow Switch Control Panels, when used with compatible sensors, auto­matically controls snow and ice melting systems, ensuring complete snow and ice melting at minimum
operating costs. Typical applications include pavement, sidewalk, loading dock, roof, gutter and down
spout snow/ice melting.
The SCP-120 provides a relay closure interface for use with energy management comput­ers (EMC). This feature can also be used for general purpose remote control annunciation and other
advanced applications. Simple remote control features are also provided by the RCO Remote Control
for the SCP-120.
All sensor and communications wiring is NEC Class 2 (high voltage). This simplifi es installation while
enhancing fi re and shock safety.

Environmental Interfaces

The SCP-120 determines when to start heater operation by monitoring the signals produced
by up to six customer supplied environmental sensors paralleled together using a three-wire bus.
Available sensors include:

• AIR-SS Aerial Snow and Ice Sensor
When used either alone or in combination, these sensors accurately determine whether or
not snow and ice melting is required. This data is communicated back to SCP-120.
Sensors measure ambient temperature and detect moisture, in any form. Snow, sleet, freez­ing rain, etc. is assumed if moisture is present at temperatures below 38°F (3.33°C). Beginning heater
operation at temperatures slightly above freezing is essential to meeting customer expectations. It
takes a long time for snow to melt at 34°F.
Sensors employ a heated interdigitated grid for moisture detection. Heat melts frozen pre­cipitation to form water which is a better conductor of electricity. Circuits detect water as a change in
resistance between the fi ngers of the interdigitated grid. The temperature of the moisture sensor is
regulated electronically.
Each sensor has its own microcontroller for signal processing, logic and control. This en­ables the use of a simple 3-wire bus to connect sensors with the control. Extension wire function and
colors follow:

• Supply(Red)

• Signal(White)

• Ground(Black)

The supply voltage is nominally 24 VAC. The signal is inverted. That is, snow produces a
ground and its absence produces 24 VAC rectifi ed. Sensors are wired together in parallel in a “wired
OR” confi guration. That is, red to red, white to white and black to black. When several sensors are
connected in parallel, any sensor asserting a ground on the signal conductor enables snow melting
heater operation. No ground on the signal line indicates the absence of snow. The SCP-120 includes
a temperature sensor for measuring pavement slab or ambient air temperature. Its signal is used to
provide an adjustable high limit temperature function.

High Limit Sensor

The calibrated 40°F to 90°F (4°C to 32°C) high limit sensor prevents excessive temperatures
when using constant wattage and MI heaters. It also permits safe testing at outdoor temperatures too
high for continuous heater operation. The temperature sensor is included.
There are two DIP switch confi gurable operation modes for the high limit thermostat. The fac­tory default operation mode uses the high limit thermostat as a slab temperature regulator, preventing
heater operation at temperatures above the set HIGH TEMPERATURE LIMIT. The optional operation
mode uses the high limit thermostat as an ambient air sensor, preventing heater operation at temper­atures above the set HIGH TEMPERATURE LIMIT until the temperature comes back within limits.
The details of operation in each mode are as follows:

Slab Regulating Thermostat Mode

• High temperature causes unit to turn off heaters, if running, and to ignore any call for heater
operation from the panel, RCO or EMC.

• High temperature continues any hold-on cycle that was initiated before the high temperature
condition. If the slab temperature drops within limits during the hold-on time the heater will be turned
back on.

• The HEATER CYCLE functions normally.

Ambient Temperature Sensor Mode

• High temperature causes unit to turn off heaters, if running, and to ignore any call for heater
operation from the panel, RCO (RCU) or EMC.

• High temperature cancels any hold-on cycle that was initiated before the high temperature
condition.

• If the HEATER CYCLE switch is operated in a high temperature condition the heater(s) will be
turned on for a maximum of 30 seconds. A new HEATER CYCLE can not be initiated for another two
minutes after that.

Initial Settings

When fi rst placing the system in service, the hold-on time should be set to an initial value.
Three to fi ve hours is suggested as a starting point. If the heaters turn off before the snow is com­pletely cleared and the melt water evaporated, increase the hold-on time by an hour or two. If the
heaters operate for several hours after the pavement is clear and dry, decrease the hold-on time by
an hour. Continue this process until satisfactory performance is achieved.

The small amount of energy wasted by a slightly excessive hold-on time is secondary to com­plete snow clearing and melt water evaporation. Incomplete operation is not desirable since
this can result in re-freezing melt water which creates a slippery ice fi lm.

If The System Turns Off Too Soon

If the system turns off before the pavement is clear and dry, toggle the heaters on for the hold­on using the HOLD-ON switch on the front panel of the control. Repeat as many times as is neces­sary until the pavement is clear and dry. If the heaters remain operating after the pavement is clear
and dry, terminate the hold- on cycle with the HOLD-ON switch. Normal operation resumes if it starts
snowing during hold-on.

Energy Management Computer (EMC) Interface

The SCP-120 interfaces an EMC via relays. Inputs from the EMC include OVERRIDE ON
which causes heater operation and OVERRIDE OFF which inhibits heater operation. These functions
are independent of weather conditions and the status of the hold-on timer. The interface provides fi ve
system status contact closures for the EMC including SUPPLY, SNOW, HEATER, ALARM and TEM­PERATURE LIMIT.
Absent signals from the EMC, the SCP-120 control panel controls the heaters based on envi­ronmental conditions. Automatic snow melting control is the default condition of the system.

SCP-120 Front Panel

The SCP-120 has indicators, adjustments and a switch for local control of the snow melting
system. Indicators include LED lights for SUPPLY, SNOW, HEAT, EMC, TEMP LIMIT and GFEP.
Adjustments provided allow for the calibrated adjustment of system HOLD-ON time from zero to ten
hours (or off) and TEMP LIMIT for the high-limit temperature adjustment with a range of 40° to 90°F
(4° to 32°C). The HEATER TOGGLE switch allows for the starting and stopping of a manual HEATER
CYCLE.
Figure 2 shows the SCP-120 front panel layout.

Figure 2: SCP-120 front

panel detail

RCO Remote Control Unit

The RCO Remote Control Unit is used with the SCP-120. It adds remote control and status
display to the SCP-120 control at a location convenient to personnel capable of observing snow
melting system operation.
Snow, slush or ice, either alone or in combination, must contact at least one sensor to start
melting. Heater operation continues until all sensors are dry. Depending on the rate of fall, snow
density, wind velocity, and other factors, heater operation must continue for a period of time after the
last sensor dries off. Slush tracked by vehicle and pedestrian traffi c along with blowing and drifting
snow are problems that are hard to predict.
The cycle timer in the SCP-120 begins when the last sensor dries off and continues for an
adjustable period of up to 10 hours to keep the heaters operational until the pavement is completely
dry. Otherwise, residual water could re-freeze and create a hazardous condition.
The RCO provides a two, four, six or eight hour CYCLE TIME adjustment that is independent of the
cycle time in the SCP-120. This allows treatment of the condition requiring an additional heater cycle
as the exception rather than the rule in order to minimize energy use.

Operating the HEATER CYCLE switch operates heaters for the CYCLE TIME which is nor­mally set to 2 hours. Operating the HEATER CYCLE switch during the cycle time stops the timer. If
the pavement or ambient temperature exceeds the SCP-120 HIGH LIMIT TEMPERATURE setting,
the heater duty cycle is reduced or disabled to prevent overheating.
Status indicators include SUPPLY and HEAT. These perform the same functions as those on
the SCP-120 front panel.

Figure 5 shows the RCO layout.

SPECIFICATIONS

All Specifi cations apply to SCP-120

General
Area of use
Approvals

Enclosure

Protection
Cover attachment
Entries
Material
Mounting

Nonhazardous locations

NEMA 3R
Hinged polycarbonate cover, lockable
SCM120: 3 × 1-1/16" entries (120 VAC Single Phase ONLY
Polycarbonate
Wall mounted

Figure 5: RCO