WarmlyYours SCP-120 - Manual (Page 2)

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, auto-
matically 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 comput-
ers (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, freez-
ing 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 pre-
cipitation 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 en-
ables 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.