The Scully ST-15 WX is a multi-point liquid-level detection system designed for high and/or low-level alarm applications in fixed storage tanks. The control units are suitable for mounting in any non-hazardous area and utilize oil-tight controls and indicators. Its internal circuitry provides intrinsically safe (I.S.) output for probe connections, rated for Class 1, Div 1, Group D hazardous areas.
Function Description
The ST-15C-WX series control units are multi-point liquid level alarm units. Each module, referred to as a "channel," monitors one sensor. For instance, an ST-15-240-WX10 can monitor 10 probes. The system is designed to provide an alarm output when liquid touches a probe (high-level alarming) or when a sensor becomes "dry" (low-level sensing, requiring factory wiring instructions).
When system power is applied, all high-level alarm channels will initially produce alarm outputs (red lamp on and alarm bus energized) until their associated thermistor probes warm up, which takes approximately 10 seconds (warm-up time varies with ambient air temperature). Once a probe is "dry" and warmed up, its associated channel's red lamp will extinguish. Two-wire optic probes do not require any warm-up time. It's important to note that the sensor is not a "switch"; the ST-15-WX control unit will only respond and indicate "dry" with a dry probe connected to its input terminals. Shorted or open probe circuits will not cause the unit to go "permissive."
Important Technical Specifications
Power Requirements:
- ST-15-115-WX models consume less than 20 watts of line power per channel, operating from 100-140 VAC, 50-60Hz mains power.
- ST-15-230-WX models operate from 200-250 VAC, 50-60 Hz mains power.
Alarm Output:
- A mains voltage alarm output is energized whenever any channel trips to an alarm position.
- The alarm output is rated at 4 amps maximum current and is protected by an internal fuse.
- An alarm buzzer is included.
Control Contacts:
- Each ST-15 control module provides a Single Pole Double Throw (SPDT) volt-free contact set.
- These contact sets are rated at 250 VAC, 5 amp resistive, and can be used to control valves, motor starters, or other electrical devices.
- The total connected load for external alarm devices (horns, bells, sirens) should not exceed 4 amps.
- For control device connections, connected loads should not exceed 5 amps resistive per contact set. Sources for these devices should be used to prevent damage to the ST-15-WX equipment through field wiring shorts or failures of controlled devices. Internal fuses do not protect these contacts.
Probe Compatibility:
The ST-15-WX is compatible with various probe models, including:
- Thermistors: SP-M, SP-MU, SP-MH, SP-BL, SP-BLU, SP-BLH (for heated products).
- Instant-On™ Two-Wire Optic: SP-TOM, SP-IO.
Probe Lead Wire Size and Maximum Run Length:
- #16 (1.5 MM²) wire: 400 (120) feet (meters)
- #14 (2.5 MM²) wire: 700 (210) feet (meters)
- #12 (4.0 MM²) wire: 1000 (300) feet (meters)
- #10 (6.0 MM²) wire: 1600 (480) feet (meters)
- #8 (10.0 MM²) wire: 2500 (760) feet (meters)
- If sensor leads are extended beyond 2500 feet, specific wiring recommendations should be consulted from the factory to prevent jeopardizing intrinsically safe provisions.
Wiring Separation:
Absolute separation of intrinsically safe wiring to sensors from power and controlled circuit wiring is critical. Separate conduit holes are provided and must be used to maintain this separation.
Usage Features
Installation:
- The ST-15-WX control unit must be installed in a NON-HAZARDOUS AREA, typically the control room of a storage tank arena.
- It can be installed outdoors in its weather-proof enclosure, provided conduit entries are properly sealed to prevent water ingress.
Controls and Indicators:
- Front Panel Indicator Lamps: These lamps indicate the status of each channel. A green pilot lamp indicates mains power is supplied. Red lamps indicate an alarm condition.
- TEST Button: This button causes all high-level alarm points to trip to their alarm condition when operated, allowing for system testing.
- ACKNOWLEDGE Button: This push button or optional key-operated acknowledge switch silences audible alarm devices when operated. The alarm acknowledging circuits reset automatically when the fault condition is corrected.
- Dynacheck® Indicator LED: Each internal ST-15 control module has a Dynacheck® indicator lamp (Scully part number 27002). This lamp flashes once every few seconds when the ST-15 Control unit is connected to a "dry" probe.
Probe Connections:
- Two-Wire Probes: When connecting to a 2-wire optic or thermistor probe, a factory-installed 680 Ω resistor on terminals G and W should be kept connected.
- Three-Wire Probes: For 3-wire probes used with heated products, the 680 Ω resistor installed between the G and W terminals must be removed. It is suggested to retain this resistor for future use.
Maintenance Features
Front Panel Indicator Lamps:
- These lamps are replaced by unscrewing the lens caps and replacing the bulbs underneath.
Dynacheck® Indicator LED:
- The Dynacheck® indicator lamp is removed by pulling it straight out of its socket.
- Burnout of the Dynacheck® indicator LED will not affect the performance of the ST-15 module.
ST-15 Modules:
- The ST-15 control modules contain no field-replaceable components other than the Dynacheck® indicator LED.
- Modules must be removed as a unit and returned to the factory for service.
- Returns to the factory must be accompanied by an RMA number, obtained from Scully Signal Company.
- Servicing or adjusting the ST-15-WX module without specific approval and instructions from Scully Signal Company is not recommended.
Relays:
- Control relays should only be replaced with equivalent types, ensuring the relays' coil voltage matches the system voltage.
Fuses:
- Fuses F1 and F2 should only be replaced with 5 amp slow-blow units.
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