NFPA 72 Understanding Supervisory Signal–Initiating Devices: Simplified Guide with Key Requirements"

Supervisory Signal–Initiating Devices

This post explains the requirements for supervisory signal–initiating devices used to monitor systems like valves, pressure, water levels, and temperature.

1. Control Valve Supervisory Signal–Initiating Device

• Purpose: Monitors the position of control valves.
• Signals:
  • Off-normal signal: Triggered when the valve moves from its normal position.
  • Restoration signal: Triggered when the valve returns to its normal position.
• Trigger Conditions: Off-normal signal is activated:
  • Within 2 handwheel revolutions or
  • Within 1/5 of the valve’s travel distance.
• Restrictions: Device must not interfere with valve operation, obstruct indicators, or prevent maintenance.

2. Pressure Supervisory Signal–Initiating Device

• Purpose: Monitors system pressure for deviations.
• Signals:
  • Off-normal signal: Triggered when pressure increases or decreases.
  • Restoration signal: Triggered when pressure returns to normal.
• Applications:
  • Pressure Tank: Detects changes of ±10 psi (70 kPa).
  • Dry Pipe Sprinkler System: Same as pressure tank.
  • Steam Pressure: Detects low pressure before it falls below 110% of the required level.
  • Other Sources: Must comply with local authority requirements.

3. Water Level Supervisory Signal–Initiating Device

• Purpose: Monitors water levels to ensure safe operation.
• Signals:
  • Off-normal signal: Triggered when water level rises or falls outside limits.
  • Restoration signal: Triggered when water level returns to normal.
• Applications:
  • Pressure Tanks: Triggered by a ±3 in. (70 mm) change.
  • Other Containers: Low water signal triggered by a drop of 12 in. (300 mm).

4. Water Temperature Supervisory Signal–Initiating Device

• Purpose: Monitors water temperature in freezing conditions.
• Signals:
  • Off-normal signal: Triggered when water temperature drops to 40°F (4.4°C).
  • Restoration signal: Triggered when water temperature rises above 40°F (4.4°C).

5. Room Temperature Supervisory Signal–Initiating Device

• Purpose: Monitors room temperature to prevent freezing.
• Signals:
  • Off-normal signal: Triggered when room temperature drops to 40°F (4.4°C).
  • Restoration signal: Triggered when room temperature rises above 40°F (4.4°C).