Application Note

Application Note: 101405A
Kayden Instruments

Fail Safe


Wiring and configuring the CLASSIC 800 Series Flow, Level, Interface & Temperature Switch & Transmitter for "Fail-Safe" operation


Kayden CLASSIC 800 Series Thermal Dispersion Flow, Level, Interface & Temperature Switch & Transmitter

The term “Fail-safe” refers to a method of wiring and programming control instrumentation commonly used in hazardous locations / applications such as a chemical plant, gas plant or refinery.

In a “Fail-safe” application the instrument is wired and configured to be “ON” (closed contacts, output(s) supplied by the instrument to the system) during normal system operations, but “OFF” (contact(s) open or de-energized, output(s) discontinued) in the event of a power failure, cable break, self-test failure, or other loss of electrical power.

A common “fail-safe” set up for No Flow Condition, Pump Protection:

  • Connect the relay contact wires to Normally Open (R1NO, R1CM)1
  • Electronic instruments such as capacitance probes, ultrasonic, and radar level controllers are bothered by vapors, bubbles, coating, require constant attention, are more expensive, and are often difficult to install.
  • Depending on application and set-up of the switch transmitter, the relay contact(s) will OPEN (deenergize):
    1. When the thermal signal decreases due to a decrease or loss of flow/level
    2. When power is lost to the switch as described above.
Classic 800 Display Panel
Display Panel

Display Panel Indicators:

Relay 1

On steady when Relay 1 is energized

Relay 2

On steady when Relay 2 is energized


Indicates a self-test error or fault condition

Set Point 1

On steady when viewing Set Point 1

Set Point 2

On steady when viewing Set Point 2

Run Mode

Flashing when switch is operating


Flashing when the Start-up Bypass Timer is active

Thermal Signal

Displays Thermal Signal

The Thermal Signal increases as:


The flow rate increases


The sensor is submerged


The sensor is submerged by the second liquid of greater thermal conductivity

Applicable CLASSIC 800 Models

Classic 810
Classic 810
Classic 812
Classic 812
Classic 814
Classic 814
Classic 816
Classic 816
Classic 828
Classic 828
Classic 830
Classic 830
Classic 832
Classic 832

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