What is the 250T Two-wire RTD Transmitter Failsafe Hook-Up Approach? – 8401068

Two-wire Transmitters and Their “Hook-up” to Achieve a Failsafe Configuration

The attached analysis references Acromag’s 250T series RTD (100 ohm platinum)

250T Two-wire RTD Transmitters Fail-safe Hook-up Approach

250T Two-wire RTD Transmitters Failsafe Hook-up Approach

This type of hook-up is typically not used because the effects of lead-wire resistance inaccuracies are additive to the RTD measurement. However, when addressing the problem of FAILSAFE, this configuration offers the most predictable output behavior. Also, H the RTD hook-up is properly done, the inaccuracies due to lead wire resistance can be minimized. The following is a list of most probable failures that could happen to the input sensor. An analysis will be performed to address each failure and predict the output of the transmitter.

  1. Lead wires +/-could break open
  2. The RTD sensor could fail open up
  3. The input could become grounded
  4. The input +/-could become shorted

Conditions/Assumptions for Failsafe Hook-up Approach:

  1. RTD type: 100 ohm platinum
  2. Lead-wire length: 50ft (or shorter)
  3. “-” & “L” connection: internal
  4. Max. temp. change for lead wires: 50°C
  5. Lead wire gauge: 17 AWG (0.005 ohms/ft)
  6. RTD break detection: UP

Based on these conditions, we can derive accuracies for this type of approach.


  1. Total lead wire resistance:
    1. 2 * 50 ft * (.005 ohms/ft) = 0.5 ohms (static value)
    2. NOTE: This lead wire resistance is a static value which means that at 25°C, R’s effect can be calibrated out of the transmitter. The value of the most concern, however, is the dynamic resistance change of the lead wires over temperature
  2. The following equation is a good approximation for copper wire resistance with temperature changes of up to 50°C.
    1. Rt= R(25°C) * [1 + (6 T)(0.004)]
    2. Rt= 0.5 * (1 + (50)(0.004)]
    3. Rt= 0.6 ohms
    4. Thus, the dynamic resistance change of the lead wires is:
      Dynamic change= 0.1 ohms (maximum temperature change= 50°C)

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