How to Calculate the Resistor Value for Open Wire Detection in BMXSDI1602 using DDP 2 or 3 wires?

Issue

Open wire detection in BMXSDI1602 requires to create artificially a leakage current (Ileakage) on the line with a resistor in parallel to the sensor. If this leakage current (0.4 mA < 1.3 mA) is out of range, the external line is considered as cut (or in a short-circuit to ground condition).

Resolution

For a dry contact sensor, set in parallel with the sensor a Rext = 33 K Ω.

However, using DDP 2 or 3 wires, the leakage current needs to fall within the limits defined above. You need to define the value of the resistor (Rext) to set in parallel with the sensor, considering

• the natural leakage current of the sensor
• the Rint (internal resistor) of the input, equal to 7.5 K Ω

Then, in order to obtain a current in the range of  :

The calculation of Rext is performed with the following formula :

where :

Released for: Schneider Electric UK