This Technical Report illustrates and explains principles of fault masking in applications where multiple interlocking devices with potential free contacts (B1 to Bn) are connected in series to one logic unit (K) which does the diagnostics (see Figures 1 to 7). It further provides a guide how to estimate the probability of fault masking and the maximum DC for the involved interlocking devices. This Technical Report only covers interlocking devices in which both channels are physical serial connections.This Technical Report does not replace the use of any standards for the safety of machinery.The goals of this Technical Report are the following:— guidance for users for estimation of the maximum DC values;— design guidance for SRP/CS.NOTE 1 Interlocking devices with integrated self-monitoring are not included in the scope of this Technical Report.NOTE 2 Limitation is also given by the diagnostic means implemented in the logic unit.NOTE 3 This Technical Report is not restricted to mechanical actuated position sensors.