Meeting fault masking standards using trapped keys: Rob Lewis looks at the options to prevent fault masking

摘要

The long discussed phenomenon of fault masking (unintended resetting) when connecting multiple volt-free contacts in series into a safety monitoring device is covered in EN ISO 14119 and an associated technical report. In simple terms, if gate switches are wired in series, then it is still possible to reset the safety circuit even after a short circuit has occurred on one part of the circuit, as one of the other gate switches can still open both circuits allowing the fault to be cleared from the safety relay or PLC.