What are the common interlock failure modes and their consequences?

In Machinery, and in general for components used in high demand mode, like Interlocking devices, Solenoid Valves, or Contactors, the component manufacturer does not normally provide the different types of failure rates.

In my experience, understanding interlock failure modes and their consequences is vital for ensuring industrial safety. Hardware failures, such as sensor malfunctions, are visible and can lead to equipment damage. Software failures, on the other hand, are less obvious but can cause unpredictable behavior in interlock systems. Human failures are the most common and varied, stemming from errors or deliberate actions, and can result in serious accidents. Implementing regular maintenance, robust software testing, and comprehensive training programs can mitigate these risks and enhance overall safety in industrial settings.

In 1985, the International Electrotechnical Commission (IEC) set up a Task Group to assess the viability of developing a generic standard for programmable electronic systems to be used for safety applications. A working group had previously been set up to deal with safety-related software. These two working groups collaborated on the development of an international standard that became known as the IEC 61508 series, published at the end of the 90s. The original scope of the Task Group, programmable electronic systems used for safety applications, was extended to include all types of electro-technical based technologies, electrical, electronic, and programmable electronic systems: the so-called E/E/PE systems.

Both IEC 62061 and ISO 13849-1 accept a testing made by an operator. That is what is defined as “Fault detection by the process,” and the DC value can go from 0 to 99%. It is a concept introduced in the new edition of both standards. The example made in both ISO 13849-1 annex E and IEC 62061 annex D, is related to a printing process. In such a process, there is one of the rollers whose rotation speed is monitored by a rotary encoder. In case of a fault of the encoder, the operator would see the printing defects and stop the line. The DC for the sensor used to monitor the rotation speed can be estimated equal to 90% and up to 99%.

Any component can fail, regardless if it is suitable to be used in a Safety system or not. Therefore, any Process Control system, for example the one that keeps the temperature in a Heat Treatment furnace under control, can fail, and the temperature may increase until it generates a dangerous situation. If you are not familiar with functional safety, you may think that the occurrence of the event is so unlikely that it can be disregarded and nothing more is needed to be able to declare my furnace safe. That is not the way Functional Safety reasons. Yes, the event has a low probability to happen, but it can happen