What is Interlocking?

Interlocking is a kind of internal automatic control mechanism which used between two or more devices, equipment or any other phenomenon. In an interlocking system, some status of the devices are defined as a precondition to control a certain device. In another words, devices cannot be controlled directly. It is designed within a system, which can create some hazardous results in a certain status combinations. Interlocking system locks the controlling of critical devices in between and allows only possible safe status sets.

The working mechanism of the interlocking can be explained with a simple example. There is an interlocking system to protect maintenance staff against electrical shock in a maintenance depot of a railway operator company in Istanbul (Istanbul Ulasim A.S.). Some components of railway vehicles are installed over the car body with some high voltage equipment. The maintenance of components can be very dangerous if the high voltage equipment are alive.

An overhead catenary system provides electrical power to trains in the depot. Maintenance staff use a platform to reach top of the trains and electrical power has to be switched off before anybody use this platform. The procedure which defined to work on the trains has to be followed by the maintenance staff when they are working on the train’s roof. However, if somebody reaches the train’s roof when the catenary line is alive, it may cause injury or death. Therefore, this problem is solved with using an interlocking system between the circuit breaker and the platform.

The electricity on the catenary system is controlled by a circuit breaker, which is equipped with a key. This key is released only when the circuit breaker is switched off and the circuit breaker cannot be switched on without this key as well. On the other side, the platform has a locked door to prevent passage of unauthorized staff. The door can only be opened with a key and it does not release the key when it is unlocked. The interlocking system is provided by these mechanism. For instance, the staff who wants to work on the trains has to use platforms but there is a locked door front of the platform steps. The only way to unlock the safety door is switching off the circuit breaker and getting the key. Conversely, it is prevented to switch on the circuit breaker when there is somebody on the platform.

To sum up, almost all possible dangerous situations are prevented with an interlocking mechanism between system equipment. In the given example, the platform door and the circuit breaker represent the critical equipment in the system. The key is used as an interlocking tool to interlock the critical equipment.

Nowadays, most of the new developed systems are based on software. However, it is still required some interlocking mechanism in safety critical systems. For this reason, some interlocking algorithms are developed by system engineers to ensure the safety in software based systems. Modern railway interlocking systems can be given as a good example of software based safety critical systems.

Reference: M. Bellek, "Design And Rams Analysis Of Railway Interlocking Based On Formal Methods: An Example Application", Master thesis, TU Dresden, Dresden, 2013.