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What is interlocking? Why is it different from locks? Why using interlocks instead of keyholes or latches? Why are interlocks mechanic and not electric or electronic?

These are some of the most common questions asked by people who approach interlocking for the first time.

We will exhaustively answer such questions in a clear and unequivocal manner, in order to highlight the importance of interlocking and their infinite possible applications.

An interlock is a closed and interconnected system of locks and elements for the sequential distribution of keys.

The fundamental element of the word “interlock” is the prefix “inter”, through which the idea of interconnection of elements and sequentiality of the key distribution is conveyed. Without interconnection and sequentiality, we lose the systemic dimension which differentiates interlocks from simple locks.

Through interconnections and sequentiality of key distribution it is possible to design infinite mandatory sequences of actions. These sequences constitute the physical expression of safety procedures for the blockage of the equipment/machinery in the occasion of inspections and maintenance.

Companies have only two options: leave the application of safety procedures to the conscientiousness of the operators, or, thanks to interlocking, integrate them into the technical system and the working environment. Interconnecting creates rigid sequences of activity and access which reduce the discretional actions of workers to zero, generating the maximum safety for both workers and equipments during blockage procedures for inspections and/or maintenance.

Maintenance is the most frequent application context for interlocks, which contribute in significantly reducing the number of injuries and deaths on the job.
Arel interlocks are exclusively mechanic or electromechanic. This choice is strictly related to their safety functions, this way reducing the error rate to the minimum, lower than electric or electronic locks, and not generating false positives. In other words they may break but, differently from electric or electronic blocks, they never allow the starting of incorrect and potentially dangerous procedures.

In order to fully comprehend the great applicative potential of interlocks it is necessary to further explore the functioning mechanisms, that is the way in which the interconnection between the elements of the system and its unequivocal dimension is generated.

Starting point of the system, which is composed by at least two elements, is the functioning mechanism of the single lock and its correct installation. Overlooking the aspects related to the installation, which are important but represent an exogenous variable, we will focus on the lock functioning and we will analytically describe the dimensions of singularity and interconnection between locks which create the uniqueness of the system.

The first dimension of uniqueness lies in the fact that every lock has its own key, expression of a variant among thousands possible combinations. For this reason each key can be considered unique as it is the only one (together with its copies, if requested by the customer) containing the sequence required to open the lock. This dimension guarantees that, during the passage from one element to the other, the “witness” is unequivocally linked to this element of the system.

The second dimension through which uniqueness and interconnection are created is constituted by the functioning mechanism of each single element of the system (the tumbler) which has two possible positions:

These two positions represent the founding elements of the interlocking language, to which are associated two states:

Through the combination of the first two positions (Open – Closed) it is possible to unequivocally define the position of the single element, and through the combination of the two states (Key trapped – Key free) it is possible to activate the interconnection with the other elements of the system.

The single elements of the interlocking system are classified in three macro-families of products, differing in their function:

The locking operation is the element allowing the isolation of the danger for the worker, being it electrical or mechanic. Through this isolating action the non-dangerous condition of the system is guaranteed.

The keys distribution is the element allowing to physically and logically interconnect the elements of the system, building connections and multiplying them between the bolt block and the door block.

The access control is the element regulating the access to the potentially dangerous equipment/machinery on which to operate the inspection or maintenance procedure.

During the course of its 50 years of history, Arel has developed it’s own language for interlocking, allowing the integration of safety procedures into the design of the systems.

The first design schemes were created towards the end of the Fifties and were born from the design tradition of French security.

The experiences of design developed during the course of years in different sectors have brought an evolution of design schemes with the development of its own alphabet in which ad hoc developed symbols represent the two sets of positions (Open – Closed; Key trapped – Key free) and their combinations with the three families of elements (Locking operation –  Keys distribution – Access control).

A correct and effective design of the maintenance safety with the application of such a language requires the integrated analysis of the following elements:

  • Schemes of the technical design of systems
  • Layout of production
  • Maintenance safety procedures

On these schemes the safety design is developed with the integration of the Arel language on the customer ‘s technical system.

An application example

The range of Arel's interlocks

During the course of its long history, Arel has developed a range of interlocks which in some cases have become the standard model in the design and quotation processes.

The international evolution of the company and the development of ad hoc solutions for many different industrial sectors have underlined the importance of diverse applicative necessities and installation traditions. For these reasons Arel has decided to extend its range of products creating, alongside its classic and renowned Light Duty series, a second series called Heavy duty.