What is an interlocking device?
An interlock associated with a guard links its state to the machine’s control system. Its function is to prevent hazardous operations when the guard is not closed and to create a safe condition if it is opened. It is not simply a matter of detecting a door: the detection must be part of a safety function designed for the specific hazard.
The assembly includes the guard, the device, its integration into the control unit, and the components that operate the machine. A suitable part installed incorrectly may not provide the necessary protection. Therefore, the selection should consider the entire machine, its operating modes, and anticipated maintenance, rather than relying solely on the sensor’s product data sheet.
Difference between interlocking and locking
The interlock can command a stop when the guard is opened, but it doesn’t necessarily prevent it from being opened. If a person can reach the area before the hazard has passed, the stop command is insufficient. The guard locking mechanism adds the function of keeping it closed while the hazardous condition persists, in accordance with the equipment’s safety design.
The comparison between access time and overall stopping time is especially relevant for machines with inertia. Other hazards may also exist that do not disappear immediately upon stopping a movement. The solution must consider the entire scenario. Simply closing the guard should not, by itself, cause an unexpected and dangerous restart: the startup logic requires specific design and validation.
How is the solution selected?
The selection process begins by identifying which hazards are being controlled, when access is needed, and what should happen when the guard is opened. Next, the safety function requirements are determined, and compatible components are chosen. Environmental conditions, opening frequency, vibrations, or mechanical deterioration can affect performance and must be considered during the design phase.
Physical integration is also important. A deformed or misaligned guard can prevent proper closure or cause repeated failures. The solution should not be to disable the device, but rather to correct the assembly and verify its safety. Any necessary monitoring, cleaning, and adjustments must be addressed to ensure that normal machine operation is compatible with maintaining the guard.
Relationship with safeguards and other measures
Machine guards provide a physical barrier, while interlocks control hazardous functions based on their status. Emergency stops serve a different purpose and do not replace this protection. Using an emergency stop button every time an area is opened is not equivalent to having a properly designed and validated access control system.
Energy lockout/tagout is not synonymous with guard locking. For maintenance or hazardous interventions, isolation measures, residual energy control, and prevention of accidental start-up may be necessary. The fact that a door is open and the machine is stopped does not prove that all energy sources in the area are controlled for any given work.
Practical example
A machine has a rotating element that continues to move after receiving a stop command. Access through a door allows the element to be reached before it stops. The assessment determines that an interlock that only commands a stop upon opening does not sufficiently control the scenario, so a solution is designed that keeps the guard locked while the hazard persists.
The technical team validates the function, timing, and release conditions. They also review how to respond to a malfunction and, if a person can enter the area, how to prevent entrapment and ensure an exit according to the applicable design. The example illustrates why simply adding a position switch is not enough to guarantee access safety.
Checking and maintenance
Equipment inspections must include the relevant protective functions, using a safe method and competent personnel. The physical condition, secure mounting, expected response, and absence of unauthorized modifications must be checked according to the instructions and established schedule. Tests must not be performed by exposing a person to danger to verify whether the machine stops.
The results should allow for the management of defects before unsafe use continues. After a repair or component replacement, it may be necessary to re-verify the entire function. Replacing a sensor with a similar one does not guarantee equivalence: characteristics, logic, assembly, and diagnostics may differ. Spare parts management must adhere to the defined safety requirements for the equipment.
Cancellation and usage issues
Disabling an interlock disables a crucial part of the safety system. If this occurs, it must be corrected and its cause analyzed, including recurring failures, adjustment difficulties, monitoring needs, or production pressures. Understanding these factors allows for improvements in design and organization without requiring the machine to operate normally with safety features deactivated.
Users need clear instructions on how to report defects and stop use when necessary. Managers must have alternative work procedures and technical support to resolve these issues. A system that penalizes reporting a faulty safety device encourages its concealment. Prevention requires prioritizing repair and testing over maintaining operations that have lost a necessary safety barrier.
Technical framework and common errors
Royal Decree 1215/1997 requires protection against dangerous moving parts by means of suitable guards or devices. INSST Technical Standards 1124 and 1220 develop criteria on guard interlocking and locking. These standards must be applied in conjunction with the relevant assessment and instructions, without converting a general description into a design valid for any machine.
Common mistakes include confusing detection with safety, omitting the shutdown time, and assuming that a closed door automatically authorizes startup. Another mistake is intervening within a zone relying solely on the interlock. The solution must demonstrate control over the intended scenario and remain effective during use, cleaning, adjustment, and maintenance, with the specific measures required for each operation.
