Machine Interlocks & Barrier Guards Expert Overview

In the broadest terms, “interlocking” in the world of machine design is simply the connection of two or more elements such that the operation of one element depends on the condition of one or more other elements. This can be applied to process control, for example: not initiating a drilling operation until the workpiece is in the correct position; or it can be part of a safety system, like automatically stopping a microwave oven when the door is opened to prevent the user from being irradiated.

Interlock Guard & Machine Safety Expert Witness

In practice, machine safety interlocking can take on many forms such as automatic shut-down of out-of-control processes or preventing out-of-sequence operations which may result in equipment failures or injuries to personnel. This article will focus on the use of interlocking in barrier guards.

Barrier guards, as their name suggests, form a physical barrier between hazards and people. This barrier:

  1. Prevents people outside the danger zone from accessing the hazards within
  2. Prevents hazards within the danger zone from escaping and causing injury to people outside
  3. Serves an awareness function, helping people identify the borders of the danger zone

The wire “cage” on a portable oscillating fan is one example of a barrier guard. The cage’s wires are close enough together that a person cannot get their fingers inside the danger zone. The cage would contain any large pieces of shrapnel if one or more blades were to fail catastrophically. And finally, the cage is visible and identifiable as the perimeter of the danger zone.

Interlock Guard & Machine Safety Expert Witness Example

Barrier guards can be fixed (e.g., panels attached with screws) or movable (e.g., doors with latches). The movable variety is where interlocking devices are often employed. What type of barrier guard to choose, and whether it should be interlocked, is a complex engineering decision in the scope of a comprehensive hazard analysis, and designers must consider factors such as:

  • The type and magnitude of the hazard
  • The frequency with which workers must access the danger zone
  • The ease of, and motivations for, defeating the guards
  • The physical environment where the interlocks must operate
  • The sophistication of the machine’s control systems
  • The economics of outfitting alternative solutions

In applications where only infrequent access by skilled workers is required, guarding the hazard using a panel affixed with fasteners requiring a tool to remove may be an appropriate solution. Augmented solutions of this variety include the use of “tamper-resistant” hardware which requires specialized tools, or simply capturing the hardware in the cover so that screws cannot be lost and then omitted on reinstallation.

Interlocked Movable Barrier Guards

When machine operators do require frequent access to danger zones, interlocked movable barrier guards can be an excellent solution.

These often take the form of hinged or sliding doors with redundant and fail-safe switches. The doors are easier to open than guards affixed with fasteners, and they are physically attached to the machine via the hinges or tracks. This discourages their outright removal and assures correct alignment of the latches and switches with each opening and closing. The switches themselves communicate with the machine controls through safety-rated relays. In the event a guard is in a position other than completely closed, the machine will either not start, or, if already running, will shut down and automatically dissipate any sources of hazardous energy.

In applications where the danger zone contains sources of hazardous energy which cannot be quickly dissipated, an augmented interlocked barrier guard may physically lock, disallowing access until the shut-down is complete. Modern residential top-loading washing machines offer a familiar example, where the lid of the machine locks until the agitator and washtub have completely stopped, either because the cycle has completed, or the user has commanded an interruption from the control panel.

Designing & Maintaining Sufficient Guards

The designers, manufacturers, and distributors of equipment have the greatest opportunity, ability, and responsibility to provide adequate guarding – explicitly when claiming conformity with various standards, and implicitly through best-practices and sound engineering principles. These entities are generally more sophisticated in the construction, foreseeable use, and reasonably foreseeable misuse of the product than any single end-user. As a result, designers, manufacturers, and distributors have an obligation to only introduce safe products into the stream of commerce.

This does not mean that end users are free of responsibility, however. Owners must maintain the guarding as recommended by the equipment manufacturer or augment the guards if warranted by a hazard analysis specific to the owner’s use case. Purchasers of equipment should therefore work closely with equipment salespeople to ensure that the product will be safe for their application.

For fixed barrier guards accessed only by experienced mechanics, analysis for sufficiency can be relatively straightforward: When held in place by the provided fasteners, does the guard perform its functions adequately?

Interlocked movable barrier guards are a more complex puzzle to solve, however. Here, the exact nature of the hazard, how quickly hazardous energy sources can be dissipated, and the specific risks to workers must be analyzed, and then the interlock system (including its failure modes and redundancies) must be shown to be sufficient.

Choosing a guarding strategy and then designing it to be sufficient is a major engineering task, and the designer(s) must be qualified to perform it. Analyzing a machine which has caused an injury can require even more expertise, as the installed safety systems, their possible insufficiencies or defeats, and any practicable alternatives, must all be analyzed.

Workplace Safety & Machine Guarding Expertise

The industrial safety experts at Robson Forensic have designed, built, maintained, and operated a broad variety of industrial machinery. Moreover, our experts have investigated countless mishaps over the years. By retaining Robson Forensic, you are securing the full weight of our collective experience, knowledge, and resources toward the resolution of your case.

For more information, submit an inquiry or call us at 800.813.6736.

Featured Expert

Richard Murray, Mechanical Engineer & Industrial Machinery Expert

Richard Murray, P.E.

Mechanical Engineer & Industrial Machinery Expert
Richard Murray is a Mechanical Engineer and industrial safety expert. His expertise is in machine design and automation; identification and control of hazards (e.g., safer alternative designs, guards,… read more.


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