Interlocking guards are devices that prevent machine elements from operating under special conditions, generally when the guard is not closed. Examples of interlocked guards include:
1. An interlocking switch on a washing machine lid that stops the rotation of the washer drum if the lid is opened;
2. A microwave oven door that will not allow you to pop popcorn when the door is open;
3. A computer numerically controlled milling machine that has a full enclosure and will not allow the spindle to operate above 750 RPM with the access doors open; and a
4. Meat grinder that will not rotate with the hopper access door opened.
Such guards are often found when one or more hazards, like the nip points found in a meat grinder, must be physically guarded and frequent access to the area is required for activities such as cleaning. ISO 14120-2002, titled “Safety of machinery – Guards – General requirements for the design and construction of fixed and movable guards”. The first sentence in the standard reads:
“This International Standard specifies general requirements for the design and construction of guards provided primarily to protect persons from mechanical hazards.”
ISO 14120-2002 defines the term “interlocking guard” in the following way:
“guard associated with an interlocking device so that:
-the hazardous machine functions ‘covered’ by the guard cannot operate until the guard is closed;
-if the guard is opened while hazardous machine functions are operating, a stop instruction is given;
-when the guard is closed, the hazardous machine functions ‘covered’ by the guard can operate, but the closure of the guard does not by itself initiate their operation”
The above definition refers to an “interlocking device” and references ISO/TR 12100-1:1992, titled “Safety of machinery – Basic concepts, general principles for design – Part 1: Basic terminology, methodology” for a definition of “interlocking device” and ISO 14119 titled “Safety of machinery – Interlocking devices associated with guards – Principles for design and selection” for further reference. The definition of “interlocking device” from ISO/TR 12100-1:1992 is reproduced below:
“Mechanical, electrical or other type of device, the purpose of which is to prevent the operation of machine elements under specific conditions (generally as long as a guards is not closed).”
ISO 14120-2002 gives the following advice on when an interlocking guard should be used:
22.214.171.124 Where access is required only for machine setting, process correction or maintenance
The following types of guard should be used:
a) Movable guard if the foreseeable frequency of access is high (for example more than once per shift), or if removal or replacement of a fixed guard would be difficult. Movable guards shall be associated with an interlock or an interlock with guard locking (see ISO 14119).
b) Fixed guard only if the foreseeable frequency of access is low, its replacement is easy and its removal and replacement are carried out under a safe system of work.
126.96.36.199 Where access is required during the working cycle
The following types of guard should be used:
a) Movable guard with interlock or with interlock with guard locking (see ISO 14119). If access is required for a very short working cycle, it can be preferable to use a power-operated movable guard…
A key phrase related to when the guard shall be fixed or moveable and associated with an interlock is “if the foreseeable frequency of access is high. The parenthetical “for example more than once per shift” offers one possible example of a situation when the foreseeable frequency of access would be considered high. “Once per shift” is not the only possible example of when the foreseeable frequency of access would be considered high, but experience has shown that it is often a good starting place to begin analyzing whether an interlocking guard may be needed.
The designer’s choice of a certain type or arrangement of machine guards must necessarily take into account the nature of the hazard being guarded against. For example even when frequent access is required, a hazardous rotating drum that cannot immediately be stopped may require a different guarding solution than an electrical hazard that can be near-instantaneously de-energized. Machine designers should carefully assess the hazards and risks of each individual machine they design using well-established risk assessment methodologies such as those to be found in ISO 12100 or ANSI B11.0. Machine designers might also wish to consider that a draft version of ISO 14120 exists that, if accepted, may change the “once per shift” phrase to “once per week”.
It is important to remember that interlocked guards require their own maintenance and adjustment to work properly, and that it may be possible to bypass or defeat an interlocked guard. Designers should take steps to avoid defeat of interlocks including but not limited to attaching the interlocking device so that it is difficult to remove and reducing the temptation to bypass by making the design comfortable and convenient to use. In the risk assessment process, designers may also wish to consider whether a hazard can simply be designed out or substituted with something non-hazardous. Whenever feasible, both solutions are usually preferable to guarding.
A number of other voluntary standards regarding interlocked guards exist from organizations such as ASTM, the American National Standards Institute (ANSI), ISO and other bodies that may be applicable to machines used either inside or outside the workplace. OHSA also publishes binding regulations that apply to employers, but that machine designers should take into account when designing machines intended for use in a workplace. In our next post on machine guarding, we will provide a brief glossary of codes and standards that can guide machine designers in creating safe, effective and useful interlocked guards for both industrial and consumer machinery. Stay tuned.
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