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Interlocks as Machine Safety Devices


     By Meridian Engineering & Technology, Inc. - Dr. Gary Hutter, P. E., CSP Safety, Mechanical, & Industrial Hygiene Engineering Expert Witness

PhoneCall Dr Gary Hutter, P.E., CSP at (847) 297-6538


Interlocks are often required by codes and standards, but sometimes, where there is no such requirement, the safety philosophy of the configuration needs to be examined. The following article, previously published in a safety journal, discusses some related issues.
Interlocks as Machine Safety Devices
Multiple Configurations, Various Types, Differing Approaches. U.S and Abroad

By: Gary M. Hutter, P.E. C.S.P. 12/07

An interlock can be defined as a device that prevents you from making an inappropriate manoeuvre, or adjusts the system to a safe state if you make an inappropriate manoeuvre.

In the context of safety, interlocks can prevent a user from making unsafe actions, or minimize the hazard of unsafe actions by rendering the machine in a safe condition when an unsafe manoeuvre occurs. For example, a guard may be interlocked to prevent machine operation when a guard is removed, or a control may be interlocked to make it non-operational if an unreasonable dangerous condition will result. Safety interlocks may have additional or combined features to reduce hazards.

Numerous product standards mandate interlocks in both industrial equipment, and in everyday consumer products. Examples of consumer products incorporating interlocks include:

- Removal of a guard on a food processor prevents the operation of motor and blade, thereby reducing the opportunity for contact with spinning blade injury (guard example of guard).
- Removal of the filter access door on a forced air furnace prevents operation of the blower motor and possible contact with blower blade and/ or combustion gas recirculation hazard (example of guard and secondary hazard of recirculation).
- Gear shift selector on a car will only allow the engine to start in Park position, Prevents the operator from starting a car's engine in gear and the possible unexpected vehicle movement (control example).
- Opening of door on clothes washer during high speed extraction cycle. May either prevent access to the spinning drum, or may stop the drum's rotation upon door opening (example of either guard or control feature).

While many often simply think an "interlock" is a safety method relying on an electro-mechanical switch (like a limit or magnetic switch) to perform the interlock feature, modern interlocking mechanism may take the form of other sensors and actuators. Many interlock switch providers currently have multi-pole magnetic switches, unique shaped key switches, and hidden features buried within structural components.

The single beam light curtain at the bottom of a garage door acts as an interlock, to reverse the door in situations of errant closing on a child or animal. The deadman control on a modern snow thrower (ANSI B71.3) acts as an interlock by placing the snow thrower in a safe condition (engine off or blade brake on) when one leaves the controls to reach into the discharge chute. The thermocouple on a gas stove prevents the release of unburned gas if there is no ignition of the gas. A light curtain, a captured lever device, and a thermocouple are examples of potential interlock sensors and actuators.

Several safety standards address both interlocked guarding and interlocked controls on industrial equipment. American National Standards Institute (ANSI) standard B11.1- 1982 for mechanical power presses defines an Interlocked Press Barrier Guard as: "A barrier interlocked so that the press stroke cannot be started normally unless the guard itself, or its hinged or movable sections, encloses the point of operation."

In paragraph 3.5.2.10 of the same standard, Press-Drive Motor Interlock addresses controls:

The clutch/ brake control shall incorporate an interlock means to prevent initiation or continued activation of the single-stroke or continuous functions unless the press-drive motor is energized and in the forward direction.

The ANSI standard for Hydraulic Power Presses, B11.2- 1995 does not define the term "interlock" while requiring interlocks on certain barrier guards. This standard also defines other terms for devices that can act as an interlock safety device. For example, a control that prevents a cycle operation under hazardous conditions could be considered an interlock device. ANSI B11.2 uses the terms below for these interlock functions:

3.3 Antirepeat: The part of the control designed to limit the press to a single cycle even though the actuating mechanism is held in the operated position.

3.10.1 Presence sensing device: A device designed, constructed, and arranged to create a sensing field or area or plane that will detect the presence of the operator's hands or other body parts.

In essence, both antirepeat and presence sensing devices can and do work as safety interlocks.

We often find machine interlocks required by standards or provided based on custom and practice. Additional exemplar domestic and foreign standards that require interlocks include:

- ANSI B11.19,Performation Criteria for Safeguarding
- ANSI/ RIA R15.06, Safety Requirements for Industrial Robots and Robot systems,
- ISO 14119 (EN 1088) Safety of Machinery- Interlocking Devices Associated with Guards

On industrial metal working machines, interlocks may be a standard required item for certain features, and an "add on" for other situations. These considerations may be based on U.S. domestic criteria, or criteria from abroad. In the publication "Guide to Machinery Safety", (Pilz Automation Technology, 6th edition) electrical control interlocks are discussed both in terms of European Union (EU) standards, and custom and practice. That publication states: "Electrical control interlocks are common where rapid or frequent access is required into a machine". (Para. 5.1.2)

In the "British Standard Code of Practice for Safety of Machinery", BS 5304, chapter nine describes "Interlocking Considerations." This standard addresses both guard and braking interlocking, and the failure mode needs of interlocks.

An example of U.S.-based non-mandatory criteria for interlocks include the National Safety Council's publication:

Safeguarding Illustrated Concepts (7th Edition, 2002) which uses the term "interlocked" as one of the three categories of Point-of-Operation guard types (other choices are "fixed", or "adjustable") that are recommended. This publication offers several examples of interlocked features on industrial machines or devices, several of which are not required by codes. Examples include an interlocked safety prop on a hydraulic press (fig. 3-115), an interlocked barrier guard on a bagging machine (fig. 3-94), and an interlocked clean-out door (fig. 3-31).

Further this NSC publication contains a brief listing of the advantages and disadvantages of interlocked guards. The safety publication identifies safety upsides of interlocks as; "maximum protection" and ease of "access": while the down sides are related to reliability and "defeatability".

The Occupational Safety and Health Administration (OSHA) publication "Concepts and Techniques of Machine Safeguarding", OSHA 3067, also has parallel discussion and ranking of interlock guarding with the NSC publication (although they list interlocks second in a grouping of four options for guards). They too list certain disadvantages of interlocks. Many machines are not required to have interlocks per OSHA criteria, but interlocks may be required by other voluntary standards or by custom and practice as referred to above. Certainly the decision to have a voluntary interlock system, or a mandatory interlock device, relies heavily on a well configured and high reliability configuration.

As noted in the product catalogue of General Electric (3682-5K-0903, 2003, GE Interlogix Industrial), the European Standards EN-954-1 and EN 1050, for risk assessment of control circuits, references the "likelihood of occurrence [of injury] if a safety interlock fails" in the risk assessment. Those standards discuss the issues of "redundancy", "self-checking", and "redundancy and self checking." One publication goes as far as including an interlock in a category of devices that also may "increase the danger of the protected system". This appreciation of the potential for poorly designed interlocks to fail is recognized in the OSHA criteria for lockouts.

Interlocks for electrical equipment may not be used as a substitute for lockout and tagging procedures.

The rational for this consideration is contained in the National Fire Protection Association "Electrical Standard for Industrial Machinery" (NFPA 79).

Collectively these various mandatory codes, voluntary standards, foreign and domestic criteria, and failsafe considerations identify some of the philosophical aspects of interlocks on industrial equipment. The reader is encouraged to consult these different sources for the application of interlocks to their industrial equipment.

ABOUT THE AUTHOR: Dr Gary Hutter, P.E., CSP
Dr. Gary Hutter is an educator and Professional Engineer, providing consulting on a variety of safety issues. He can be contacted at 847-297-7538

Copyright Meridian Engineering & Technology, Inc. - Dr. Gary Hutter, P. E., CSP

More information about Meridian Engineering & Technology, Inc. - Dr. Gary Hutter, P. E., CSP


While every effort has been made to ensure the accuracy of this publication, it is not intended to provide legal advice as individual situations will differ and should be discussed with an expert and/or lawyer.
For specific technical or legal advice on the information provided and related topics, please contact the author.

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