Machinery has always had one purpose, to simplify the lives of users.
While advancing technology allows for improved production, it also leaves room for potential safety risks. These risks can range anywhere from a broken finger to complete loss of life. In fact, the Bureau of Labor Statistics rates manufacturing as the second most dangerous workplace, second only to healthcare. In order to mitigate these dangers, components such as limit switches began to be implemented for safety regulation.
(Read the full ‘Workplace Injuries and Illnesses’ report here.)
So what exactly is a limit switch (AKA Travel Switch)?
OSHA defines a limit switch as an electromechanical device designed to “cut off the power to the motor and apply the brake to stop the carrier if a loaded step passes the terminal landing.” They can determine the presence or absence, passing, positioning, and end of travel of an object. Originally used to simply define the limit of travel of an object, they normally consist of an actuator mechanically linked to a set of contacts.
Scientific studies reviewing workplace accidents conclude:
“Results show that violations are the main type of human error in the automotive manufacturing industry with 62.66% of frequency…Also, violations were associated mainly with the contributing factor of machinery and equipment belonging to the same category.” (Want to read the study? Find it here)
You can find limit switches in a variety of common consumer products such as garage doors and refrigerators; you know that little button that makes the light shut off when you close the door? More safety oriented applications like handheld chainsaws utilize a limit switch to stop the blade from moving as soon as the user’s hand is no longer holding the on switch in place.
Learn about the differences between Limit/Position, Safety Interlock and Coded Magnetic Switches here!
In 1989, the European Union established the Machinery Directive 89/392/EEC
This required any machinery in the EU market had to be: “designed and constructed so that it is fitted for its function; and can be operated, adjusted and maintained without putting persons at risk when these operations are carried out under conditions foreseen, but also taking into account any reasonably foreseeable misuse thereof.”
Easily triggered switches may work for individual cases, but what happens when the application is a large-scale production with thousands of pounds of moving equipment?
We circle back to how dangerous manufacturing can be and acknowledge how imperative it is to reduce the risks as much as possible.
How does a manufacturer avoid putting a person at risk during operations? By establishing universal safety regulations, implementing redundancy and adding tamperproof safeguards as needed.
First, a switch is only considered “safe” if it fails strictly in the OFF position. If the product can fail in the ON position, running machinery will have the opportunity to harm operators.
Second, it’s important to remember that redundancy is the name of the game in safety. By diversifying the design and/or technology of the switches you install, you can avoid your components all failing due to the same issue.
Third, the switch needs to be resistant to tampering. Devices designed to reduce the defeating of guards are required to follow regulations which are consistently being updated. In 2013, the new standard for interlocking devices (EN ISO 14119) was established laying out the requirements for the design and installation.
The EN ISO 14119 standard defines the four different types of interlocking devices as:
Type 1 Interlocking Device
Device that is mechanically actuated by an uncoded actuator.
- Rotary, Linear Cam and Roller Actuators
- Hinge Actuators
- Plunger
Type 2 Interlocking Device
Device that is mechanically actuated by a coded actuator.
- Key-Interlock Actuator
- Trapped Key Actuators
Type 3 Interlocking Device
Device that is contactlessly actuated by an uncoded actuator.
Type 4 Interlocking Device
Device that is contactlessly actuated by a coded actuator.
What is a coded actuator?
According to the EN ISO 14119, coded actuators are defined as:
- Coded actuator: actuator specially designed for use with a specific interlocking device
- Low level coded actuator: coded actuator for which 1 to 9 variations in code are available
- Medium level coded actuator: coded actuator for which 10 to 1000 variations in code are available
- High level coded actuator: coded actuator for which more than 1000 variations are available.
Graph courtesy of Pizzato’s ‘Introduction to Safety‘ whitepaper.
Determining the design and degree of protection your switch needs boils down to performing a proper risk assessment.
Safety has its own set of standards and the IEC 61508 Functional Safety Standard was the first to quantify the safety performance of an electrical control system and introduce the concept of lifecycle. IEC 61508 developed the SIL concept (Safety Integrity Level) which established a way to indicate the probability of failure on demand (PFD). Increasingly manufacturers and users are requiring a SIL certification as a way to confirm the safety standards of any product.
As your project’s SIL increases, the more important it becomes to install the correct switch with corresponding safety features.
As Mike Ladd from Pizzato puts it, “In the end, you’ll be the one defending your choices when it comes to safety.” While SIL standards are becoming more popular, there are a number of safety standards that electrical components can fall under. It’s important to know which regulations are required for each project.
What is IEC 62061 and IEC 61508?
IEC 61508 is the international reference standard on functional safety of electric, electronic and programmable electronic systems. This standard consists of seven sections, the first three specifying the safety requirements for hardware and software, the rest acting as informative and supportive supplements for the correct application of the former.
IEC 62061 derives from and retains the features of IEC 61508, but simplifies safety requirements (of both hardware and software) adapting them to the specific needs of industrial machinery.
IEC 61508 Safety Integrity Level Definition:
Safety Integrity Level Risk Assessment:
So let’s review!
- First and most importantly, safety standards must be established on any manufacturing floor to prevent potential disasters.
- Thorough risk assessments defined by regulated safety standards provide your engineers with the information needed to determine which type of switch will be optimal.
- In addition to safety, the actual function of the device will determine the type of actuator is needed.
Limit Switch
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Keyed Interlock Switch
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Coded Magnetic Switch
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RFID Safety Switch
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We’ve covered a brief history of how switches became a standard in manufacturing safety and hopefully answered some questions you didn’t even know you had! Check out our wide selection of safety switches today.
Visual thinker in a digital spectrum, or in layman’s terms….I make all the visual content for Marshall Wolf Automation 🙂 With a background in video advertisement and film production, I work with MWA’s marketing department to keep our customers reading our blogs and viewing our products.
