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Accessing the hidden savings of safety interlocks with proper selection, installation, and maintenance

A deeper understanding of safety interlock technology can help overcome challenges related to installation and maintenance while also cutting costs in machine design and operation.


Photo: Omron

Every piece of equipment poses some type of challenge when it comes to maximizing machine uptime and overall equipment effectiveness (OEE). In pursuing increased OEE, however, engineers often overlook the impact of machine safety technology, especially safety interlocks, on maximizing machine uptime. The proper selection and implementation of these devices can offer cost savings beyond their primary function of keeping people safe, while the wrong safety interlock or poor installation can leave workers unprotected and reduce equipment availability.

When manufacturers follow best practices in selecting and installing safety interlocks, and users follow best practices in maintaining them, the technology can increase overall production efficiency. In this article, we’ll discuss a few of the most common challenges related to safety interlocks that prevent companies from reaping their full benefits.

 

Challenge #1: Selecting the right technology

Choosing the right safety interlock for a machine might sound like an easy task. However, as in many engineering decisions, you need to balance multiple factors like cost, safety requirements, and production needs. In weighing these factors, a risk assessment should guide your safety interlock selection. Relaying on a risk assessment will ensure that (a) all phases of the machine cycle have been evaluated; (b) the risk level based on hazard types, severity of potential injuries, and the probability of their occurrence are fully understood, and (c) the performance level requirements (PLr) for the safety function are well known.

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Using the results of the risk assessment to guide your selection will ensure that you neither under-specify nor over-specify your safety interlocks. Selecting one that fails to the meet PLr (i.e., under-specifying) will leave your workers inadequately guarded, since the protection level will be insufficient. On the other hand, selecting an interlock that wildly exceeds the performance level requirement (i.e., over-specifying) will certainly protect your operators but could also increase the machine cycle time and decrease machine availability. This is due to the additional safety hurdles that operators and maintenance teams will need to navigate in their daily routines.

An example of the latter scenario is the usage of a safety interlock with door monitoring and guard locking functionality when the stopping time of the overall system is lower than the time required to reach the hazardous area. In other words, people will never be exposed to overrun movements because the machine will stop before they reach the danger zone. Also, as door switches with guard locking and guard monitoring require additional wiring and specific programming in the safety controller, the maintenance and troubleshooting of this device can make it more difficult to determine the root cause of a fault in the safety system.

From an operational point of view, a device that exceeds functional safety requirements typically adds extra steps to the process, such as the need to press a designated button to request access and then open the movable guard. Similarly, when the guard is closed and the machine is ready to run, the operator will need to confirm in the system that the guard is closed and that the safety interlocks can be locked. This will only serve to increase the number of steps in an operation, increase the cycle time, and ultimately affect the system’s OEE.

To find more information about these considerations, users can refer to ISO 14119:2013 (“Safety of machinery — Interlocking devices associated with guards — Principles for design and selection”). This safety standard offers principles for the selection of interlocks associated with guards and additional guidance to prevent tampering.

Challenge #2: Installing safety interlocks correctly

During the machine design stage, failing to consider such basic elements as the location, purpose and installation methodology for safety devices can often become an inadvertent contributor to unplanned equipment downtime, or in the worst-case scenario, to an accident. Used on routinely accessed movable guards and doors, safety interlocks are subject to same stresses, risk of damage and potential abuse as any other moving component on the machine.

Selecting the location of the safety interlock is crucial to ensuring its long-term functionality. For example, placing a safety sensor and actuator at the bottom or middle of a movable guard may make simplify installation and maintenance, but that location also puts the safety device at risk of foreseeable damage from operators moving material in and out of the machine. Similarly, it is often tempting to have the safety interlock do double duty as a doorstop to limit the travel of the guard. Some interlocks are rated as doorstops, but not all are. Using a non-rated interlock as a doorstop may work initially, but there is a risk that the safety interlock will fail as the guard is cycled, ultimately causing unplanned machine downtime.

After selecting the proper location for the interlock, it is important to follow the manufacturer’s recommendations for the best installation methods using the appropriate fasteners. According to ISO 14119:2013, Clause 5, one of the biggest opportunities for improving installation practices is to correctly fasten the interlocks to minimize the possibility of them coming loose or changing their position relative to the actuation system during their expected lifetime. While it may seem like a simple thing, using the correct type of screw to install the safety interlock can be the difference between trouble-free operation and repeated unplanned down time.

Challenge #3: Minimizing the risk of defeat

Unfortunately, with any safety device, but especially with safety interlocks, you need to worry about operators and maintenance teams intentionally manipulating or defeating the safety function. Safety device manipulation can lead to serious and potentially fatal accidents in addition to causing unplanned downtime and raising costs. The first line of defense against workers defeating the safety functions is a safety strategy based on a risk assessment done in parallel with machine design. Aligning these two activities ensures that the selected safety interlock technology is suitable for the machine type and risk involved and is installed in a way that prevents tampering.

Safety devices on machinery, like safety interlocks, are typically defeated only if it interferes with the production cycle. Designing a safety solution that minimizes the impact on production reduces or eliminates the incentive of workers to bypass the safety device so that they can “get their jobs done.” While there are situations in which a complex solution is necessary to preserve production, often the design considerations can be as basic as ensuring that the interlock does not block the operator’s view of the process.

Similarly, selecting a good location of the safety interlock can help element the temptation or opportunity to defeat the safety device. Simply locating the interlock at the top of the guard or in an inconspicuous location helps create the attitude of “out of sight, out of mind.”  One of the most common mistakes that designers make is to locate the safety device on the “wrong side” or exterior of the guard.  This placement allows operators access to the device without opening the guard, providing them the opportunity to bypass the safety interlock without stopping the machine.

 Challenge #4: Avoiding fault masking

Fault masking in one of the least understood potential hazards that can affect the overall performance level of a safety system and make it unsuitable for protecting operators. The term refers to the unintended resetting of the safety system even when using a safety-rated control system. It can occur in the series connection of safety switches, including electromechanical, magnetic and guard locking switches.

The risk of fault masking in the conventional series connection of the safety switches restricts the performance level that can be achieved. In some cases, depending on the performance level required, the risk of fault masking and its impact on the diagnostic coverage capabilities makes the solution unsuitable as part of a risk reduction strategy. Connecting more devices in series shortens the time to a dangerous failure when doing mean-time-to-dangerous-failure calculations per ISO 13849-1. The probability of fault masking can also be influenced by the frequency of device activation, the distance between devices, the accessibility of moveable guards, and the number of operators.

Summary

Having a comprehensive safety strategy helps users and manufacturers select the right safety interlock for their applications and install it in accordance with best practices to prevent tampering and fault masking. By doing so, companies can reduce downtime related to manipulation of safety devices as well as damage that results from incorrect installation.

Is this enough to protect operators and keep machines running? Not completely. Safety solution knowledge, just like safety devices, requires maintenance and upkeep over time, and it is important to continue training employees to prevent accidents and minimize safety device manipulation. A comprehensive safety strategy that not only considers the right safety technology, but also includes safety training, will help your team understand why safety devices – including interlocks – are important and determine the best ways to select, install, and maintain them.


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