May/June 2011

Factory Automation

Implementing safeguards creates cost-effective machine safety solutions

Safety light screens, interlocked guards, safety controllers can become ideal additions to any comprehensive solution

Fast Forward

  • Contact with machines is a leading cause of work-related injuries.
  • Technological advances provide safety solutions for guarding multiple hazardous machine applications.
  • Safety light screens, interlock guards, and safety controllers and modules offer comprehensive protection against equipment damage and employee injuries that are flexible, user-friendly, and cost-effective.
By Mike Carlson

Switches are interfaced with the machine's control system to react when the guard is moved; the switch sends a stop signal to the machine to discontinue operation while the guard is open.

As technology and economic demands increase, so does reliance on automation and machinery for consistent, cost-effective productivity. While the use of machinery is extremely advantageous-from an efficiency standpoint and a reliability standpoint-these powerful machines can be extremely hazardous, resulting in injuries ranging from impact and crushing to amputations or even death. Generally, the more frequently employees interface with machinery, the higher the chance for potential injury-that could result in not only pain and suffering to the employee, but also in excessive costs and liability for a company. Implementation of the appropriate safety devices and procedures will limit exposure to machine hazards and will also help avoid loss of production and preserve company resources in the process.

The adage "a penny saved is a penny earned" has never been truer than when regarding machine safety and injury prevention tactics. Each year, companies nationwide will spend an estimated $170 billion on direct and indirect costs associated with work-related injuries. According to the American Society of Safety Engineers for every $1 spent on injury prevention, as much as $4.41 was returned to the company. A single work-related injury has significant consequences beyond the mere direct expenses. The ratio of indirect to direct costs of an accident is an estimated 2.12:1; this means for every $1 spent on a direct cost to an injury, a company will expend an extra $2.12 in indirect expenses. Indirect expenditures encompass everything from increases in insurance premiums and human resources costs to the time lost quarantining the machine involved and hours of administrative tasks.

Loss of productivity is also a critical concern after injuries occur. The average work-related injury will cause an 11.1-day absence from work per incident. Additionally, many employees have to devote time to the requisite accident report, which causes a decline in productivity from the witness and the interviewing manager.

Pure monetary losses and the pain and suffering to the employee are not the only issues involved with workplace accidents. The impact to co-workers, family, and friends of the victim ripples outwards like a stone dropped into calm waters. These ripples can have emotional, financial, and mental consequences that are nearly impossible to predict or track.

To ensure worker safety and conserve company assets, simple yet effective measures can be taken to increase machine safety and equipment preservation. Safeguarding solutions proven to be effective in providing superior machine safety include safety light curtains, interlock guards with safety interlock switches, and safety controls and modules.

Safety light curtains

Common threats posed by interaction with machinery by personnel include amputation and trapping/crushing by equipment. According to the Bureau of Labor Statistics, 24% of all work-related injuries arise from contact with machinery, roughly 28 cases per every 10,000 full time employees-making it a primary source of fatal occupational injuries. By strategically placing safety light curtains (often referred to as safety light screens) around hazards, companies can prevent harm to employees and damage to expensive equipment. Safety light screens allow safe, unhampered production for a variety of machine-related applications ranging from automated processes to hand-fed machines.


Safety light curtains are composed of an LED array, which emits beams of light to phototransistor receivers. This creates an optical "wall of light" guarding access to a hazard. The curtain is designed to react when an opaque object (such as a hand, ankle, or torso) passes through the beams of light, creating an incomplete optical path between emitter/receiver, which then results in sending a stop signal to halt all hazardous motions or remove the hazardous situation before an injury occurs. The primary purpose of the safety light screen is to reduce the risk of harm by preventing the machine from operating once an object has entered the sensing field. Once the field is clear, the safeguard will allow operation to resume by actuating the normal means of cycle start. However, safety light screens are not appropriate for machines that cannot be stopped quickly, produce ejected materials, or are subjected to intense heat conditions.

Safety light screens offer an abundance of features allowing for a customizable safeguard. When selecting safety light screens, considerations should include: response time of the safety light screen and machine, sensing capabilities of safety light screen (resolution), the height of the sensing field, and the distance between the emitter and receiver. Determining the required resolution (minimum object sensitivity or detection capability) is vital in selecting the optimal safety light screen for an application. High-resolution safety light screens provide increased sensitivity to enable detection of a finger while low-resolution capabilities identify larger obstructions, such as a torso. Resolution determines the "depth penetration factor"-the distance that you can reach into the sensing field before the safety light screen can reliably detect the intrusion. This distance (also called the "C" adder in International/European safety standards) is added to the distance that results from multiplying the hand-speed constant (63" per second) by the total time it takes to stop or remove the hazard. These two distances added together determine the distance between the safety light screen sensing field and the hazard. If the safety light screen has a higher resolution, the safety light screen can typically be located closer to the machine.

The versatility of safety light screens allows for multiple application and mounting options. Large hazardous areas can be contained with parameter guarding, which uses safety light screens with large resolutions or individual multiple beams systems, sometimes called "grids," installed at entryways to prevent machines from functioning if workers pass through. Safety light screens can also be used as an effective replacement for safety mats by mounting safety light screens horizontally and low to the ground. This can produce the same safeguarding function as a safety mat, but eliminates the reliability issues of physical damage to the mat.

Safety light screens have many advantages that minimize the risk of injury, including the flexibility to allow easy access to the machine for maintenance or repair, thus reducing the motivation to bypass safeguarding for frequently accessed machinery. Since there is no physical obstruction, the safety light screen permits the hazardous area to be readily visible. The lack of a physical barrier, which can create physical strain when moving, also makes safety light screens very ergonomic.

Safety interlock guards, switches

While safety light screens provide "invisible" protection, some machinery applications require a more noticeable protective measure. In these cases, physical barriers or guards can be used to prevent inadvertent contact, thus providing protection against injury and damage.

Guards are popular due to their simplicity, and depending on the type of protection intended, there are several options to consider. Fixed guards are physical barriers that can completely surround a hazard. This is ideal for machines that do not require frequent access, as these guards can be heavy or difficult to move. Adjustable guards are another option, while less permanent than fixed guards, these guards require adjustments and constant supervision. Alternatively, interlocked guards typically can be easily moved-opening like doors, gates, or covers-allowing frequent access for feeding, adjustments, or minor maintenance.

Safety interlocking switches are used in combination with interlocked guards to monitor the position of easily moveable guards. Switches are interfaced with the machine's control system to react when the guard is moved; the switch, or switches depending on the required level of performance, sends a stop signal to the machine to discontinue operation while the guard is open. Interlock switches will prevent the machine from operating while the guard is open; however, "locking-style" safety switches can prevent a guard from being opened before the machine has effectively ceased operation.

Once the guard has been closed, the switch reactivates, and the machine is allowed to resume operation. To be considered an effective protective measure for injury prevention, guards must be sized in a way that will eliminate any worker from reaching around, under, through, or over the guard to access the hazard. However, interlock guards are not appropriate for all situations, and typically, they are not utilized when visibility or frequent access is required.

To meet application-specific requirements, there are multiple styles of switches to choose from; options include electro-mechanical, fiber optic, RF transponder, and magnetic switches. Electro-mechanical switches are the most common and function by physical contact with the switch. When the interlock guard is moved, mechanical force is applied to the switch to create a positive opening force, which opens electrical contacts to power down the machine until the guard is closed. Fiber optic, RF transponder, and magnetic switches are non-contact, requiring no physical force for operation. For instance, fiber optic switches create a beam of light that is broken with movement of the guard and causes the machine to stop. Non-contact options are more practicable for high contamination, heavy vibrations, or washdown environments.

Interlocked guards and safety light screens are highly effective protective measures for safeguarding employees and protecting equipment. Averting accidents not only helps to reduce injury rates, but also, according to the Occupation Safety and Health Administration, directly benefits company profit. Consider the direct and indirect costs a company will incur for a single occurrence of a commonplace injury. An employee with an amputation will average direct and indirect costs with an estimate of $101,467, and the additional sales needed to cover these expenses would be roughly $3,382,233. Another instance of a worker crushed or trapped by a machine would be an average total cost of $95,071 with an additional sales requirement of approximately $3,169,033. These examples illustrate the significant impact that implementing proper safeguarding solutions have on any company utilizing machines in every-day production, which is why taking machine safety precautions is vitally important to protect employees, equipment, and the company.

Safety modules, controllers

In addition to safeguarding solutions, such as safety light screens and interlock guard switches, a company can enhance its protective measures with safety modules and safety controllers. Safety modules and safety controllers are among several options for interfacing and monitoring safeguarding devices. Other options include safety buses and safety programmable logic controllers (PLCs), which can be costly substitutes to safety controllers and modules. Safety buses are used to monitor multiple safeguarding devices, which can be expensive, complex, and unnecessary for a single stand-alone machine. Alternately, safety PLCs are viable for monitoring the function of safeguards linked to complex machine systems, but again, can be costly to install. In place of these methods, safety modules and controllers provide a flexible and cost-efficient solution. Safety modules typically monitor or perform one or two functions that guard against a failure or a fault that could result in the loss of the safeguarding function. Safety controllers are used to integrate multiple safety devices or replace multiple safety modules to economically meet the required level of safety performance.


Safety controllers can provide an integrated approach to risk reduction by managing the operation of multiple safety devices. For example, if a machine has emergency stop buttons, two-hand control, safety light screens and interlocked guard switches, a safety controller can be used to manage all of these operations at a fraction of the cost of multiple safety modules, safety BUS, or safety PLCs. Safety controllers can also provide several safety-related functions, such as muting or bypassing of a safety light screen, Presence Sensing Device Initiation, external device monitoring of devices under control, and monitored manual reset functions.

The extensive costs that result from each machine-related injury have lasting and devastating effects. Utilizing safeguards like safety light screens, interlocked guards, and safety controllers can become ideal additions to any comprehensive safety solution, providing superior protection for employees from injury and equipment from damage.


Mike Carlson is a safety products manager at Banner Engineering.