Safeguarding: Make Intelligent Safety Light Curtain Choices

As machinery safety standards and safety light curtains evolve to meet new application demands, users are faced with more choices and responsibilities than ever before.

A new breed of light curtain is gaining recognition in the United States. Developed in Europe and classified as "Type 2," it is a lower-cost, reduced-capability alternative to the more robust "Type 4" control-reliable safety light curtains typically used to safeguard machinery in the United States. The terms and definitions of the product "Type" are derived from the international standard for light curtains, IEC 61496. Understanding the capabilities and differences between these two types of machine safeguarding devices will help users determine which is right for their application.

Three Differences

In most instances, Type 2 and Type 4 safety light curtains look much the same. However, these photoelectric safeguards are designed to satisfy vastly different safety requirements. Essentially, the Type 2 products are designed to a lower level of safety integrity and must not be used in applications where the Type 4 control is the appropriate choice. Although the differences are technical and based on various industry standards, these devices differ in three significant areas:

1. Fault Detection Circuits. Type 2 light curtains lack the redundant automatic self-checking circuits employed in Type 4 light curtains. As a result, the Type 2 light curtain does not meet the OSHA or ANSI standard for control reliability (more on this later). Type 4 safety light curtains are designed to immediately detect the failure of a single component within a defined response time. This is not true of Type 2 light curtains.

2. Optical Angle. Traditional Type 4 safety light curtains have an effective optical angle of ±2.5 degrees, while Type 2 devices have an effective optical angle of ±5 degrees. The wider optical angle increases the possibility of reflective surface interference, where a reflective object near the sensing field of the light curtain causes an optical "short circuit." As a result, an object in the sensing field may not be detected, as the light "bends" or reflects around the object. This possibility demands users take great care during installation and alignment to ensure proper operation of the Type 2 device. Fortunately, there are simple tests to detect this potential hazard. The tests must be performed during installation and periodically afterwards for any light curtain.

3. Price. The third difference is price. Type 2 devices are typically 15 percent to 30 percent less expensive when compared to an equivalent Type 4 device. The cost difference stems from the less precise optical angle and fewer fault detection circuits. In addition, Type 2 light curtains typically have fewer available features, such as exact blanking, floating blanking, MPCE monitoring (Machine Primary Control Element) and MTS (Machine Test Signal).

Control Reliability

Required for safety-related machine control systems by the Occupational Safety and Health Administration (OSHA) and the American National Standards Institute (ANSI), control reliability is defined as follows:

"The device, system or interface shall be designed, constructed and installed such that a single component failure within the device, interface or system shall not prevent normal stopping action from taking place, but shall prevent a successive machine cycle." (ANSI B11.19-1990, 5.5)

Examples of machines that require control reliability include machine tools, such as power presses, shears, press brakes and robots.

A Type 4 safety light curtain employs self-checking circuitry to monitor itself for internal faults. If it detects an internal fault, the Type 4 safety light curtain immediately sends a stop signal to the guarded machine and the light curtain enters a lockout condition. Only after replacement of the failed component, and an appropriate system reset, will the Type 4 light curtain and the machine it is guarding be restored to operating condition.

Because Type 2 light curtains lack the redundancy of internal fault detection circuits, they should not be considered control-reliable and, thus, not suitable as a safeguarding option on machinery that needs to stop in a preset time to ensure safe operation or where OSHA or ANSI requirements or a risk assessment requires control reliability. Type 2 light curtains simply cannot be modified to be made into control -reliable devices.

A Question of Interpretation

Because of these differences, Type 2 light curtains are intended for use in machine-guarding applications where the worst-case injury resulting from an accident may be remedied by simple first aid.

The social, legal and political cultures of Europe and the United States are distinct, resulting in different interpretations of "simple first aid." These differing interpretations impact a user's decision as to whether to apply a Type 2 or Type 4 device in a given application.

In Europe, first aid is measured, in part, on the amount of time an employee misses from work. For example, if a worker is injured and must go to the hospital for stitches or other medical procedures, yet returns to work the same day or the next day, it would be considered simple first aid.

In the United States, these injuries would be considered much more serious due to the nature of the injury itself, lost machine and worker productivity (the "gawk" factor, lower employee morale, investigating the cause of the injury, etc.), required injury reports, preparing insurance claims and so on.

In the United States, first aid is defined in OSHA 1904.12 as any one-time treatment, and any follow-up visit for the purpose of observation of minor scratches, cuts, burns, splinters, and so forth, which do not ordinarily require medical care. Such one-time treatments and follow-up visits are considered first aid, even though they are provided by a physician or registered professional personnel.

Application Considerations

Both Type 2 and Type 4 devices have a role to play in keeping workers and their machinery safe and productive. But it is important to review each application, and its potential for injury, when applying these or any other machine safeguard.

Differences in safety performance demand that the decision to use a Type 2 or Type 4 safeguard be based on a thorough and complete risk assessment for each machine. Users must also consider the interpretation of an injury based on the type of first aid required and possibilities of infrequent exposure, as described in the American National Standard for Industrial Robots and Robot Systems, ANSI/RIA R15.06, and ANSI Technical Report for Machine Tools, ANSI B11.TR3.

Type 2 light curtains offer an effective, low-cost safety solution in applications where a thorough and complete risk assessment determines that injuries can be remedied by simple first aid and require no professional medical attention outside of the definition as stated in OSHA 1904.12.

After low risk has been assessed, Type 2 light curtains have a number of potential safety and non-safety applications, including semiconductor equipment, storage and conveying equipment, small textile equipment, packaging equipment (with the exception of palletizers), process protection, parts counting, tooling guarding and inspection equipment.

As a general rule, if a risk analysis determines that the severity of the potential injury is high, regardless of its probability, a Type 2 device is not an appropriate machine safeguard. In addition, Type 2 must not be used where regulations require the use of control reliability circuits Any potential for an accident that will result in a severe injury should be treated with the most conservative approach to safety equipment selection and application.

Last and simply, when considering the application of a Type 2 light curtain, ask, "Would I think twice about putting my hand in this area?" If the answer is yes, then by U.S. standards any resulting accident would require more than simple first aid, and should qualify the use of a Type 4 safety light curtain in the application.

Sidebar: Application Examples Type 2 or Type 4?

Determining when to use a Type 2 or Type 4 safety light curtain may best be demonstrated by reviewing a couple of examples. While it is essential to perform a complete risk assessment on all machines, only the severity of the potential injury needs to be considered to decide between a Type 2 and Type 4 safety light curtain.

First, a pharmaceutical company's packaging department uses index tables, conveyors, filling and labeling equipment and a multitude of moving parts.

In the assessment process, the user determined that the size and force of the motors used on the index table and conveyor was insufficient to cause serious injury. The worst-case injury was defined as a potential bruise requiring simple first aid.

In this application, a Type 2 light curtain is the light curtain of choice. It serves as an appropriate safeguarding device, while doubling as a process control device.

In our second example, the assembly department of a gas and pneumatic regulator manufacturer requires that an operator continuously interface with the assembly equipment. The workstation consists of a small pneumatic press and an automatic self-feeding screwdriver. The operator must insert a multitude of parts that need to be compressed while the driver inserts screws.

The worst-case injury would require off-site medical attention. In this application a Type 2 device is not an appropriate safeguarding choice, but rather a Type 4 light curtain is recommended.

Sidebar: Don't Roll the Dice Perform a Risk Assessment

Conducting a thorough risk assessment requires the user follow a formal procedure that considers many factors when looking at machinery hazards. A risk assessment must be applied in a consistent manner across all plant machinery. This will enable the user to logically evaluate safety hazards and hazard guarding solutions. The process considers all hazards and each type of safety hazard on a given machine.

The risk assessment analyzes each hazard by breaking it down into three components: frequency of exposure, probability of injury, and severity of the potential injury.

An operator, for instance, typically has a high level of exposure, while someone performing maintenance does not. Probability considers machinery speed, and compares it to a person's typical reaction time - so a fast-cycling machine will have a higher probability injury than one that is a relatively slow. The user must also estimate the type of potential injury in terms of severity, ranging from a simple pinch on the low end, to loss of a digit or even life at the other extreme.

Severity of injury should always dictate the assessed risk level. If the severity of potential injury is high, but exposure and probability are low, a Type 2 device is not an appropriate safeguarding option. Type 2 devices are not intended for use where ANSI B11.19, OSHA 1910.212 or 217 apply, and should never be used on a mechanical power press. Type 2 devices are not and cannot be made control reliable.

Based on the risk assessment results and the type of hazard, the user can work with a safety expert to determine the most appropriate machinery safeguards for each application.

Remember that a light curtain, whether Type 2 or Type 4, may not be right for every machine safety application. Other safety equipment, such as safety mats, safety switches, hard guards or a combination of equipment may offer the optimum solution.

Machine users should reference ANSI B11.TR3, ANSI/RIA R15.06-1999, and/or EN1050 prior to beginning their formal risk assessment. A comprehensive discussion of safety strategy and risk assessment, including formal procedure documentation, is available at http://www.sti.com/safety/index.htmwww.sti.com/safety/index.htm.

Joseph J. Lazzara is president and CEO of Scientific Technologies Inc. (STI), the largest provider of automation safeguarding solutions in North America (www.sti.com). He has a bachelor of Environmental Engineering degree from Purdue University and a masters in Business Administration degree from Santa Clara University. He is chairman of the Safety, Health and Environmental Committee for the Association of Manufacturing Technology (AMT). He is also a member of the board of directors of the American Electronics Association (AEA).

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