Machine builders and end users can now enjoy the benefits of new machine guarding technologies endorsed by international safety standards. From the most sophisticated manufacturing operation to the simplest relay-based system, economical and effective choices are now available to enhance machine safety. In the 1970s, machine builders began to change the way they wired machines by replacing relays with automatic sequencers, known today as programmable logic controllers (PLC). However, machine builders addressed safety as an afterthought. Regulators excluded PLCs from machine safety, and little attention was paid to safety by end users.
Today it is a different story.
It is no longer necessary to attach steel cables to operators’ wrists to keep hands out of machine pinch points, for example. Safety is now automatic. The latest options include integrated, networked safety systems that use reliable safety PLC technology. Designed and built according to IEC guidelines and tested by nationally recognized testing laboratories, such as Technical Inspection Association (TUV) and Underwriters Laboratories (UL), PLCs, buses, I/Os, and other components are replacing traditional hardwiring on machines.
The most important benefit of integrated, automated machine safety is enhanced operator protection. According to one study, machine-related injuries are among the most common in the workplace. The National Institute for Occupational Safety and Health (NIOSH) indicates the fatality rate from machine-related accidents was second only to motor vehicle-related accidents and recorded higher fatality rates than from homicides, falls, and electrocutions. In addition to protecting machine operators, machine builders and end users alike are realizing other benefits to enhancing machine safety:
- Lower cost of controls
- Speed time to market
- Decrease machine downtime
- Reduce litigation
Incorporating safety into machine designs begins by understanding recent changes in international safety standards and regulations. Then, it is simply a matter of applying the appropriate safety options to meet these requirements. When considering these options, it is important to include total lifecycle costs in the decision-making process.
How increased safety standards reduces operating cost
In a challenge to conventional wisdom that increasing safety adds cost, new research and evidence reveals the opposite. Companies that implement safety functions, perform functional safety evaluations, and implement safety in manufacturing processes are finding benefits where few expected to: on the bottom line. Where companies once saw up-front costs, downstream revenues are being tallied. For example, the potential for greater human interaction with the automated equipment can improve productivity.
There are other financial benefits of implementing safety standards. First, safety compliance opens up markets or keeps them open. It helps align with the EU’s Machinery Directive. EN/ISO 13849-1, Safety of machinery-Safety-related parts of control systems-Part 1: General principles for design, which is harmonized under the EU Machinery Directive. Second, insurance companies are inclined to look favorably on enterprises that have implemented safety products and processes, and may lower premiums or discount rates accordingly.
The importance of NFPA 79
Recent developments to machine safety include changes in standards from the National Fire Protection Agency (NFPA). In the fall of 2002, NFPA 79 was republished with application guidance for failsafe, safety-rated PLCs, and safety-rated buses in functional safety applications. It also established requirements for a risk analysis to be performed on all machinery and described E-stops as a part of the safety design. All safety-rated devices could thus be installed on a safety-rated bus.
The code text changes appear as the following:
- Original NFPA 79 1997
Where a Category 0 stop is used for the emergency stop function, it shall have only hardwired electromechanical components. In addition, its operation shall not depend on electronic logic (hardware or software).
- NFPA 79 2002
Allows PLC use in safety-related functions. 11.3.4 use in safety-related functions. Software and firmware-based controllers to be used in safety-related functions shall be listed for such use.
- Annex to NFPA 79 2002, A.11.3.4 IEC 61508
Provides requirements for the design of software- and firmware-based controllers for use in control systems performing safety-related functions.
- NFPA 79 2007
Drives or solid-state output devices designed for safety-related functions shall be allowed to be the final switching element, when designed according to relevant safety standards.
- NFPA 79 2012
New definitions and revised rules for expanded wireless and cableless technology that align with IEC 60204-1. The new NFPA 79, now the benchmark for industrial machinery safety, is aligned with the NEC and NFPA 70E.
These changes allow manufacturers to develop powerful solutions that replace hardwired relays with PLC safety circuits that have built-in safety functionality. This built-in safety greatly reduces cost, requires less time to implement, and increases machine uptime.
Other important standards
- ANSI B11.0: 2010
ANSI B11.0 Safety of Machinery (General Requirements & Risk Assessment) may be considered an extremely significant document for machinery safety. The scope of the standard states that it applies to new, modified, or rebuilt power-driven machines, not portable by hand, used to shape and/or form metal or other materials by cutting, impact, pressure, electrical, or other processing techniques, or a combination of these processes. There are many standards for machines in a variety of industries, and many machines in all industries for which there is no industry standard. Therefore, this B11.0 standard could have very far-reaching impact.
- ISO 12100: 2010
Particularly important for European and global applications, ISO 12100:2010 (ISO 12100) is the international standard that will help designers identify risks during the design stage of machine production, reducing the potential for accidents. Guidance is given on the documentation and verification of the risk assessment and risk reduction process. Overall, ISO 12100 applies to the system level, but specific elements trace down to the product or component level. Many specific machines have no associated type C (application-specific) standard. In these cases, ISO 12100 applies to identify hazards and risks not yet identified by a type C standard.
Benefits of risk assessment
To ensure compliance with the essential health and safety requirements, a risk assessment must be carried out for machinery and partially completed machinery. The manufacturer or authorized representative may delegate the performance of the risk assessment to a third party, but retains responsibility for its completeness and accuracy. Risk assessment is part of the required technical documentation. Failure to perform one can result in a design error and/or incorrect instructions. It protects against allegations of negligent or even culpable action and, in some cases, against possible consequences under criminal law. Transparency of processes, decisions, and results helps to defend against liability claims. If applied correctly, it can result in cost savings. So following the risk assessment standards is a major step in overcoming liability issues.
The risk analysis reference in ISO 12100 formalizes what has been a requirement all along, but was not an absolute standard. It was once assumed that people would follow due diligence and engineering principles to provide a safe workplace, as required by OSHA.
Now, a formal process must be followed to evaluate risk potentials throughout all modes and operations of a given machine. This process identifies all risk levels that could injure the operator, maintenance personnel, or even individuals walking past a machine. The person conducting the risk assessment must be trained and understand how machinery operation and production is affected by applicable codes and standards.
The following are the most important factors taken into account during a risk assessment:
- Severity of foreseeable injuries
- Probability of occurrence
- Frequency of exposure to hazard
- A list of actions required to meet applicable standards (showing appropriate actions to ensure personnel safety)
By evaluating the machine and the environment around it for safety, a risk assessment lets a manufacturer know what needs to be changed to meet applicable codes. It also significantly lowers the risk to machine operators. If an injury occurs, OSHA will ask what the employer has done to make the area safe.
A risk assessment shows that the employer took steps to understand and correct any associated standard violations.
Looking beyond the factory doors and into the global marketplace, OEMs and machine builders familiar with the risk assessment regulations are expanding sales internationally. All machines shipped to Europe are required to provide complete risk assessment documentation.
Safety strategy options
The right safety strategy can provide a competitive advantage for the machine builder and manufacturer. The recent changes in safety standards mentioned above have opened the door to new solutions that would not have been permitted under the old rules. Choosing the right option can result in quicker time-to-market, with higher product throughput and lower total cost of ownership for safety systems, improving both overall equipment effectiveness (OEE) and return on assets (ROA).
Here are four safety options to consider:
1. Dedicated safety relays
The mainstay of safety circuits for decades, dedicated safety relays continue to be used. However, while they may help meet machine safety standards, the overall costs may outweigh the benefits. These relays significantly limit the ability to monitor and troubleshoot machines. For example, a single machine might incorporate 20 emergency stop buttons, any one of which must shut it down. Traditionally, E-stops are wired in series to decrease the cost of wiring.
The challenge comes when a fault occurs and the operator needs to detect the problem. Using this conventional wiring method, many manufacturers often spend 10-20 minutes diagnosing the problem on the machine.
Also, many operators using machines that rely on this configuration often bypass the safety relays with short pieces of wire so that opening the cage door or breaking the beam on the light curtain will not shut down the machine. They claim this action increases efficiency while minimizing downtime because a maintenance person can get to the machine much more quickly and work on it even if it is still running.
Despite the perceived advantages, this “jumpering” produces a dangerous condition and a major safety violation. Newer solutions cannot be manipulated in this manner and therefore provide an extra level of safety.
2. Networked safety relays
These devices can significantly lower the cost of single- and multi-zone applications by allowing one device to be wired to the entire safety circuit, networking to each individual device. This configuration significantly lowers the cost of wiring, allows individual safety incidents to be monitored, and permits fast troubleshooting. It is also a very good solution for safety systems with relatively low complexity (controlling two or three safety zones, for example). However, networked safety relays may not be the best solution for highly complex systems where minimizing control programming is required.
3. Dedicated safety PLC
When a PLC is already in place and controlling a machine, a safety PLC may be added to the system. This solution can add greater monitoring capabilities to the system if the safety PLC is networked to the control PLC and allows use of the existing PLC program. Challenges with this solution, however, are twofold: the added safety PLC is a new expense to the system, and it introduces another programming language to learn, implement, troubleshoot, and maintain.
4. Integrated, simplified safety system
Combining the functionality of a control system and a safety system into one PLC allows manufacturers to greatly reduce lifecycle costs on a machine. Many PLCs, for example, combine integrated control and safety into one controller, saving significant overall costs. This type of integrated safety system allows all data to flow to the human-machine interface (HMI) for fast and easy troubleshooting. This approach simplifies machine control and safety system coordination, from design to installation to troubleshooting. Design and implementation are simplified by using the same programming language for control and safety circuits. Wiring is simplified by using safety networks to monitor and/or control each device on the safety circuit.
Troubleshooting is often cut by 60 to 80 percent since each networked safety device communicates via the same HMI as the rest of the control system. These advantages significantly reduce downtime and the costs associated with failures. An integrated safety system also makes it nearly impossible to bypass the safety circuit by jumpering out a safety device, including a door switch or light curtain.
Technological trends for machine builders and integrators with additional cost-saving opportunities include the following:
- Larger numbers of safety devices can now be connected on the network, such as failsafe motor starters and safety drives. Most of these devices have traditionally been hard-wired and provided only a minimal level of diagnostics.
- Wireless safety is a growing trend. In automotive and aerospace assembly operations, warehouses, distribution centers, or material handling applications, it is difficult and expensive to do all the wiring required to integrate the safety systems. So wireless is the perfect fit. Moreover, the technology is easier and faster to implement and uses a minimal amount of floor and cabinet space. Wireless is at the cutting edge of technology for safety systems.
- Mobile HMI panels with E-stop functionality are taking safety and safety networking to a higher level. They provide tremendous flexibility for control and safety to the operator on the plant floor, allowing uninterruptable changes between access points.
Integrated safety saves lifecycle costs
When considering the right safety option for the application, take into account the entire lifecycle cost of the product or system, not just the purchase price. Consider design, wiring, and production efficiencies.
Design: How much time can you save by implementing networked safety and control into one system? Mechanical, electrical, and programming issues are greatly simplified with a single PLC.
Wiring: Installing an integrated system costs far less than hardwiring. By transporting safety and regular production data on a single network (such as PROFINET, PROFIBUS or AS-Interface), this architecture requires the use of only one cable instead of hundreds or thousands of wires.
According to the electrical project engineer for a packaging machine manufacturer, “a more complex, discrete wired machine can take six electricians more than 368 hours to wire and start up. Integrating distributed I/O and safety I/O eliminates manufacturing redundancy and reduces complexity. Two electricians can wire up that version of the system in just 96 hours.”
In addition, manufacturers can now integrate electronic and programmable safety systems directly into servo drives, permitting axis movement at safe speeds while an operator is in the working envelope. This change further reduces the number of cables and connections, again lowering safety system complexity, design, commissioning, and installation costs.
Production efficiencies: If a manufacturer’s downtime costs $10,000 an hour, it does not take long to justify a low-cost, integrated system that saves 30 minutes each time a safety circuit is activated.
Overall cost savings from integrating safety on a standard OEM machine can be considerable. Significant savings can be seen, when new and old methods of safety and automation system implementation are compared. Five key aspects that affect the overall cost are hardware design, software design, hardware costs, build and assembly, and field wiring. Savings are found in every major area, up to 35 percent in total cost of ownership (TCO) for the automation and safety implementation on each machine built.
Integrated safety saves
Unless a machine employs only one or two safety relays, a networked safety system using a single PLC will deliver far greater benefits than traditional hardwired methods. Improved machine safety, reduced time to market, and lowered lifetime cost may be achieved by working with an experienced automation and safety advisor.