Machine safety pays off
Plant engineers are taking advantage of new machine guarding technologies endorsed by international safety standards. From the most sophisticated manufacturing operation to the simplest relay-based system, machine manufacturers and end users now have economical and effective choices to enhance machine safety.
The latest options include integrated, networked safety systems using reliable safety PLC technology. Designed and built according to IEC guidelines and tested by a nationally recognized testing laboratory, safety PLCs, buses, I/O and other components are replacing traditional hardwiring on machines.
The most important benefit of integrated, automated machine safety is enhanced operator protection. The National Institute for Occupational Safety and Health (NIOSH) reports that fatality rates from machine-related accidents were second only to motor-vehicle-related accidents, and recorded higher fatality rates than homicides, falls and electrocutions.
Safety standards drive changes
Recent developments to machine safety include changes in standards from the National Fire Protection Agency. In the fall of 2002, NFPA 79 was re-published providing application guidance for failsafe, or safety-rated PLCs and safety-rated busses to be used 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.
These changes allow manufacturers to develop powerful new 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.
The risk analysis
In 2004, ANSI B11 TR4-2004 was approved. It provides application guidance for safety-rated hardware and software-based devices in functional safety applications. These and other standards reference IEC 61508, 62061, 60204-1 and EN 954-1 (which will be phased out next year) %%MDASSML%% many requiring a formalized risk analysis to establish risk reduction methodologies. I SO 13849-1 2006 will replace EN 954-1 .
The risk analysis formalizes what some companies had been doing all along %%MDASSML%% even though the assessment wasn’t considered 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 risk assessment process, as described in the ANSI Z10 standard from 2005 titled Occupational and Health and Safety Management Systems must be followed to evaluate risk potentials throughout all modes and operations of a given machine. In addition, ANSI PMMI B155 2006 and S2 for the semiconductor industry to mention a few, have been updated to more clearly call out the risk assessment requirement and reference other standards for more detailed information.
The risk assessment process identifies 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.
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 the employer has taken steps to understand and correct any associated standard violations.
Looking beyond the factory doors and into the global marketplace, OEMs and machine builders know that machines shipped to Europe are required to have complete risk assessment documentation.
The right safety strategy can provide a competitive advantage for the manufacturer. The recent changes in safety standards have opened the door to new solutions that would not have been permitted under the old rules. Choosing the right option can result in a quicker time-to-market with higher product throughput, and a lower total cost of ownership for safety systems, improving both overall equipment effectiveness and return on assets.
Safety options to consider include:
Dedicated safety relays: The mainstay of safety circuits for decades, dedicated safety relays continue to be used even today. However, while dedicated safety relays 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 %%MDASSML%% any one of which must shut it down. Traditionally, E-stops are wired in series to decrease wiring cost. 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 to 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 of this bypass process, this “jumpering” produces a dangerous condition and a major safety violation. Newer solutions cannot be jumpered in this manner, and therefore provide an extra level of safety.
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, providing 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. For example, an ASIsafe network offers hundreds of device options that can be cost effectively networked for safety control and monitoring.
Dedicated safety PLC: When a PLC is already in place and controlling a machine, a safety PLC may be added to the system to provide any additional safety functionality. This approach can add greater monitoring capabilities to the system if the safety PLC is networked to the control PLC and uses the existing PLC program. However, challenges with this solution are the new expense when a safety PLC is added to the system, and the introduction of another programming language to learn, implement, troubleshoot and maintain.
Integrated, simplified safety system: Combining the functionality of a control system and a safety system into one PLC allows manufacturers to greatly reduce life cycle costs on a machine. For example, the latest safety PLCs combine integrated control and safety into one controller. Already implemented in many applications, manufacturers are saving millions in overall costs. The integrated safety system allows data to flow to the 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% since each networked device communicates via the same HMI. 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 %%MDASSML%% including a door switch or light curtain.
Integrated safety saves life-cycle costs
When considering the right safety option for the application, take into account the entire life-cycle cost of the product or system %%MDASSML%% not just the purchase price. Considerations include:
Design: How much design time can be saved 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 ASIsafe or PROFIBUS), this architecture requires the use of only one cable instead of hundreds, or thousands, of wires and connections.
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. Typically, two electricians can wire that version of the system in just 96 hours.
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 reduces the number of cables and connections further %%MDASSML%% again reducing safety system complexity and lowering design, commissioning and installation costs.
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.
|J.B. Titus is a senior safety consultant and safety product specialist with Siemens Energy & Automation in Norcross, GA. Since starting with SE&A in 2003, he has organized and launched the safety initiative and business focus for the U.S. discrete manufacturing markets. He has also consulted with clients over the past 25 years regarding machine functional safety compliance. Titus holds a BBA from Oklahoma University in Industrial Management and an MBA from Case Western Reserve University in marketing and finance. He is a professional member and Certified Safety Professional (CSP) of ASSE and is OSHA-certified in machine guarding. Titus also serves on several ANSI, NFPA and NEMA national safety and health standards committees.|