Machine safety is serious business, and understanding relevant machine safety standards should be considered a starting point. With so much at stake—from employee safety to litigation costs—an original equipment manufacturer (OEM), machine builder, system integrator, or end user needs to learn as much as possible. Machine safety is regulated by a host of national and international safety standards and enforced by government agencies.
In the U.S., the Occupational Safety and Health Administration (OSHA) and National Fire Protection Association (NFPA) are the primary agencies, while the International Electrotechnical Commission (IEC) and International Organization for Standardization (ISO) originate in Europe but hold increasingly global influence. All these regulations, standards, and agencies cause confusion for machine builders and end users.
When an accident occurs and a worker is injured or killed, lawsuits and OSHA investigations often follow. Attorneys and OSHA will ask the question: What did the company do to ensure that the machine was as safe as possible? In most cases, if a lawsuit occurs, the machine builder, automation equipment manufacturer, and the systems integrator may also be named in the lawsuit.
Fortunately, machine safety regulations are becoming more standardized. In 2012, the NFPA brought NFPA 79 into alignment with the European Union’s Machinery Directive (EN/ISO 13849-1). NFPA 79 deals with safety-rated programmable logic controllers (PLCs) and safety buses, so this was a major step. Today, if equipment vendors, machine builders, systems integrators, and end users follow EN/ISO 13849-1, it’s a good start toward protecting workers and surviving lawsuits. Machine safety remains a complex and subtle task, but the rules are becoming clearer and safety equipment more capable.
Since the 1970s, when few safety regulations existed, machine safety regulations have followed a difficult, contentious path toward standardization. The major problem was that the standards were not keeping up with safety technology. The original NFPA 79 regulations required the use of hardwired components, such as emergency stop pushbuttons. It was amended in 2002 to allow the use of safety PLCs and software-based controllers, and in 2007 to allow drives and other equipment designed for safety to be used as switching elements. Finally, in 2012, NFPA 79 adopted new rules and regulations that brought it into alignment with IEC standards and the National Electrical Code (NEC). Meanwhile, similar events were happening in Europe with the EN 954-1 safety standard, which had been in place for many years, but did not address programmable electronic safety equipment nor consider failure probabilities. Efforts were made to replace EN 954-1 with the new EN ISO 13849-1 and EN 62061 safety standards as part of the European “Machinery Directive” as far back as 2009. Since December 2011, all machine and process safety systems sold in Europe must conform to EN ISO 13849-1 and EN 62061 safety standards. The accompanying chart shows various elements of the standards that apply to machine safety.
ISO/IEC Machine Safety Standards
EN ISO 13849-1 Safety-related parts of control systems: Principles for design
EN ISO 13849-2 Safety-related parts of control systems: Validation
EN ISO 12100 General principles for design - Risk assessment and risk reduction
EN IEC 60204-1 General requirement for electrical equipment of machines
EN ISO 11161 Integrated manufacturing systems
EN IEC 61508 / 62061 Functional safety of electrical/programmable electronic safety related systems
EN 13849-1 has wide applicability as it applies to all technologies including electrical, pneumatic, hydraulic, and mechanical. This standard provides requirements for the design and integration of safety-related parts of control systems, including some software aspects. The standard applies to a safety-related system but can also be applied to the component parts of the system.
Common terminology
Explanations of common machine safety terms follow.
PLr: Performance Level required
DCavg: Diagnostic Coverage average
MTTFd: Mean Time To Failure dangerous
CCF: Common Cause Failure – failure of many components from one event
B10: Time by which 10% of a population of a product will have failed
A full and detailed study of EN ISO 13849-1 is needed before it can be correctly applied. The following overview provides 15 steps to EN ISO 13849-1 requirements:
1) Create a technical file: Documentation of every step in the process must be maintained for both the end user and possible future litigation.
2) Design with safety in mind.
3) Determine the limits of the machinery.
4) Identify all potential hazards.
5) Perform a risk assessment: The manufacturer has to perform a risk assessment for the machinery. Based on the outcome of this risk assessment, the risk level can be determined. Risk reduction and residual risk can be estimated.
6) Perform risk reduction, by design and/or safety measure. Example safety measures are: guard, light curtain, door.
7) Identify residual risks: Document the residual risks in the manual.
8) Determine the PLr (Performance Level required): PLr is used to denote what performance level is required by the safety function. To determine the PLr, the standard provides a risk graph into which the application factors (Severity of injury, Frequency of exposure, and Possibility of avoidance) are input. The output is the PLr (a, b, c, d, or e).
9) Choose the appropriate Category (B, 1, 2, 3, 4) and architecture you need to achieve the required PL. Clause 6 of ISO 13849-1 provides the definitions of the categories.
i. Categories B and 1 are single-channel with no monitoring.
ii. Category 2 includes monitoring at certain times (start-up, new cycle, etc.).
iii. Categories 3 and 4 are dual-channel with monitoring.
1. Cat 3 detects some, but not all faults.
2. Cat 4 must detect every fault. Also see: “Safety Categories - related PL (Table 7 from EN ISO 13849-1:2006).”
10) Choose the components for this Category architecture. Need B10 values of components used – get official document from the supplier.
11) Calculate MTTFd (Mean Time To Failure dangerous), based on B10 values. Need following information:
i. Days of operation per year
ii. Hours of operation per day
iii. Time between successive cycles in seconds
iv. B10 value from supplier of component
v. Expected life time of the machinery
12) Evaluate your safety system design
13) Validate your designed machinery using EN 13849-2. This must be done by a person other than the designer of the safety systems. This doesn’t mean that a third party has to be involved. A colleague who has no involvement with the design can perform the task as evaluator.
14) Create an overview of the Essential Health and Safety Requirements (EHSRs) you have filled.
15) Create a user manual with the appropriate information. This should show how to transport, commission, use, service, adjust, dismantle, and scrap in a safe way.
Free machine safety software tool
SISTEMA is a software tool for EN ISO 13849-1 implementation. SISTEMA stands for Safety Integrity Software Tool for the Evaluation of Machine Applications. Its use will greatly simplify the implementation of the standard. It was developed by the BGIA in Germany and is free for use. It requires the input of various types of functional safety data, which is done automatically when using a manufacturer’s SISTEMA data library. It also helps create the documentation package.
This synopsis of relevant machine safety standards should be considered as a starting point to understanding machine safety. With so much at stake, from employee safety to litigation costs, those involved need to learn as much as possible about machine safety and related standards.
- Michael Guelker is a Festo Corp. product manager. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering and Plant Engineering, mhoske(at)cfemedia.com.
