Understand the jargon for new Machine safety Standards with RS
20 November 2009
The new machine safety standards, BS EN ISO 13849-1 or BS EN 62061, bring new approaches and terms. Understanding what these mean will ease the transition process when the legislation goes live from the December 29 2009. RS can help you understand the legislation, and find the right solution for your application
For many years BS EN 954-1 has been used as the standard of conformity to the Machinery Directive 98/37/EC. With the new European Machinery Directive 2006/42/EC coming into force on December 29 2009 two new standards, BS EN ISO 13849-1 or BS EN 62061 come into play and these standards introduce the notion of not only if, but how likely faults will occur.
For example, EN 13489-1 uses a quantitative rather than a qualitative process meaning that more effective assessments of risk can be made, which now take into account the components used. With a probabilistic element in compliance that must be quantified, machine builders must be able to determine levels of safety integrity or performance. This is to verify that their safety circuits meet with the determined safety integrity level (SIL) or performance level (PL). This means that panel builders and designers need to be aware of the performance of the components used in safety circuits (including safety detection components, safety logic solvers and output devices like contactors).
The terms in the legislation can be a minefield with new reference terms and knowing what each acronym means can be complicated. Here are the main points that machine builders need to understand for each standard.
BS EN ISO 13849-1 terms:-
MTTFd (Mean Time To Dangerous Failure)
The average period before failure of components in the safety circuit causes harm to a worker. MTTFd is classed as high (low risk, 30-100 years), medium (10-30 years) or low (high risk, 3-10 years) however, if the component's MTTFd is 100 years, it does not mean it will last this long without fault.
DC (Diagnostic Coverage)
A component or circuit's ability to detect/diagnose a fault within itself (a short circuit, for example). The higher the DC, the lower the probability of hazardous hardware failures.
PL (Performance Level)
Made up of the circuit architecture (category B, 1, 2, 3, 4 as in BS EN 954-1) as well as MTTFd and DC. PLa represents the highest failure probability, and PLe represents the lowest failure probability. If a manufacturer states a specific PL for a component (such as a safety relay) it means this is the highest PL a circuit incorporating that component could achieve.
CCFs (Common Cause Failures)
Failures due to a common issue, such as a short circuit, or due to a single event not based on mutual causes. Steps can be taken to prevent common cause failures such as using different components driven in different modes in dual channel systems.
BS EN 62061 terms:-
SIL (Safety Integrity Level)
The discrete level for the determination of the safety integrity requirements of the safety related control system. Level one is low, while three is high. If a manufacturer claims a specific SIL for a component (such as a safety PLC) it means this is the maximum SIL that can be claimed for a system that uses this component as a subsystem.
SILCL (SIL Claim Limit)
Applies to subsystems within a safety system. A subsystem is defined as a part of a safety system/circuit, which if it fails, will bring about a breakdown of the safety function. SILCL is the maximum SIL that can be claimed in relation to architectural constraints and systematic safety integrity.
PFHD (Probability of Dangerous Failure per hour)
A measure of the dependability of a component, subsystem or entire safety system/circuit - in the same way that MTTFd is in BS EN ISO 13849-1
SFF (Safe Failure Fraction)
Represents the share of failures in the total rate of failure subsystem, which does not lead to a dangerous failure. For compliance with both standards B10 and B10d are associated with electromechanical components.
B10
The number of operations at which 10% of the population will have failed and
B10d
The number of cycles after which 10% of the population has failed to a dangerous state.
Electromechanical components do not have published MTTFd or PFHD figures, since failure rates depend upon the hourly actuation rate, which is application specific. However, designers can use B10 or B10d with known machine data (guard switches might activate for a known number of times per hour for machine loading purposes, for example) to calculate the MTTFd or PFHD of subsystems containing these components.
Finding the right product
Whilst these new standards require more diligence, machine builders do not need to buy new products, just ensure that the standards have been applied. Knowing the range of potential solutions and machine guarding equipment can help decide which product is right for the application. With the widest range of machine guarding from mechanical guards to complex light curtains, mats and bumpers, RS has your solution, available now.
For more explanation of the standards as well as thousands of components from all the market leading brands visit rswww.com/automation
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