or use of the SIL concept out of context such as, "This is a SIL 3 heat exchanger" or "This software is SIL 2". According to IEC 61508, the SIL concept must be related to the dangerous failure rate of a system, not just its failure rate or the failure rate of a component part, such as the software. Definition of the dangerous failure modes by safety analysis is intrinsic to the proper determination of the failure rate. SIL is for electrical controls only and SIL numbers do not relate to the numbering associated with the Categories in EN 954-1. Certification The International Electrotechnical Commission's (IEC) standard IEC 61508 defines SIL using requirements grouped into two broad categories: hardware safety integrity and systematic safety integrity. A device or system must meet the requirements for both categories to achieve a given SIL. The SIL requirements for hardware safety integrity are based on a probabilistic analysis of the device. In order to achieve a given SIL, the device must meet targets for the maximum probability of dangerous failure and a minimum safe failure fraction. The concept of 'dangerous failure' must be rigorously defined for the system in question, normally in the form of requirement constraints whose integrity is verified throughout system development. The actual targets required vary depending on the likelihood of a demand, the complexity of the device(s), and types of redundancy used. PFD (probability of dangerous failure on demand) and RRF (risk reduction factor) of low demand operation for different SILs as defined in IEC EN 61508 are as follows: For continuous operation, these change to the following. (PFH is Probability of dangerous Failure per Hour) Hazards of a control system must be identified then analysed through risk analysis. Mitigation of these risks continues until their overall contribution to the hazard are considered acceptable. The tolerable level of these risks is specified as a safety requirement in the form of a target 'probability of a dangerous failure' in a given period of time, stated as a discrete SIL. Certification schemes are used to establish whether a device meets a particular SIL. The requirements of these schemes can be met either by establishing a rigorous development process, or by establishing that the device has sufficient operating history to argue that it has been proven in use. Electric and electronic devices can be certified for use in functional safety applications according to