The SAFEC project

Submitted by root on Wed, 05/07/2014 - 23:18

Contract No CT98-2255 Determination of safety categories of electrical devices used in potentially explosive atmospheres (SAFEC)

Background

Existing CENELEC standards cover different types of electrical apparatus for use in potentially explosive atmospheres. The EU ATEX 100A Directive 94/9/EC has introduced Essential Safety Requirements and a categorisation system. EN 954, under the Machinery Directive, has a different categorisation system for safety-related devices. A categorisation system needs to be developed which is compatible with these and with standards for safety-critical control systems, such as IEC 61508.

Objectives

To draft a description of appropriate subdivisions of safety devices. To define all safety devices which are used in the context of electrical equipment for use in potentially explosive atmospheres and study their characteristics and performance in terms of the defined subdivisions. To draft a method for identifying when a particular subdivision should be used, taking into account the application and working environment of the equipment. To determine the correspondence between the proposed subdivisions and the relevant essential safety requirements.

Work programme

Task 1 is to derive target failure measures in the context of the ATEX requirements. These may be in terms of reliability and/or number of faults required to cause failure on demand. Task 2 is to assess standards such as EN 954 and IEC 61508 for suitability in specifying and certifying that the required target failure measures have been achieved. Task 3 is to identify the types of safety devices which are currently in use. Task 4 is to study these safety devices to determine their characteristics and performance in relation to the target failure measures. Task 5 is to determine a methodology for testing, validation and certification. Task 6 is to prepare the current report and proposals for standardisation.

Results and Achievements

A number of problems have been identified with the use of EN 954 for certification of safety devices because the defined categories are not hierarchical in terms of reliability/integrity. IEC 61508 is therefore preferred. Safety integrity level (SIL) as defined by IEC 61508 is a suitable target failure measure for definition of safety devices. However, it will also be necessary to define additional fault tolerance requirements to conform with the ATEX Directive. SIL targets for safety functions and hence safety devices have been calibrated by considering individual risk criteria, accident statistics and the performance of existing safety devices. Good agreement was achieved between these different calibration methods. SIL requirements have been defined for the safety function of explosion prevention for each hazardous zone, i.e. SIL4 in zone 0; SIL3 in zone 1 and SIL1 in zone 2. The SIL target for a particular safety device may be less than this as the requirement can be allocated between the safety device and the rest of the equipment. IEC 61508 is the most suitable concept for certification. A certification scheme has been proposed based on the safety lifecycle approach of IEC 61508.

CONTENTS

Introduction
1. Background
2. The SAFEC project
3. Scope
4. Liaison with CENELEC and CEN
Identification of safety devices
Review of control system standards
1. EN 954-1 requirements
2. IEC 61508 requirements
3. Summary of the standards with respect to the ATEX

Directive

Choice of target failure measures
1. Types of target failure measure
2. Discussion
Calibration of SIL requirements
1. Introduction
2. Use of individual risk criteria
3. Use of accident statistics
4. Estimation of SILs for existing safety devices
5. Discussion and calibration of SIL targets
Methodology for testing, validation and certification
1. Introduction
2. Requirements of certification scheme
3. Selection of a concept for certification
4. Certification scheme
Conclusions
References