Functional safety in application of programmable devices in power system protection and automation

This paper summarises work carried out by the health and safety executive (HSE) with the electricity industry in the UK. An assessment was made of the extent to which programmable electronic (PE) technology applied to power system protection and automation was safety related and whether appropriate techniques and measures are being used in design, verification and maintenance of such systems. The approach was to examine a number of schemes, firstly to determine if they were safety related, and then to compare the practice of design, verification and maintenance of these systems. The benchmark used to compare was the international standard IEC/BS/EN 61508 "Functional safety of electrical/electronic/programmable electronic safety-related systems". This is a risk based standard incorporating the concept of a safety lifecycle from initial concept to decommissioning. The safety integrity of the scheme is proportional to the degree of risk reduction required. The safety integrity is categorised into four levels called SIL (safety integrity level), with SILI giving lowest and SIL4 giving highest integrity. The SIL is achieved by demonstration of hardware reliability, techniques and measures used in design, verification, maintenance and operation, and overall functional safety management appropriate to the SIL level. This standard is little understood in the electricity transmission and distribution industry and there is still some debate about its applicability.