1. CRR 428/2002. Principles for proof testing of safety instrumented systems in the chemical industry. Billingham: ABB Ltd for the Health and Safety Executive (HSE), p.17, part 4.2.1(a).
2. CRR 428/2002. Principles for proof testing of safety instrumented systems in the chemical industry. Billingham: ABB Ltd for the Health and Safety Executive (HSE), p.5, part 4.1.1(a).
3. CRR 428/2002. Principles for proof testing of safety instrumented systems in the chemical industry. Billingham: ABB Ltd for the Health and Safety Executive (HSE), p.5, part 4.1.1(b).
4. Dearden HT. Functional Safety in Practice. Llandudno: Time Domain Solutions Ltd., pp.144–158.