Energise or De-Energise to Trip?

"De-energise to trip" is a long established principle because of the danger of common cause failures. Although there is little published on this topic it is covered in the section "protection systems (trips and interlocks)" in the HSE technical measure document for COMAH sites, but the quality of UPS, diagnostics, etc. is now very different from the last century. Architecture, failure modes and frequency, design policies (for example, overrides and diagnostic coverage), testing policies, repair policies and operating policies all affect the number of spurious trips, failures to danger and common cause failures. These effects are illustrated with practical examples from the energy industry: oil and gas production and power stations.