Pressure relieving devices generally require more frequent inspection and maintenance than the rest of process plant. By providing isolation, devices can be removed with the plant remaining in operation. However isolation introduces the risk of inadvertent disconnection of a pressurised system from its relieving device; such isolation therefore needs to be rigorously controlled. This publication offers guidance on the design, installation, management and operation of isolation systems, identifying the key points to be considered. The second edition of EEMUA 184 revises and updates the original 1996 edition. It is a companion volume to EEMUA Publication 188 Guide for Establishing Operating Periods of Safety Valves.