HRU is designed to detect immersion in water, in order to release marine emergency equipment. Sealed inside the plastic casing is one small hermetically-sealed pressurised primary Lithium – Manganese Dioxide cell. The product also contains a very small quantity of explosive composition, used to drive a blade which cuts a plastic retaining bolt where it passes through the product. Immersion in water (to a depth of up to four metres maximum) is required to cause activation.
The HRU Replacement Kits provide for in-service maintenance of a range of marine emergency distress beacons. The Kits consists of one or two HRUs together with labelling materials, installation hardware, and instructions. The EPIRB Kits contain one HRU. The S- VDR Kits contain two HRUs.