The helicopter was hovering at a height of 35 ft as part of a training exercise to provide a doctor and a paramedic with experience in winching procedures. The doctor, who was in the sling suspended on the winch cable, was raised to within 5 ft of the helicopter when there was an uncommanded firing of the explosive cable cutter. The doctor fell 30 ft and sustained serious injuries. Investigation revealed the uncommanded firing had resulted from a short circuit in the winch electrical system caused by a blown power resistor in the winch motor circuit. This allowed a substantial current flow in the cable cutter, sufficient to fire the squib. The current flow was not contained within the body of the hoist because of poor bonding between the winch motor and its mounting. The resistor failed due to inadequate heat dissipation at high current flow. The Civil Aviation Authority (CAA) subsequently issued a mandatory requirement to remove pyrotechnic cable cutting devices from helicopter hoists when used to raise or lower personnel. Additionally, both the CAA and the manufacturer have issued mandatory requirements to rectify the bonding problem pending a technical solution to preclude power resistor overheating.