The seventh unit shows how we carry out a risk assessment of a fire or explosion scenario. It includes how to assess, in a semi-quantitative way, the frequency of potential ignition sources. The ignition sources covered include electrostatic discharges, hot surfaces, electrical equipment, rotating machinery, electromagnetic radiation and lightning. The ignition frequency depends the flammable gas or dust, the extent of the zone where a flammable mixture is present and the type of equipment in that zone.