Electrostatic precipitators operating according to the two-stage Penney principle are distinguished by high precipitation efficiency, even in the range of very fine dusts and aerosols, and an extremely low pressure difference. Thus, they are exactly the right equipment for use in room air systems and for the separation of harmful substances as occupational safety measure in production processes. The directive VDI 3678, page 2, comprehensively describes the principle of this waste gas cleaning process as well as the criteria to be taken into account when setting up and operating these plants. This description is completed by references to future developments and the energy-efficient use. The separation of solid and/or liquid particles from process or waste gases is described in the directive VDI 3678, page 1. This directive is an important document for the work of manufacturers, operators and relevant authorities.