FISRE special report: Insulation resistance of electronic circuits in smoke - quantification of the problem and needed countermeasures

Acute effects of smoke on insulation resistance of electronics used in programmable automation circuitry in nuclear power plants were studied experimentally. Real and mock-up circuitry was exposed to smoke in four tests. Relevant physical parameters of smoke and electrical performance of circuitry was measured online for a period of an hour. It was noticed that insulation resistance decreased by three orders of magnitude due to soot deposits on surfaces on uncoated circuitry. For real commercial circuits, coated by a protective lacquer layer, no electrical changes were observed. A quantitative model for smoke exposure and deposition on surfaces is proposed based on existing physical models of aerosols. A consistent picture of the relevant processes was found, and an easy calculation method for smoke hazard estimation for PSA-work is proposed for insulation resistance deterioration. This model allows also estimation of the required performance of protective coatings to prevent parasitic leakage currents between different parts of circuitry.