Effects of cable fire smoke on electronic boards

The consequences of a fire are twofold: an increase in temperature and heat fluxes, and smoke propagation at the source of the fire and in adjacent rooms. These may lead to malfunctions of fire safety electrical equipment in nuclear power plants. A survey of relevant literature has shown that the equipment most affected by smoke particles and gases are electronic boards. Smoke causes electronic boards to malfunction in four ways: loss of metallic contact due to chemical corrosion, increased short circuiting between the conductors, loss of mechanical contact conductivity, and reduced capacity for movement in electromechanical systems as a result of particle deposits. These effects can occur over different timescales: – In the short term, malfunctions that have critical impact in terms of plant safety occur – such as electrical short circuiting and increased contact resistance, – Mid to longer term, corrosion effects can appear. A test procedure based on the repeatability of a 20-minute duration cable fire was set up. Temperature and smoke composition in terms of both gaseous products and soot were controlled. Tests were performed on boards designed to sum test and regulation signals, or to control an analog chain fitted with relays. Although the behaviour of the equipment may be dependent upon its construction material and equipment layout, no electronic board malfunctions occurred during the 24-hour tests. Furthermore, no malfunctions were detected in the boards when they were retested six months later under normal conditions and without any clean-up following the smoke exposure. KEY WORDS: smoke; reliability; electronic boards; cable fires; malfunction criteria