Recent results of an experimental study on the impact of smoke on digital I & C equipment

A program to assess the impact of smoke on digital instrumentation and cont rol (I & C) safety systems began in 1994, funded by the US Nuclear Regulato ry Commission Office of Research. Digital I & C safety systems are likely r eplacements for today's analog systems. The nuclear industry has little exp erience in qualifying digital electronics for critical systems, part of whi ch is understanding system performance during plant fires. The results of t ests evaluating the performance of digital circuits and chip technologies e xposed to the various smoke and humidity conditions representative of cable fires are discussed. Tests results show that low to moderate smoke densiti es can cause intermittent failures of digital systems. Smoke increases leak age currents between biased contacts, leading to shorts. Chips with faster switching times, and thus higher output drive currents, are less sensitive to leakage currents and thus to smoke. Contact corrosion from acidic gases in smoke and stray inductance or capacitance are less important contributor s to system upset. Transmission line coupling was increased because the smo ke acted as a conductive layer between the lines. Permanent circuit damage was not obvious in the 24 h of circuit monitoring. Test results also show t hat polyurethane, parylene, and acrylic conformal coatings are more effecti ve in protecting against smoke than epoxy or silicone. Common sense mitigat ion measures are discussed. Unfortunately we are a long way from standard t ests for smoke exposure that capture the variations in smoke exposure possi ble in an actual fire. (C) 1999 Elsevier Science S.A. All rights reserved.