4. COMMON CAUSE OF FAILURE (CCF) The fourth fundamental states the system must be resistant to the most common failures. For categories 2, 3 and 4, the system must obtain 65 points. The table below can help calculate the worth of each measure. Common measures include separation of redundant signals, use of NO and NC contacts at the input, and use of certified components according to the correct voltage and current. Protection against transient over voltage and electromagnetic interference is also required. It's critical to follow temperature, humidity and environmenal specifications, and to implement proven components and state-of-the-art techniques. CANADIAN AUTOMATION VOLUME 4, ISSUE 2 15