This is just an Excerpt from a larger document, click here to view the entire document.Safety Factors / Derating
These terms refer to the limiting of the nominal stresses on all parts to levels below their specified maximum. The use of safety factors in structures is common (e.g., a column meant to support a five ton load might be designed for ten tons). Similar policies in electronics engineering (e.g., a power transistor rated at 25 watts may be operated at 20 watts) are called derating. The effect is to shift the stress curve in Figure 1 to the left, reducing the area of stress-strength overlap. Critical parameters will differ from part to part (e.g., wattage for a power transistor vs. voltage for a capacitor) and some parts should not be derated (e.g., aluminum electrolytic capacitors). Derating guidelines are tabulated in the Reliability Toolkit: Commercial Practice Edition, available from RIAC and also in MIL-STD-975K, Notice 2, NASA Standard Parts Derating, the Rome Laboratory Technical Report RL-TR-92-11, Advanced Technology Component Derating, and the Naval Sea Systems Command TE000-AB-GTP-010, Parts Derating Requirements and Application Manual. Structural safety factors are recommended in civil and mechanical engineering handbooks and prescribed in building codes.