Environmental effects
The effect of temperature on a switch will be to make changes in its rated characteristics. The normal temperature range for switches is typically -200oC to +800oC, with some rated at -50oC to +100oC. This range is greater than is allowed for most other electronic components, and reflects the fact that switches usually have to withstand considerably harsher en- vironmental conditions than other components. The effect of very low temperatures is due to the effect on the materials of the switch. If the mechanical action of a switch requires any form of lubricant, then that lubricant is likely to freeze at very low temperatures. Since lubrication is not usually an essential part of switch maintenance, the effect of low tem- perature is more likely to be an alteration of the physical form of materials such as low-friction plastics and even contact metals.
Plastics that are used as low-friction bearings can become sticky at very low temperatures, causing irregular mechanical action. The metals that are used for contacts can alter their crystal shape, causing a large increase in contact resistance. This change of crystal shape is very sharp, and will occur at a clearly defined temperature. The best-known change of this type is the change of tin from its metallic form to the form of a dull-grey powder at a temperature of about -20oC, but other metals will exhibit less dramatic changes at various temperatures. At the high-temperature end of the scale, the most serious threat is to insulation resistance, which can decrease very considerably at the higher temperatures. Contact resistance can also be increased because of the growth of layers of oxide on contact surfaces, and if a switch is to be used at constant high temperatures, near its rated limit, a wiping action is desirable (see above). For electronics purposes, exposure of a switch to high temperatures is more likely than exposure to very low temperatures.