3-39 Table 3-2.—Typical Measured Statics Charges (in volts) RELATIVE HUMIDITYITEM LOW (10-20%) HIGH (65-90%) WALKING ACROSS CARPET 35,000 1,500 WALKING OVER VINYL FLOOR 12,000 250 WORKER AT BENCH 6,000 100 VINYL ENVELOPES FOR WORK INSTRUCT. 7,000 600 POLY BAG PICKED UP FROM BENCH 20,000 1,200 WORK CHAIR PADDED WITH URETHANE FOAM 18,000 1,500 Metal oxide semiconductor (MOS) devices are the most susceptible to damage from ESD. For example, an MOS field-effect transistor (MOSFET) can be damaged by a static voltage potential of as little as 35 volts. Commonly used discrete bipolar transistors and diodes (often used in ESD-protective circuits), although less susceptible to ESD, can be damaged by voltage potentials of less than 3,000 electrostatic volts. Damage does not always result in sudden device failure but sometimes results in device degradation and early failure. Table 3-2 clearly shows that electrostatic voltages well in excess of 3,000 volts can be easily generated, especially under low-humidity conditions. ESD damage of ESDS parts or circuit assemblies is possible wherever two or more pins of any of these devices are electrically exposed or have low impedance paths. Similarly, an ESDS device in a printed circuit board, or even in another pcb that is electrically connected in a series can be damaged if it provides a path to ground. Electrostatic discharge damage can occur during the manufacture of equipment or during the servicing of the equipment. Damage can occur anytime devices or assemblies are handled, replaced, tested, or inserted into a connector. Technicians should be aware of the many sources of static charge. Table 3-3 lists many common sources of electrostatic charge. Although they are of little consequence during most daily activity, they become extremely important when you work with ESD material.
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