MIL-HDBK-274A
4.4.1.3.3.1 Charge accumulation. It is generally considered that all parts of the skin are sufficiently moist to allow only negligible amounts of charge to be formed between the skin and the garment next to the skin. Provided the person does not remove any garments, the only effect of the charged clothing can be to cause an attraction between the layers (since opposite charges attract) or for the clothing to cling to the body.
4.4.1.3.3.2 Removal of outer garments. If a person removes their nylon parka, a charge of up to 27 kV can reside on the outside surface of the newly exposed wool sweater. The positive charge on the wool sweater and the negative charge on the parka are developed due to static effects. The opposite charges on the sweater and parka may produce a spark from one to the other.
4.4.1.3.3.3 Worst-case friction static electricity. The 27 kV value is used herein as representative of the worst-case friction static electricity hazard levels produced by servicing personnel. Using Equation 2, U = i C·V2, with C= 500 pF for body capacitance, the amount of available energy is 0.18 J or 180 mJ. This is above the threshold levels for fuel vapor ignition, component damage, electroexplosive device (EED) ignition, and reflex action shock as discussed further in 4.7 and shown in Table IV.
4.4.1.4 Static Summary. Various sources of static electricity, such as triboelectric,
induced charge and friction have been discussed. In general, direct effects of static electricity are not serious, although such shocks can cause involuntary reflex movements resulting in injury to the affected person or others nearby. On the other hand, sparks generated in the vicinity of a fueling operation can have disastrous results. Catastrophic results can occur from discharge of static electricity into a squib, cartridge, or EED. Care in ensuring that the aircraft, weapon system, fueling system, and personnel are kept at ground potential will minimize the dangerous effects of static electricity. A ground of up to 10,000 ohms is considered an adequate safety measure.
4.4.2 Power. Various types of electrical power such as 115 volts alternating current (VAC) and 28 volts direct current (VDC), ground faults, lightning, and RF electromagnetic energy are discussed in the following paragraphs. The term power used in this context assumes electric current flow measurable in amperes, in contrast to static electricity whose current flow, typically, is in the range of thousandths (0.001) of an ampere. The current generated by ground faults, lightning, or RF energy sources can be substantial.
4.4.2.1 Operational power. During aircraft flight, all electric power required by onboard equipment is supplied by generators driven by the aircraft engines. Two basic types of electrical voltage are used: 28 VDC and 115 VAC, at 400 Hertz (Hz). The 28 VDC system is a two-wire system, positive and negative, with the negative side of the system connected to the aircraft structure. The 115 VAC is a four-wire system consisting of phases A, B, C, and neutral (N),
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