The Circuit Designer's Companion - diagramas.diagram...

Electromagnetic compatibility 265Area ClassAverage rate ofoccurrence(transients/hour)100rIndustrial 17.5Business 2.8Domestic 0.6Laboratory 2.3101n ∝ V pk−3Table 8.2 Average rate ofoccurrence of mains transients0.1Source:Transients in Low Voltage SupplyNetworks, J.J. Goedbloed, IEEETransactions on ElectromagneticCompatibility, Vol EMC-29 No 2, May1987, p 1070.0130 100 300 1k 3kFigure 8.1 Relative number of transients vs.maximum transient amplitudeMains lines (V T 100V)Telecomm lines (V T 50V)(Figure 8.1) are the load dump, which occurs when the alternator load is suddenlydisconnected during heavy charging; switching of inductive loads, such as motors andsolenoids; and alternator field decay, which generates a negative voltage spike when theignition switch is turned off.ESDElectrostatic discharge is a further source of transient upset which most often occurswhen a person who has been charged to a high potential by movement across aninsulating surface then touches an earthed piece of equipment, thereby dischargingthemselves through the equipment. The achievable potential depends on relativehumidity and the presence of synthetic materials (Figure 8.3), and the human body isroughly equivalent to a 150pF capacitance in series with 150Ω resistance, so thatcurrents of tens of amps can flow for a short period with a very fast (sub-nanosecond)rise time. Even though it may have low energy and be conducted to ground through theequipment case, such a current pulse couples very easily into the internal circuitry.Determining and specifying the effects of interferenceRequired reliability of operation depends very much on the application. Entertainmentdevices and gadgets come fairly close to the bottom of the scale, whereas computersand instrumentation for control of critical systems such as fly-by-wire aircraft andnuclear power systems come near the top. The purchasers and authorities responsiblefor such systems have recognised this for some years and there is a well-established raftof EMC requirements for them.Actually determining whether a piece of equipment has been affected by