LIGHTNING BOLTS Surge arrestors can prevent lightning from sparking a bad day.

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1 HOWTO BEN LIGHTNING BOLTS Surge arrestors can prevent lightning from sparking a bad day. TEVENS Phoenix Contact Inc.,/ nce of protection ag as recently driven home for one company when its factory ect hit. The strike shattered huge insulators in the electrical distriea and sprayed the pieces over An exceptionally good kept heavy industrial maharmed, but not so for the the plant. Massive surge currents vaporized traces on some circuit boards. The damage would have been worse but for the fact that the firm disconnected uch of its low-level process monitoring ectronics during thunderstorm season. The National Weather Service reports at some areas in the continental U.S. experience as many as 100 days of thunderstorms every year. It is no secret that the lightning that accompanies bad weather can spell trouble for electronics. Engineers can gain a healthy respect for lightning strikes from a review of their electrical qualities. Studies have shown that 90% of all lightning has a crest current exceeding 8 ka. Typical current rate-of-rise is greater than 2 Wps, with a single stroke lasting between 0.1 to 0.6 msec.. Damaging voltage fields of 70 V/m can be present a mile away from lightning. And high electromagnetic fields created by lightning strikes can cause appreciable current surges in electrical wiring. Moreover, lightning needn't strike the ground or strike close by to cause damage. Even cloud-to-cloud lightning can cause current surges that have a long duration, often exceeding several milliseconds. Though such surges may take Place at voltages below those caused by or- dinary switching transients, they contain more energy because of their longer duration. The higher energy can heat up electrical components and cause damage. One form of damage takes place when the rate of current from a lightning strike exceeds the earth's ability to absorb it. The current finds alternate paths to ground, often through unprotected equipment. Lightning that reaches the earth can also raise the electrical ground potential for the wiring of a whole building, causing potentially harmful conditions in the process. Other ill effects arise from magnetic fields that accompany the lightning bolt. These fields can have varying frequencies, with the highest known component on the order of 1 khz to 5 MHz. The greater the frequency, the higher the voltages created. As lines of magnetic flux cross conductors, they Various kinds of electrical and electronic components experience damage at differing levels of surge energy. JUNE 25,1993 MACHIN O S/GN 51

2 6rounding rods have a finite resislance to earth that asymptot- /ca//y approaches 2552, But twt /y long rods, on fhe order of 50 ft or more, may be needed to get low resistance in mwlm&nonconductlve sand or ssb. Use of multiplej shorfer m& fsprefeneqln such Instances to reduce the mqulmlrod Iengfh. create potentially damaging surge voltages. The magnetic field caused by lightning-induced current produces other difficulties through inductive coupling to nearby conductors. As an example, a 500A, 50psec transient produces about 30V in a six-inch length of unprotected cable sitting next to it. Inductive coupling from nearby power lines can be equally pernicious. For example, consider how a IOOkA/ps surge in a power line can affect a one-meter loop of wire located 50 m away. The induced voltage will be about 8 kv. Lightning strikes also create strong local voltage fields that can induce damaging inrush and outrush currents as well. Heading off trouble There are three ways of minimizing the chances of lightning problems: using good grounding practices, shielding wiring and components, and using surge arrestors. Grounding and shielding practices are well known and can be found in standards (IEEE Std ). But even use of standards cannot guarantee protection from every conceivable scenario. It is useful to examine the benefits of good grounding. Grounding effectively lowers the resistance of the electrical path to earth and thus allows surge arrestors to handle higher levels of current. Lower ground circuit resistance also increases the life span of surge arrestors by reducing the heating that these devices experience. Such effects become clear by considering the power handling capacity, P, of the surge arrestor: P = 12R, where I = surge current, A; and R = ground circuit resistance, SZ. It is also interesting to calculate the effectiveness of specific kinds of grounding conductors for a severe lightning strike. Consider, for example, a strike having a 25-kA crest current and a current rate-of-rise of 25 ka/psec. An event with these qualities would be among the top 10% of recorded lightning strikes in terms of intensity. To calculate the voltage developed across a grounding strap, use dl V-IRi-Ldt where I = lightning crest current, A; R = dc resistance of the grounding wire itself, Q; L =wire inductance, ph; and d/dt(z) = current

3 - 1. cause different soil types and moisture content influence grounding effectiveness, grounding rods must have different minimum lengths depending on the type of soil. The longer the rod, the lower the impedance, up to a point. For example, in heavy clay, the resistance to earth asymptotically approaches 25Q as grounding rod length exceeds 9 or 10 ft. But longer rods would be required to reduce resistance to this level in less conductive soil. Clay and other dense soils such as loam, shale, or gumbo are relatively conductive, having an average resistivity of 4kQ/cm3. In contrast, rocky or dry, sandy soil measures 50kQ/cm3 and fill dirt or ashes hit 200k!21cm3. Unfortunately, it may be difficult to install sufficiently long grounding rods. In one case, a plant built on sandy soil needed a ground rod 80 ft long. The preferred technique in such cases is to use multiple, shorter rods. The spacing of these rods is important. They should be positioned so that the distance between them is twice their length. The reason is that grounding rods dissipate 90% of their surge current in a cylindrical volume having a diameter of approximately twice the rod length. The resistance zones hetween rods should not greatly overlap, but should touch. Protecting circuits One thing to note is that instrument loopgrounds commonly provided on test equipment and industrial gear provide little protection from the huge currents and voltages that accompany lightning strikes. Instrument-loop grounding wires typically run for long distances and are a small gauge. Thus, they present a relatively high inductance and a correspondingly high impedance to surge currents. Only low-impedance, earthgrounding techniques are effective against high-energy transients. Equipment used to guard against surge currents generally employs several different kinds of protective circuit elements connected in parallel. This is the preferred Surge arrestom designed to protect the main electrical service entrance often contain hoth MOV and surfacedischarge, surge-arresting elements as depicted in the schematic of a Powertrab surge protector. The device includes a test light and mechanical microswitch contact for actuating a local or remote alarm. It can divert up to 100 ka of surge current, An inductor (not shown in the schematic) sits in series with the MOV. In the presence of 8 surge, the inductor presents a large impedance tu the initial rapidly rising current crest, allowing the surface discharge device to fire and protect the MOV from current that exceeds its maximum ratings. The remaining current shunts through the MOV. The destructive effects of a lightning strike become evident from comparisons with electrical qualities of ordinary switching transients. JUNE 25,1993 MACHlNE D S/GM 53

4 circuits, and sometimes on low-level data lines such as RS-232 circuits. The surge arrestor at the ac service entrance is the system s primary protection against a lightning strike. Surge arrestors designed for this sort of duty generally contain two surge elements, a surface-discharge device and a metal-oxide varistor (MOV), between each power line and ground. The surface discharge device handles the initial jolt. The MOV then further reduces the voltage and current levels. Surface-discharge devices can clamp 3,000 V and withstand currents of 100,000 A. They consist of two electrodes with a special plastic insulator sandwiched between. A sufficiently high voltage forces an arc across the insulator, thereby generating a pressure surge that results in a small thunder clap. The MOVs packaged in these surge arrestors typically handle up to 40 ka. Because MOVs have a finite life span, they must be packaged in a special way. One technique is to solder one end of the MOV to a spring-loaded contact using low-temperature solder. The MOV heats up as it approaches the end of its life, melting the solder, disconnecting the MOV, and springing the contact. This actuates a visual warning indicator. The main arresting element in some surge arrestors placed at distribution panels is a gas-filled tube. These tubes operate in a way that is analogous to surface-discharge arrestors. Instead of an insulating plastic, they use argonheon gas between two electrodes. qey handle currents up to 10 ka and have a response time on the order of one microsecond. One point to note about gas-filled tubes, though, is that they arc over at a voltage that depends on the rate-of-rise of the surge voltage. The faster the voltage rate-of-rise, the higher the voltage at which they respond. Typical response times are on the order of a microsecond or less. Surge arrestors for electrical distribution branch circuits may also employ MOVs either in conjunction with gas tubes or as their primary arresting element. Arrestors aimed at protecting branch circuits and low-level signal lines frequently employ a combination of gas tubes, MOVs, and surge-suppressing diodes. The diodes conduct current at a specific breakover voltage. They respond in much less than a microsecond and always break over at the same voltage. In a typical configuration, the three types 54 MACH/[ D S/GM JUNE 25, _I.

5 of components are connected in parallel. The gas discharge tube takes the initial surge followed by the MOV. The suppressor diode then reduces the transient to a safe voltage level. In some devices, however, the MOV and diode may be connected between signal lines with a gas tube connected between signal lines and ground. This style of device may be used in the case of balanced lines or RS-422 current loops that are not referenced to ground. The diode and MOV limit the voltage difference developed between the balanced lines, while the gas tube heads off damaging line-to-ground voltages. I JUNE 25,1993 MACHINE DESIGN 55