Kamis, 27 Oktober 2016


How To Climb a Cell Tower

I initially wrote this as a humorous how-to essay for a class assignment.  I sent a copy to my employer, and it's since circulated the company.
How to Climb a Cell Phone Tower
            Cell phone towers are almost invisible until you know what they are.  Then they’re everywhere.  I remember the first time I noticed one.  When I was little, my dad told me someone got paid to climb up there and change the light bulbs, and I thought that was the coolest job ever.  Little did I know, years later, it’d be my job.   I was complaining about working for a vet one day – getting paid eight dollars an hour to get bitten by dogs – and my friend Charlie told me I should come work with him on the towers.  I looked at him incredulously.  “You really think I can do that?”  He replied, “Sure, it’s not that hard.”  He was right.
            I’m about to tell you how to climb towers, but I’m going to tell you how to do it right.  There’s a lot of prep and safety.  So – if you think I’m wasting your time, and you can just go out in the middle of the night and free climb, remember: it’s a class A misdemeanor, and there’s a good chance you’ll die.
            If you’re thinking about climbing, the first thing you have to know is that you’re not afraid of heights.  Or in my case, be pretty sure you’re not.  I wasn’t really sure until I got to the top and looked around.
            The second thing you have to do is find a reputable tower crew.  I don’t mean a crew full of macho young men who race each other to the top.  That’s not safe.  And it’s definitely not where you want to start.  Find a crew that’s mostly fat old men.  Persnickety, safety-first, by-the-book, do-it-right, old men.  Men who are old enough to have seen and done lots of things and most importantly, old enough not to foolishly show off. 
Getting on the crew isn’t too hard - they’re usually happy to have fresh blood on the crew, as long as you’re willing to work.  Did I mention, tower work is work.  You’ll probably never have a harder job in your life.  But you’ll probably also never have as much fun.  Tower work isn’t rocket science.  You should know instinctively which way to turn a wrench and how to operate a hand-held DeWalt drill, but a degree in mechanical engineering isn’t necessary.  You also don’t have to be super buff.  Being in good shape is a plus.  At the very least, you should be able to walk across a parking lot without any trouble. 
Once you get on the crew, you’ll work on the ground for a little while before they send you up.  You’ll learn important things like how to handle ropes and how to use radios to talk to whoever is on top.  When the climbers give you a hard time, don’t take it personally.  Climbers are notorious for saying things like, “Complaint department is on the top floor,” or “Come to my office and let’s talk about it.”  You’ll also learn the difference between corrugated and smooth-wall cable, and you’ll learn to cuss every time you have to deal with Commscope. 
After a few weeks, it’s your big day.  Boss says you’re going up.  First, harness.   Before you ever put it on, examine it closely.  You’ll be putting your life in this harness, so everything has to be right.  No nicks, cuts, or frayed straps.  Now inspect the dee rings for cracks or rust.  These are the metal pieces that hold you when you’re dangling two hundred feet off the ground.  Kind of important.  Check the back ring and the chest ring again.  Check under the nylon straps.  Check the back side and check the front side again.  And again. And again.  If you ever fall, those are what will catch you, so they have to be perfect.  Now, check your lifelines.  We call them “hundred percents.”  They’re hanging from your back dee ring and look like a pair of oversized bungee cords inside two red socks, with big metal hooks on the free ends.  Examine the hooks for rust or scratches.  Open and close them a few times and make sure they operate smoothly, and that they close securely.  Run your hands over the nylon coverings and make sure there are no cuts or frayed spots.  Remember, if you ever fall, that red bungee cord might be the only thing between you and the ground, so when you’re satisfied that they’re okay, check them again.
Now, putting it on.  You’ll be wearing it for a while, and you’ll move in ways you didn’t know you could, so it should be comfortable.  A properly fitted harness is like your favorite pair of shoes. Only it’s more complicated than putting on rollerblades, so ask an experienced climber to help you get it on right.  Slide your arms through the shoulder straps and let the weight of the harness hang off your shoulders.  It feels pretty light.  Buckle the belt as tight as you comfortably can.  Reach through your legs and grab a leg strap.  Buckle it into the buckle at your hip and tighten it dog-collar tight.  Now do the same on the other side.  Clip the free ends of your hundred percents into the big dee rings at your hips, one on each side.  They’ll hang down in loops almost to your knees, but you’ll hardly know they’re there.   Right now, that harness feels pretty good.
Now, you need gear.  Clip a couple extra carabiners to the small dee rings on the back of your belt.  There’s no telling what for, but you’ll probably need them.  Call them “beeners,” because everyone will look at you funny if you keep calling them carabiners.  Ask for a spreader bar and have your buddy show you where the rings for your boatswain’s seat are, because the spreader bar is great as long as it’s attached to the right rings.  Better yet, say “chair” instead of “boatswain’s seat.” That way, everyone will know what you mean. 
Grab another beener and clip it through the hole in the middle of your spreader bar.  While you’re grabbing, get a 3-foot positioning rope.  It’s a short piece of one-inch diameter white rope with elongated flat clips on each end.  These ropes take a lot of abuse, so check them carefully for cuts and frays, and check the metal clips for rust or cracks. Open and close the clips a few times to make sure they work.  When you’re satisfied, clip one end into the beener on your spreader bar and the other into one of your side dee rings. Get another 3-foot or a 4-foot positioning rope and after you’ve checked it, clip both ends to one of your back dee rings.  You never know when you’ll need it.  Now, use another beener to attach a lad-safe to your chest ring.  That harness is starting to get heavy.  Don’t worry.  You’re not done.
Tools.  That’s right – you’ve been so focused on getting ready to go up that you completely forgot that you have to do stuff while you’re up there.  Again, have your climber buddy help you get everything you’ll need for the job at hand.  Make sure you have a tower wrench, a pair of small channel locks, two small crescent wrenches, a box-cutter type knife, a flat-head screwdriver, half a paper towel, and a roll each of skinny tape and fat tape. That will do for most jobs, and if you need anything else, the ground crew will send it up on the rope.  Tuck all these tools securely into your tool pouches.  Make sure they’re secure.  The last thing you want to do is drop anything off the top of a tower.  A screwdriver falling from 200 feet can break a windshield.   Puncture a roof.  Someone’s head.  So pay attention and don’t miss your pocket.
Speaking of rope, the lead climber should carry the rope.  Tonight, that’s not you.  You’ll have an experienced climber going up with you to teach you what you need to know, and he or she will probably carry the rope and snatch block.
Even without the rope and snatch block, your harness now weighs about 60 pounds.  It’s well-distributed, but you definitely feel the weight.  Slip your hands into a pair of weight-lifting gloves.  It sounds strange, but they’re perfect.  The leather palms and half-fingers protect your hands from the galvanized surface of the tower.  The wrist wraps support your wrists while you climb and the cut off fingers gives you the dexterity you need to open connections.
Don your helmet and tuck a bottle of water into the top of your right-hand tool pouch.  It’s time to climb.  Walk up to the ladder.  If you’re feeling at all nervous, don’t look up. There’s a cable running up the middle of the entire ladder.  That’s called a safety climb.  Your lad-safe will attach your chest dee ring to that cable so if you fall, you aren’t going to hit the ground.  Get your climber buddy to show you how to put the lad-safe on it.  Practice taking it off and putting it on.  Make sure it rolls smoothly up the cable, cause you’ll be dragging it up all the way. 
Now, practice resting.  Of course it sounds stupid.  Trust me, this is one of those things you’re going to want to know.  Remember the positioning rope that’s hanging off your spreader bar?  Unclip the end from your side dee ring.  Wrap that rope one time around the ladder rung at chest level.  Clip the free end back into the spreader bar.  Now, lean back into the rope.  You’ll feel your chair pressing against the back of your legs just like the seat of swing.  Sit down into it.  Go ahead and trust that the harness will hold you.  If you don’t believe that by now, quit.
The most important thing about climbing is that you trust your equipment.  As long as you know your harness will catch you, there’s no reason to be afraid.  Oh, sure, you can get hurt.  But you won’t die.  Not from hitting the ground anyway.
You have your helmet.  Gloves.  Water.  Tools.  Positioning ropes.  Extra beeners.  More extra beeners.  Put your foot on the first rung and start up.  Use your legs to propel your body upward.  Just use your hands to hold yourself to the ladder.  If you try to climb with your arms, you’ll be half dead before you’re half up.
So, you’re climbing.  For the first little bit, just pay attention to each step.  When your arms get tired,   it’s time to rest.  Hold onto the ladder with one hand and use the other to tie your positioning rope to the ladder.  Lean backward against the rope and sit in your chair.  Harder to do than it was when you were standing on the ground.  Stare at your hands and make your fingers let go of the ladder.  Stretch your arms.  Now, you can look around.  Treetops.  Rooftops.  Buzzards flying.  Below you.  When you’re ready, stand back up, unclip one end of the rope, unwrap it, and keep on climbing.
            When you get to the top of the ladder, you’re going to have to get off the safety climb so you can go do whatever it is you’re up there to do.  Secure yourself with your positioning rope like you were going to rest.  Look around you and find something really sturdy.  Something you could hang a car off of.  Unhook one hundred percent from your hip and hook it to that strong something.  Now look at it and make sure it’s secure.  Look again.  You are now “tied off.”  If you fall, that hundred percent is going to catch you 9 feet straight down.  Time for new pants but you’re alive.  So look again and make sure.  Look down at the ground.  Long way down.  Now look at that hundred percent again.  Once you’re really sure, unfasten your lad-safe.  It can be done with one hand, but you’re welcome to use both.  You are now free to move about the tower.  Move around,  get stuff done – but before you unlatch one hundred percent, make completely sure the other one is secure.  When you get done, reattach your lad-safe, hook your hundred percents back to your hip rings and climb down.  That’s usually harder than going up.  You’re tired.  Gravity is pulling you.  You have to use your arms more.  And your lad-safe will get stuck at least twelve times.  Climb up a rung and pull it down to you.  When you finally reach the ground, your legs are shaking.  You’re tired.  And relieved.  And giddy.  And excited.  And already looking forward to the next climb.


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Lightning Safety in the Mining Industry

Lightning Safety in the Mining Industry

By Richard Kithil, President & CEO, NLSI

1. Abstract

Investigations at various mine locations worldwide disclose many power quality and safety issues traceable to ignoring lightning safety. Some examples are:
  • Mine-wide mobile radio communications fail following a lightning strike to a central radio tower. Surge protection was not adequate. (Canada)
  • Five maintenance workers are injured when lightning strikes a stationary vehicle. The lightning early warning system is low cost and old technology. (Peru)
  • A central computer system is damaged by a lightning strike. Important geological data is lost. Grounding methods are in violation of codes and standards. (U.S.)
  • More than $US one million is spent on some 350 unconventional-design lightning rods. The product is not approved by U.S. or international lightning protection codes and standards. Safety to all buildings is compromised. (Peru)
  • A mine has no policies/procedures in effect for lightning safety. Accidents under dangerous conditions result in production delays and liabilities for management. (Australia)
  • Miners are killed when lightning exploded methane gas inside a subsurface mine. A comprehensive approach to lightning safety is absent. (U.S., Russia, and China)
  • Four of eight gas turbine generators suffer lightning-induced failures. Production losses at the mine are $700,000/day. The power shortage lasts three months. (Papua New Guinea)
  • Lightning causes failure of a primary water pumping system. Underground flooding closes the mine for 45 days. (South Africa)
  • Smelter potline is “frozen” after lightning hits a substation. 164 pots have to be dug out by hand. Production is shut down for seven weeks. (U.S.)
  • Haul truck is struck by lightning with consequential tire explosion. Grenade-like fragments kill two workers. (Mexico)
  • Mine superintendent standing outside during a thunderstorm is killed by lightning. (Australia)
  • Radio operator inside a building is killed by lightning. Grounding is poor. (Indonesia)
  • Mine worker next to a tree is killed and another is injured. (Dominican Republic)
  • Two workers are killed and seven are injured in two separate incidents in gold and copper mines. (Laos)
  • Lightning disables two substations near an underground mine. 275 workers are trapped two kilometers below the surface. Production is suspended. (South Africa)
  • Two exploration crew members struck by lightning are airlifted to hospital. Hand-held detector fails to warn of threat in time for evasive action. (Peru)
With such examples, it is difficult to support a position that lightning incidents are rare. This paper suggests that lightning is responsible for significant economic and personal costs in the industry. How to defend against the lightning hazard at mining operations? We recommend adoption of a lightning hazard mitigation and safety planning process that can be applied to most facilities.

2. Lightning Behavior and Characteristics

2.1. Physics of Lightning

Lightning’s characteristics include current levels approaching 250 kA with the 50% average being about 25kA, temperatures to 15,000 C, and voltages in the hundreds of millions. There are some 10 cloud-to-cloud lightnings for each cloud-to-ground lightning flash. Globally, some 2,000 on-going thunderstorms generate about 50-100 lightning strikes to earth per second. Lightning is the agency that maintains the earth’s electrical balance.
The phenomenology of lightning flashes to earth, as presently understood, follows an approximate behavior: the downward leader (gas plasma channel) from a thundercloud pulses toward earth. Ground-based air terminators — such as fences, trees, blades of grass, corners of buildings, people, lightning rods, and power poles — emit varying degrees of induced electric activity. They may respond at breakdown voltage by forming upward streamers. In this intensified local field, one or more leaders likely will connect with one or more streamers. The "switch" is then closed and the current flows. Lightning flashes to ground are the result. A series of return strokes follow.

2.2 Lightning Effects

Thermal stress of materials around the attachment point is determined by:
  • Heat conduction from arc root
  • Heat radiation from arc channel
  • Joule heating
The radial acoustic shock wave (thunder) can cause mechanical damage. Magnetic pressures — up to 6,000 atmospheres for a 200 kA flash — are proportional to the square of the current and inversely proportional to the square of the diameter of struck objects. Voltage sparking is a result of dielectric breakdown. Thermal sparking is caused when melted materials are thrown out from hot spots. Exploding high current arcs, due to the rapid heating of air in enclosed spaces, have been observed to fracture massive objects (e.g., concrete and rocks). Voltage transfers from an intended lightning conductor into electrical circuits can occur due to capacitive coupling, inductive coupling, and/or resistance (i.e., insulation breakdown) coupling. Transfer impedance, due to loss of skin effect attenuation or shielding, can radiate interference and noise into power and signal lines. Transfer inductance (mutual coupling) can induce voltages into a loop, which can cause current flows in other coupled circuits.

2.3 Behavior of Lightning

Absolute protection from lightning may exist in a thick-walled and fully enclosed Faraday Cage; however, this is impractical in most cases. Lightning “prevention” exists only as a vendor-inspired marketing tool. Important new information about lightning may affect sensitive facilities. First, the average distance between successive cloud-to-ground flashes is greater than previously thought. The old recommended safe distance from the previous flash was 1-3 miles. New information suggests that a safe distance should be 6-8 miles (Lopez & Holle, 1998, National Severe Storm Center). Second, some 40% of cloud-to-ground lightnings are forked, with two or more attachment points to the earth. This means there is more lightning to earth than previously measured (Krider, 1998, International Conference on Atmospheric Electricity). Third, radial horizontal arcing in excess of 20 meters from the base of the lightning flash extends the hazardous environment (Sandia Laboratories, 1997). Lightning is a capricious, random, stochastic and unpredictable event. At the macro-level, much about lightning is understood. At the micro-level, much has yet to be learned.
When lightning strikes an asset, facility, or structure (AFS), return-stroke current will divide up among all parallel conductive paths between attachment point and earth. Division of current will be inversely proportional to the AFS path impedance, Z (Z = R + XL, resistance plus inductive reactance). The resistance term will be low, assuming effectively bonded metallic conductors. The inductance, and related inductive reactance, presented to the total return stroke current will be determined by the combination of all the individual inductive paths in parallel. Essentially lightning is a current source. A given stroke will contain a given amount of charge (coulombs = amp/seconds) that must be neutralized during the discharge process. If the return stroke current is 50kA, that is the magnitude of the current that will flow, whether it flows through one ohm or 1,000 ohms. Therefore, achieving the lowest possible impedance serves to minimize the transient voltage developed across the AFS path through which the current is flowing [e(t) = I (t)R + L di/dt)].

3. Lightning Protection Designs

Mitigation of lightning consequences can be achieved by the use of a detailed systems approach, described below in general terms.

3.1 Air Terminals

Since Franklin's day, lightning rods have been installed upon ordinary structures as sacrificial attachment points, intending to conduct direct flashes to earth. This integral air terminal design does not provide protection for electronics, explosives, or people inside modern structures. Inductive and capacitive coupling (transfer impedance) from lightning-energized conductors can result in significant voltages and currents on interior power, signal, and other conductors. Overhead shield wires and mast systems located above or next to the structure are suggested alternatives in many circumstances. These are termed indirect air terminal designs. Such methods presume to collect lightning above or away from the sensitive structure, thus avoiding or reducing flashover attachment of unwanted currents and voltages to the facility and equipments. These designs have been in use by the electric power industry for over 100 years. Investigation into applicability of dielectric shielding may provide additional protection where upward leader suppression may influence breakdown voltages (Sandia Laboratories, 1997). Faraday-like interior shielding, either via rebar or inner-wall screening, is under investigation for critical applications (U.S. Army Tacom-Ardec).
Unconventional air terminal designs that claim the elimination or redirecting of lightning (DAS/CTS - charge dissipators) or lightning preferential capture (early streamer emitters - ESE) deserve a very skeptical reception. Their uselessness has been well-described in many publications, including: NASA/Navy Tall Tower Study, 1975; R.H. Golde, 1977, “Lightning”; FAA Airport Study, 1989; T. Horvath, 1991, “Computation of Lightning Protection”; D. MacKerras et al, 1997, IEE Proc-Sci Meas. Technol, V. 144, No. 1; National Lightning Safety Institute, 1997, “Royal Thai Air Force Study”; A. Mousa, 1998, “IEEE Trans. Power Delivery, V. 13, No. 4; International Conference on Lightning Protection - Technical Committee personal correspondence 2000; Uman & Rakov, 2002, “Critical Review of Nonconventional Approaches to Lightning Protection,” AMS Dec. Radioactive air terminals have been banned in Europe following investigations by reputable scientists (R.H. Golde, op. cit. and C.B. Moore, 2000, personal correspondence).

3.2 Downconductors

Downconductor pathways should be installed outside of the structure. Rigid strap is preferred to flexible cable due to inductance advantages. Conductors should not be painted, since this will increase impedance. Gradual bends always should be employed to avoid flashover problems. Building structural steel also may be used in place of downconductors, where practical, as a beneficial subsystem emulating the Faraday Cage concept.

3.3 Bonding

Bonding assures that unrelated conductive objects are at the same electrical potential. Without proper bonding, lightning protection systems will not work. All metallic conductors entering structures (e.g., AC power lines, gas and water pipes, data and signal lines, HVAC ducting, conduits and piping, railroad tracks, overhead bridge cranes, roll-up doors, personnel metal door frames, hand railings) should be electrically referenced to the same ground potential. Connector bonding should be exothermal and not mechanical, wherever possible, especially in below-grade locations. Mechanical bonds are subject to corrosion and physical damage. HVAC vents that penetrate one structure from another should not be ignored as they may become troublesome electrical pathways. Frequent inspection and cross-joint measuring (maximum 10 milliohms) of connectors to assure continuity is recommended.

3.4 Grounding

The grounding system must address low earth impedance as well as low resistance. A spectral study of lightning's typical impulse reveals both a high- and a low-frequency content. The grounding system appears to the lightning impulse as a transmission line, where wave propagation theory applies. A considerable part of lightning’s current responds horizontally when striking the ground: it is estimated that less than 15% of it penetrates the earth. As a result, low-resistance values (25 ohms per NEC) are less important that volumetric efficiencies.
Equipotential grounding is achieved when all equipments within the structure(s) are referenced to a master bus bar, which, in turn, is bonded to the external grounding system. Earth loops and consequential differential rise times must be avoided. The grounding system should be designed to reduce AC impedance and DC resistance. The use of buried linear or radial techniques can lower impedance as they allow lightning energy to diverge as each buried conductor shares voltage gradients. Ground rings connected around structures are useful. Proper use of concrete footing and foundations (Ufer grounds) increases volume. Where high-resistance soils, poor moisture content, absence of salts, or freezing temperatures are present, treatment of soils with carbon, Coke Breeze, concrete, natural salts, or other low-resistance additives may be useful. These should be deployed on a case-by-case basis where lowering grounding impedances are difficult and/or expensive by traditional means.

3.5 Corrosion

Corrosion and cathodic reactance issues should be considered during the site analysis phase. Where incompatible materials are joined, suitable bi-metallic connectors should be adopted. Joining of aluminum downconductors together with copper ground wires is a typical situation promising future troubles.

3.6 Transients and Surges

Electronic and electrical protection approaches are well-described in IEEE 1100. Ordinary fuses and circuit breakers are not capable of dealing with lightning-induced transients. A surge protection device (SPD, also known as TVSS) may shunt current, block energy from traveling down the wire, filter certain frequencies, clamp voltage levels, or perform a combination of these tasks. Voltage-clamping devices capable of handling extremely high amperages of the surge, as well as reducing the extremely fast rising edge (dv/dt and di/dt) of the transient, are recommended.
It is suggested to protect the AC power main panel, all relevant secondary distribution panels, and all valuable plug-in devices, such as process control instrumentation, computers, printers, fire alarms, and data recording and SCADA equipment. Protecting incoming and outgoing data and signal lines (such as modems and LANs) is essential. All electrical devices that serve the primary asset, such as wellheads, remote security alarms, CCTV cameras, and high-mast lighting, should be included.
Transient limiters should be installed with short lead lengths to their respective panels. Under fast rise time conditions, cable inductance becomes important and high transient voltages can be developed across long leads. SPDs with replaceable internal modules are suggested.
In all instances, the use of high-quality, high-speed, self-diagnosing SPD components is suggested. Transient limiting devices may use spark gap, diverters, metal oxide varistors, gas tube arrestors, silicon avalanche diodes, or other technologies. Hybrid devices, using a combination of these techniques, are preferred. SPDs conforming to the European CE mark are tested to a 10 x 350 us waveform, while those tested to IEEE and UL standards only meet a 8 x 20 us waveform. It is suggested that user SPD requirements and specifications conform to the CE mark, as well as ISO 9000-9001 series quality control standards.
An uninterrupted-power supply (UPS) provides battery backup in cases of power-quality anomalies — brownouts, capacitor bank switching, outages, lightning, etc. UPSes are employed as backup or temporary power supplies. They should not be used in place of dedicated SPD devices. Correct Category A installation configuration is as follows: AC wall outlet to SPD to UPS to equipment.

3.7 Detection

The marketplace provides good/not good/poor products for lightning detection. They are available at differing costs and technologies and can be useful to provide early warnings. They acquire lightning signals, such as RF, EF, or light from cloud-to-cloud or cloud-to-ground or atmospheric gradients. Users should beware of overconfidence in detection equipment. It is not perfect, and it does not always acquire all lightning all the time. Detectors cannot “predict” lightning. Detectors cannot help with “bolt from the blue” events. We recommend the redundancy of a network system, a professional grade detector, and a high-quality handheld detector. A notification system of radios, sirens, loudspeakers, telephone, cell phone, or other communication means should be employed with the detectors. See An Overview of Lightning Detection Equipment for a more detailed treatment of detectors.

3.8 Testing and Maintenance

Modern diagnostic testing is available to “verify” the performance of lightning conducting devices as well as to indicate the general route of lightning through structures. With such techniques, lightning pathways can be inferred reliably. Sensors that register lightning current attachments can be fastened to downconductors. Regular physical inspections and testing should be a part of an established preventive maintenance program. Failure to maintain any lightning protection system may render it ineffective.

4. Personnel Safety Issues

Lightning safety should be practiced by all people during thunderstorms. If thunder is heard, the accompanying lightning is within one’s hearing range, and evacuation to a safe location should be immediate. Measuring lightning's distance with detection equipment (see above) is useful. The National Lightning Safety Institute recommends suspending all outdoor activities when lightning crosses a 10-mile radius. Activities should not be resumed until 20-30 minutes has expired from the last observed thunder or lightning. This is a conservative approach — perhaps it is not practical in all circumstances. Safety is the prevailing directive.
Specifically for outdoor workers, we suggest an action protocol of:
  • Yellow Alert – Lightning is 20-40 miles (30-60 km) distant. Be cautious.
  • Orange Alert – Lightning is 11-19 miles (16-30 km) distant. Be aware.
  • Red Alert – Lightning is 0-10 miles (0-16 km) distant. Suspend activities and go to shelter.
When lightning threatens, standard safety measures should include the following: avoid water and all metal objects; get off the higher elevations, including rooftops; avoid solitary trees; stay off the telephone. A fully enclosed metal vehicle — van, car, or truck — is a lightning-safe refuge because of the quasi-Faraday Cage effect. (Go to YouTube’s “Faraday Cage” for a demonstration.) A large permanent building can be considered a safe place. Shipping containers (MilVans/Conex) can be converted to safe shelters. In all situations, people should avoid becoming a part of the electrical circuit.
Every organization should consider adopting and promulgating a lightning safety plan specific to its operations. An all-encompassing general rule should be: “If you can hear it (thunder), clear it (evacuate); if you can see it (lightning), flee it.”

5. Codes and Standards

In the U.S. there is no single lightning safety code or standard providing comprehensive assistance. NFPA-780 is a general installation guideline for lightning protection systems. U.S. government lightning protection documents should be consulted. The Federal Aviation Administration's FAA-STD-019d and the U.S Air Force's AFI 32-1065 are valuable. The IEEE 142 and IEEE 1100 Recommended Practices are suggested. Other recommended federal codes include military documents MIL HDBK 419A, Army PAM 385-64, NAVSEA OP 5, AFI 32-1065, NASA STD E0012E, MIL STD 188-124B, MIL STD 1542B, MIL STD 5087B, and UFC 3-570-01. The International Electrotechnical Commission’s IEC 62305 series for lightning protection is a comprehensive reference document for the lightning protection engineer. Adopted by many countries, IEC 62305 is a science-based document applicable to many design situations. Spanish-speaking mining operations should consult the Mexican lightning protection code NMX-J-549-ANCE. Beware of the French NF C17-102 and the Spanish UNE-21186 codes, which are produced by enthusiastic salesmen, not by objective engineers and scientists.

6. Conclusion

Lightning has its own agenda and may cause damage despite application of best efforts. Any comprehensive approach for protection should be site-specific to attain maximum efficiencies. In order to mitigate the hazard, systematic attention to details of grounding, bonding, shielding, air terminals, surge protection devices, detection and notification, personnel education, maintenance, and employment of risk management principles are recommended.
Areas of vulnerability at typical mines include: security; communications and radio towers; IT operations; gated entrances; conveyors and crushers; drilling; loading; hauling; maintenance buildings; administrative buildings; laboratory buildings; cafeterias; weigh stations; compressor and pumping stations; guard crossings; exploration crew activities; diesel generator stations; as well as other locations specific to particular mine activities.

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Senin, 24 Oktober 2016

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Rabu, 12 Oktober 2016


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