De-energized Lines. 1. Buzzing. 2. Light Emitting Devices. Methods for Detecting Induced Voltages. 3. Noise Producing Devices

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1 De-energized Lines De-Energized Work A de-energized circuit is NOT dead circuit. Mohamed A. El-Sharkawi Department of Electrical Engineering University of Washington elsharkawi@ee.washington.edu (06) f the de-energized circuit at a worksite is not effectively grounded, the adjacent energized circuit can NDUCE LETHAL OLTAGE at the worksite Buzzing Methods for Detecting nduced oltages Buzzing (Fuzzing): a method to test for the presence of high voltage in power lines. Buzzing may be accomplished by a variety of tools and devices such as live line tools and noisy testers. Touching the conductor with the metal cap at the end of a live line tool puces a buzzing sound if the voltage is high enough Buzzing may not detect low voltage hazards Light Emitting Devices Glowing Neon or Light Emitting Diode For low voltage detection. Small neon lamps typically require about 90 volts to start glowing, but they can continue to glow at about 60 volts. Light emitting diodes can be activated at even lower voltage. Good methods to detect low voltages Light is hard to see in bright sunny conditions. 3. Noise Pucing Devices Hot stick Power line High potential gradient at the Sharp end Corona Discharge n high voltage circuits, sharp edges have high electric field strength that could ionize surrounding air. onization crates leakage current in air to lower potential points Corona emits light and the ionization process has a distinct high frequency noise. This is the buzz that linemen use to identify the presence of high voltage. The method is not accurate for detecting hazardous low voltages

2 4. Direct oltage Measurement oltage Detectors: A voltmeter that can be fit into a hot stick. Highly accurate method. Can cover a wide range of voltage Newer Devices: emote oltage Detector AC HotStick provides warning of exposed high voltage AC from a safe distance. audible beeping visual (flashing LED) No need to contact t the surface being tested For 50K system, it can detect the presence of voltage from as far away as 150 m (500 ) Hard to tell which line is pucing the coupling 8.9 Main Protection Techniques for personnel Working on De- energized Equipment 8.10 Main Methods 1. Establish Equipotential zone at the work site. Maintain Minimum Approach Distance (MAD) and use live-line tools 3. nsulate the worker and use gloves/sleeves or barehand techniques. 1. Equipotential Zone t ensures that any two points at the worksite within direct or indirect reach by the worker have the same potential. Any person inside the zone is protected from electric shocks. The equipotential zone is often grounded t is constructed by using temporary grounds that are also known as Protective grounds Jumpers Personal grounds Working grounds

3 Fundamental Procedures Line Work Static Wire Lockout/Tagout of the breakers feeding the work area Breakers on both ends of the line for line work Breakers surrounding a work area in substations The terminals of the line or equipment being worked on are grounded to prevent accidental energization at the site All caps are discharged or disconnected. Protective grounds are installed at the work site to establish Equipotential zone. CB Ground Equipment Elevated Equipment CB CB CB CB Ground Grid Ground Grid Substation xfm Substation CB CB Busbar Bus Disconnect Circuit Breaker Feeder

4 Energized System Lockout Work Area Work area Work area Substation ing Design Lockout Work Area Work area Work area Maintain MAD 3. nsulate Worker Unless an equipotential zone is established, the site must be considered hot due to electromagnetic coupling from other energized circuits. All worker must stay outside a well established distance from the hazard called Minimum Approach Distance (MAD). MAD is a function of the operational voltage at the site; not the expected induced voltage. workers can reach the work area from outside MAD by using insulated tools such as hot sticks. The hot stick can be fitted with various tools to perform the job (cutting tool, bending tool, etc.) 8.3 The worker s body is insulated from the ground potential. n this case, the body of the worker is floating at the potential of the work site, much like a bird on a power line. The worker can then use his barehand to perform the needed work. Barehand work of all personnel is performed on one phase at the time

5 Purpose of Grounding a Worksite Grounding of De-Energized site (Grounded Equipotential Zone) To provide a means to carry electrical current into earth under NOMAL and FAULT conditions. Thus providing a bypass for the current away from the workers. Grounded equipotential zone must also guarantee that other personnel at the worksite, not directly involved with the line work, are not exposed to the hazard of electrical shock (truck operators, site preparers, etc.) Unprotected Worker Worker Protected by Temporary Ground Path of current Path of current Purpose of Protective Grounds To protect the worker by maintaining a sufficiently low voltage across his/her body even if the line is energized. Energization can occur due to several reasons: Electromagnetic coupling Accidental closers of switches Downed conductors Lightning Example A 0 km de-energized line is accidentally energized by a k source while a line is working at the far end of the line. The line resistance is 0.1 /km, and the worker body resistance plus his ground resistance is 3000 Ohm. Compute the current through the line and the voltage across his body Assume a 0.01 temporary ground is installed between the line and ground in parallel with the worker. Compute the current through the line and the voltage across his body

6 Solution: Without Temporary Ground line Solution: With Temporary Ground line line ground-wire line ma k 8.31 ground wire ground wire line 0.01 ground wire ground wire 000 line 995 A line 0.01 Safe ma Example of Erroneous Practice Solution f =1000A The maximum available fault current at a worksite is 1,000 amp. Workers weight is 165 lbs and his body resistance is 1,000 ohms Maximum accepted levels l at a given utility: Maximum voltage across worker during fault is 100 Maximum current through worker is 1/3 the maximum heart fibrillation level (Heart fibrillation threshold is 86 ma) Compute the maximum resistance of the temporary ground. tem-ground = 86/3= 95 ma Heart fibrillation ill threshold h is 86mA; -max = 86/3= 95mA max max tempground max * faultmax 95* max max m 1 Temporary ground must be lower than 7.8 m Example of Better Practice Compute the resistance of the temporary ground that would limit the current through the worker during fault to 0mA (just below the respiratory tetanus level of 3mA). Solution: * 0* tempground 1.67 m 1 fault This low value of temporary ground may require large cross section ground wire or parallel grounds EEE std.1048: Personal Ground A portable device designed to connect (bond) a de-energized conductor or piece of equipment, or both, to an electrical ground. Distinguished from a master ground in that it is utilized at the immediate site when work is to be performed on a conductor or piece of equipment that could accidentally become energized. Synonyms: protective ground; working ground mmediate site is interpreted as across the worker body

7 Protective Grounds Protective Grounds Capable of conducting the maximum fault current for the time it takes to clear the fault by the protection devices ka is common, but it can be as high as 100 ka Adjustable jaw Clamp Protective grounds shall have an impedance low enough to cause immediate operation of the protective devices in case of accidental energization of the line Twist eye to adjust jaw by hot stick Low resistance Cable Type 1 C clamps Serrated type 3 flat clamps for Towers and flat surface Adjustable jaw Designed to bond with conductors Cable insert Twist eye to adjust jaw by hot stick Busbar Clamp nstalling Temporary protective Ground by Hot Stick

8 esistance of Temporary Ground Assembly surface clamp cable clamp surface The main components of the temporary ground resistance are Surface contact of both ends of the ground esistance of the clamps esistance of the cable Protective Ground Assembly f you allow 5 across the worker. For a fault current of 10kA, the assembly resistance must be less than 5.5 m 10,000 Multiple cables may be used to increase the fault current capacity Use adequate length, longer cables means higher resistance Example of Utility Guideline k rating Maximum Fault Current (Amperes) Fault Clearing Time (Sec.) Copper Grounding Cable Minimum Size 30k 33, /0 30k 6, /0 30k 6, /0 115k 4, /0 115k, /0 34.5k 18, /0 6k 3, /0 6k 1, /0 6k 7, /0 13k 39, /0 13k 30, / Diameter Copper esistance AWG (inch) (mm) (mω/m) (mω/ft) 0000 (4/0) (3/0) (/0) (1/0) Withstand Ampacity for Copper Cable in KA AWG 15 cycles 30 cycles 45 cycles 60 cycles 0000 (4/0) (3/0) (/0) (1/0) Common terminal connected to grounded point Ground Cluster Terminals to three different points

9 Grounding Cluster Traveler Ground (EEE std. 1048) A portable device designed to connect a moving conductor or wire rope, or both, to an electrical ground. Primarily used to provide safety for personnel during construction or reconstruction operations. Synonyms: block ground; rolling ground; sheave ground Protective Grounds (EEE std. 1048) The protective grounds are insulated conductors that must be capable of carrying the current, and withstanding the mechanical forces for at least as long as the current lasts. the accessible voltage drop across the ground set cables (i.e., between connecting points) must not be hazardous. The rating of the grounding set depends on the following: 1. The current-carrying capacity of the cable. The current-carrying capacity of the cable clamps and their connection to the cable 3. How well it is connected at its ends (i.e., surface preparation and tightness) 4. The configuration it is being used in 5. The resistance of the complete grounding system 8.51 Considerations for nstalling Protective Grounds Protective grounds cannot be used without cleaning its connectors to reduce the surface contact resistance; high resistance compromises workers safety Cannot be installed over galvanized, painted or rusted framed; high resistance compromises workers safety Cannot be coiled; inductance increases the impedance of the protective grounds, thus compromises workers safety nstallation and removal of the protective grounds must be made in a specific sequence. The loose end is at the ground potential 8.5 Example Solution f =1000A The available fault current at the worksite is 1,000 amp. Worker s weight is 165 lbs and his body resistance is 1,000 ohms. Compute the current through the if the temporary ground resistance is 5m epeat the computation assume that due to poor installation, the contact resistance of the temporary ground is 0.1 tem-ground With temp ground tempground ma With temp ground tempground tempground ma tempground

10 Ground or Ground Electe (EEE std. 1048) A that is driven into ground to serve as a ground terminal copper-clad solid copper galvanized iron galvanized iron pipe. Ground provides low resistance path to ground. The resistance depends on: Soil type Soil Moisture Depth of into ground adius of the Worksite Hazards: Aerial Work EEE Std 1048: f the conductor that is being contacted by workers becomes energized for some reason, the voltage rise at the work site depends on a number of factors such as the following: 1. Fault current available at that location. The location of grounding sets relative to the work site and the fault current source 3. The number of phases that are grounded 4. The integrity of bonding between the conductor and the surface on which the worker is standing Worksite Hazards: Ground Work EEE Std 1048: A worker on the ground who happens to be touching grounded parts of the structure or conducting equipment attached to them, would be exposed to a voltage that depends on the method of connection to earth as well as on 1. Fault current available at that t location. The location of grounding sets relative to the work site and the fault current source Unfortunately, optimum conditions of protecting aerial worker touching the conductor generally result in poor conditions for the worker on the ground, and vice versa. Note: This doesn t mean that all workers cannot be protected Main Types of Grounding Systems Bracketed grounds: Distributed grounding for wide work areas. Used with line work Work can be performed quickly May not provide effective protection to all workers Worksite grounds: mmediate grounding at the worksite. Provides the best protection to the workers Takes longer time to establish Bracketed Grounding Systems Bracketed Grounds Exposed to lethal voltage Step and Touch Potential Danger

11 Bracketed grounding system The conductors are grounded at adjacent towers on either side (or both sides) of the work site. With no static wire (overhead ground wire OGW), most of the current will flow through the bracketed grounding sets and will result in high GP. The magnitude of the GP is determined by the current and the tower footing resistance. Since the work site tower is located at essentially remote ground, full tower rise voltage will appear between the work site tower and the conductor, and thus be applied to any line who has contact with both. Why use it? t requires less work, thus convenient for large areas Example For a bracketed grounding system, assume that the grounding is at the two adjacent towers. Assume =1000, the ground resistance of each tower is 15, and the temporary ground resistance is 0.0. For a fault current of 1000A. 1. Compute the current through the. Compute the voltage across the 3. Compute the GP at tower 1 4. Assume that the bracketed ground is installed at tower 1 only, compute the voltage across the Solution Solution (Part 1 and ) =1000A // 3 * // 1 * A * k 1 = gw1 + tower1 = tower 3 = gw3 + tower3 Unsafe grounding, Lethal condition GP Solution (Part 3) A 1 * * k Solution (Part 4) 1 * 1 * A * k Unsafe GP When only one tower is grounded, the voltage across the almost doubles

12 Example Assume that the bracketed ground is on the two adjacent towers, repeat the solution when a jumper of 0.0 ohm is connected between the conductor at tower and the tower structure below the worker. =1000A 1 Solution gw = gw1 + tower1 = tower 3 = gw3 + tower Solution (Part 1 and ) 1 // 3 * // gw // gw * gw * 6.67 ma * A Solution (Part 3) 1 GP A 1 * *15 5 k Less GP, but still unsafe Safe condition Worksite Grounds for Line Work Worksite Ground

13 Worksite Ground for equipment Worksite Grounds Equipment Properly installed protective grounds will result in the minimum obtainable impedance path in parallel with worker s body. Creates a grounded equipotential zone t requires more time to install as compared with bracketed grounding system Ground Grid Grounding of Metal Towers Single Circuit Grounds of a Double- Circuit: Metal Tower Grounding of nsulated Structure Equipotential Zone OSHA (n.3): Temporary grounds shall be placed at such locations and arranged in such a ner as to prevent each employee from being exposed to hazardous difference in potential. All grounds and grounded equipment at the work site must tbe bonded dtogether th to a single point ground system. The equipotential zone must be bonded to the best ground system at the site. Multiple points ground system can lead to hazardous conditions due to variations in the GP

14 Static ground wire Static ground wire 6 3 nsulators 3 nsulators High oltage circuit High oltage circuit 1 Personal ground 7 moves with worker Boom truck with a person inside conductive bucket Personal ground 6 moves with worker Boom truck with a person inside conductive bucket Substation Grid Busbars 1 High oltage circuit 1 Personal ground 6 moves with worker Boom truck with a person inside conductive bucket Keep in Mind f other workers at the site are not protected during faults or accidental energization, the protective grounds assembly do not form the equipotential zone. Workers on the ground near the site Workers standing on ground and touch the truck Workers touch the tower structure Establishing Equipotential zone Long protective cable Problems: Long path from phase to ground Neutral is used as a path for fault current

15 Adequate length Elevated Equipment 7 8 Eq quipment Ground Grid Elevated Equipment 7 8 Eq quipment Ground

16 Static ground wire Long protective cable 3 nsulators 4 High oltage circuit Boom truck with a person inside conductive bucket static 4 Static ground wire Case Study 7.1 A crew established a grounded equipotential zone for de-energized lines. An accident occurred when the de-energized line comes in contact with another high voltage circuit resulting in a fault current of 5 ka in the line being maintained. Assume the following data: The ground resistance of the ( ) = 30 5 The ground resistance of the tower ( gt ) = 15 The resistance of the tower structure ( tower ) = truck truck static 6 g truck g The ground resistance of the static wire including the wire resistance ( static ) = 0.01 The resistance of any ground wire ( gw ) = 0.00 The body resistance of the ( ) = Case Study 7.1 Solution: Part 1 1. Draw the equivalent circuit. Compute the current through the line and the voltage across his body 3. Compute the potential of the truck 4. Assume the temporary ground wire 6 is not present, repeat parts and Assume that the temporary ground wires and 6 are not present, repeat parts and 3 static g tower 6 static gt

17 Solution: Part Solution: Part ma 30 ma truck Solution: Part truck Solution: Part 5 Case Study 7.1 truck 77.7 ma n the previous case, assume a person is touching the truck. Assume that the body resistance of the person is 1000 Ohm and his foot resistance is 1000 Ohm. Compute the voltage across the, and the current through his body static Solution: Part Solution: Part 9 ma 9 3 static g tower gt -g truck Solution: Part

18 Solution: Part 4 5 ma Solution: Part ma truck 50 truck Summary of Currents Safety Dilemma Part Part 5 Man inside basket 5.46 ma 77.7 ma More protection to the person inside the basket will result in less protection to the person standing on ground. Man on ground 9 ma 1.15 ma Since both workers are not protected, the protective ground arrangement don t form an equipotential zone Since both workers are not protected, the protective ground arrangement don t form an equipotential zone How to address the problem? f the current through the worker touching the truck is high, we don t have equipotential zone We have three options: to create a barricade around the truck where no person is allowed to enter the barricaded area To isolate the worker by having him stand on isolated mat (least desirable option). to have a conductive mat under the truck where the worker stands on the mat and the mat is connected to the truck by a low resistance jumper. Example 1 Assume the line is accidentally energized at 15k. Assume the truck is conductive and all temporary grounds are 0.0. The tower ground resistance is 15 and the resistance is 30. The ground resistance of the worker is 1500, and the worker resistance is 000. Compute the current through the two workers

19 f 1 Solution f 1 Solution tower -g tower -g Ground is not protected eq f // 1 //( g ) 0.0 // //( ) eq 15,000 / A f f ma g m A Ground Mat To protect the worker on ground Ground Mat To protect the worker on ground Ground mat Ground mat Ground Mat Example Compute the current through workers When ground mat is used. Assume the ground mat resistance is 30. Ground mat

20 f 1 Solution f 1 Solution tower eq f mat 3 // // // 1 // 0.0 // // 30.0 mat eq ,000 / A tower 1 f f ma ma f mat m A Both men are protected Case Study 7. (mportance of Personal ground) A line working on a de-energized line from a conductive boom. The line used one ground wire between the tower and the conductor. Assume an accident occurred where the de-energized line became in contact with another high voltage circuit. This resulted in a fault current of 5 ka. Assume the following data: Ground resistance of the ( ) = 30 Ground resistance of the tower ( gt ) = 15 Ground resistance of the static wire including the wire resistance ( static ) = 0.01 esistance of any ground wire ( gw ) = 0.0 Body resistance of ( ) = 1000 Case Study s the inside the basket protected? Lethal condition during fault (Existing grounds) Large fault current go through the structure ground Most of fault current go through the static wire Fault current 1.7 A of current go through the worker Case Study 7. epeat the previous case, but assume an additional ground wire is installed between the conductor and the basket

21 Safe condition during fault (Equipotential grounds) The worker is protected mainly because of the personal ground 1 Most of fault current go through the static wire Fault current Negligible amount of fault Current go through the worker Case Study 7. Assume the line did not secure wire to the tower. When the fault current flows, wire is dislodged from the tower side. gnore the resistance of the tower structure, and evaluate the grounding system Answer 100 ma Single-Phase vs. 3-Phase grounding (EEE std.1048) The magnitude of three-phase short-circuit currents may be higher than that of a single-phase short Especially when the ground resistance is high. n fact, the single-phase fault current of a three-phase distribution line, grounded only on one phase through a high-resistance ground, may be insufficient to cause the line circuit breaker to open. Protective grounds applied to all phases will therefore provide more certain, and generally more rapid, operation of breakers when ground resistance is high. n addition, three-phase grounding also means that only a small part of the fault current of a three-phase fault would flow to ground at the structure, thereby reducing the step and touch potentials at the base of the structure. Single-Phase vs. 3-Phase grounding (EEE std.1048) f ground resistances are low enough to ensure consistent fault clearing, and the step and touch potentials are within acceptable limits or can be guarded against, then grounding of only one phase of a three-phase line might be permitted. However, working clearances must always be maintained for the ungrounded phase conductors Special Attention to Overhead Ground Wire (OGW) OGW could be insulated (for obstruction lights) or bonded to the structure of the tower. n either case, the connection of the OGW to the tower structure must be made by a temporary protective ground to ensure the proper grounding of the tower. Parallel Grounds Why connecting grounds in parallel? For extra safety; in case one is dislodged To increase the current carrying capability of the grounding system When two or more sets of grounds are placed in parallel at a specific worksite, care must be taken to ensure that the sets are identical in cable lengths ensure that the sets are identical in cable sizes ensure that the sets are identical in clamp sizes Each set is applied to clean conductors. The division of current among parallel grounds depends upon the total impedance of each ground set, including connection impedance

22 Example A fault current of 10KA flow through two ground jumpers. Assume the resistance of each jumper is 0.0, and the current carrying capability of each is 6kA. Compute the current through each jumper jumper Solution f 5kA Example Now assume that one of the jumpers is 0.0 and the other one is Compute the current through each jumper jumper1 jumper f f jumper1 jumper1 jumper jumper1 Solution jumper jumper kA kA Exceeded the rating Case Study: Unexpected Fault 8.19 CB Substation Ground Site description The worker was performing maintenance on a conductor that was de-energized and grounded at the substation. The worker placed two ground wires on both ends of the work area. These two ground wires were attached to a ground cluster that was connected to a ground by a third ground wire. The wires were long and one of them was coiled All ground wires were for inductive grounding Site description The worker operated from a conductive truck and boom. The basket was made of a mix of insulated and conductive materials. The truck was grounded through another ground

23 Accident CB Sequence of events Substation Ground A fault occured at nearby circuit Part of the fault current pass through the ground of the truck then to the truck itself and eventually to the worker on its way to the ground point at the substation. The worker died iolations The ground wires were long and their resistances were about 15 mω each. This is too high for personnel safety. The ground wires are for inductive grounding only; inadequate during fault conditions Grounds were not bonded to a single point The s were inserted only 0.75m into ground. The ground resistance of the was over 660Ω, too high to protect site workers. There was no personal ground installed. This alone could have saved the worker s life Prevention CB Substation Ground Grounding During the nstallation of Overhead Transmission Line Conductors EEE Std 54a-1993 To pull pulling lines and conductors during stringing operations. t normally incorporates one or more pairs of multiple-groove bullwheels. Equipment: Bullwheel Puller/Tensioner

24 Equipment: Traveler Ground Equipment: ollers Equipment: Sagging Tractor arious Lines at Worksite Pilot line: A lightweight line, normally synthetic fiber rope, used to pull heavier pulling lines that, in turn, are used to pull the conductor. Pulling line: A high strength line, normally synthetic fiber rope or wire rope, used to pull the conductor. When a conductor is being replaced, the old conductor often serves as the pulling line for the new conductor. Conductor: The power line being installed or replaced Pulling and Tensioning Sites Puller equipment should be effectively grounded to a driven or other suitable grounding sources in the following order of preference: a) substation ground grids b) ground grid mats where deemed necessary c) tower or steel pole grounds d) pole ground Grounds at Pulling/Tensioning Site The first tower away from the puller should use traveler grounds The puller should use a running ground bonded to the grounding system When pulling bundled conductors, the subconductors should be bonded together. Clearly marked warning barriers should be used at the puller site to identify the hazardous areas. ubber gloves and protectors (overgloves) should be worn by personnel working on the ground in the designated area during stringing operations

25 Grounds at Pulling/Tensioning Site Grounds at Sagging Site Anchoring Site The location along the line where anchors are installed to temporarily hold the conductors in facilitating splicing, pulling, or tensioning. n addition to the grounding g system at the anchor site, the first tower in either direction should have grounds in use until clipping-in has been accomplished. All subconductors and all phases should be jumpered together at the anchor site and connected to the ground source. Bypass jumpers should be installed to tie each end of each phase together. Splicing Site The location along the line where the conductors are temporarily anchored to join the conductors together to form a splice. Splicing vehicles should be effectively grounded prior to making splices in the conductors. The ground system should remain in place until the spliced conductors are raised to clear the splicing site Grounding at Splicing Site nstallation of pilot and pulling line During installation of conductive pulling lines, running grounds should be installed on the pulling line at the tension and puller sites. Traveler grounds should be used on the first tower away from the tensioner or puller or both, on either side of energized line crossings Traveler grounds should also be installed at intervals of not more than two miles along the line being strung. The tensioner and puller machines should be effectively grounded and bonded to the wire rope and reel stands The end of the conductor should be bonded to the machine through brushes and slip rings

26 Effective Grounding: American National Standard (ANS) EEE Std 54a-1993 (Supplement to EEE Std ) Key Standards for Work on De- energized Lines n order to have a totally effective grounding system, the grounding system shall 1. Protect personnel from step and touch voltages by providing an equipotential zone(s) or a low impedance path to ground, thereby reducing step and touch voltages to an acceptable level to protect personnel and equipment.. Withstand and dissipate fault and surge currents. 3. Provide rugged mechanical properties Effective grounding: American National Standard (ANS) EEE Std 54a-1993 (Supplement to EEE Std ) Section 7.1.3: t is very important to give attention to restraint of grounding jumpers to minimize possible severe mechanical movement should a fault occur. Equipotential Zone The worker is protected only when an equipotential zone is encompassing the work site. OSHA (n)(3) "Equipotential zone. " Temporary protective grounds shall be placed at such locations and arranged in such a ner as to prevent each employee from being exposed to hazardous differences in electrical potential. f the worker is outside the equipotential zone, the worker is exposed to hazardous voltage OSHA ()(C): Appendix C 1) The creation of an equipotential zone will protect a worker standing within it from hazardous step and touch potentials. Such a zone can be puced through the use of a metal mat connected to the grounded object. n some cases, a grounding grid can be used to equalize the voltage within the grid. Equipotential zones will not, however, protect employees who are either wholly or partially outside the protected area. Protective Grounds f the boom and bucket are conductive, the equipotential zone must be grounded. Grounding must also be made to protect the worker under fault conditions, not just under the normal operating condition. OSHA (n)(4)(i) Protective grounding equipment shall be capable of conducting the maximum fault current that could flow at the point of grounding for the time necessary to clear the fault. OSHA (n)(4)(ii) Protective grounds shall have an impedance low enough to cause immediate operation of protective devices in case of accidental energizing of the lines or equipment

27 Protective Grounding: American National Standard (ANS) EEE Std 54a-1993 (Supplement to EEE Std ) All equipment, conductors, anchors, and structures within a defined work area must be bonded together and connected to the ground source. The recommended procedures of personnel protection are the following: 1. Establishment of equipotential work zones. Selection of grounding equipment for the worst-case fault 3. Discontinuation of all work when the possibility of lightning exists which may affect the work site Protective Grounding: American National Standard (ANS) EEE Std 54a-1993 (Supplement to EEE Std ) The value of a grounding system depends on a low resistance path. All equipment shall be kept in excellent condition. All surfaces to which grounding clamps are to be connected shall be cleaned to ensure proper contact. Frequent inspection of all components is essential Grounding Surface: EEE Std Provisions must be made in the work instruction for proper surface preparation at the connection points to ensure low contact resistance to prevent the clamps from being blown off by mechanical forces. Failure to remove the high-resistance oxide layer at the connection point can lead to excessive resistance heating and consequent melting at the connection, resulting in loosening and dislodging of the clamp. A brittle, corrosive layer could also cause the clamp to loosen. When tightening clamps, always follow the ufacturer s recommendations Summary: Equipotential Zone 1. Equipotential zone must encompass the work site to protect Workers inside the bucket Workers on the ground touching the truck Workers on the ground touching the tower Workers walking in the immediate vicinity of the tower. Any worker inside the zone must have his/her body bypassed by a very low resistance, much lower than the body resistance of the worker. Short jumpers Adequate cross section that withstands the maximum fault current at the worksite for as long as it takes to clear the fault by the CB

28 Summary: Equipotential Zone 3. Equipotential zone must protect worker even during faults. 4. All grounds much be bonded to clean surfaces. 5. The type of the ground clamp must match the shape of fthe bonding surface. 6. The clamps must be mechanically secured and must not be dislodged even by the severe force of fault current. 7. Grounds must be bonded and connected to a single ground point (the best ground at the site). Summary: Equipotential Zone 9. ncluding the three phases of the circuit in the equipotential zone is preferred t reduces the ground current during fault (lower step potential) t ensures quick action of the CB as the fault current of 3-phase system is much higher than the single phase fault. 10. The jumpers shouldn t be long to hit the workers should fault current pass through it 11. nstallation and removal of the grounds must be made by live-line tools