Harger Lightning & Grounding. *Grounding and Bonding* The Foundation For Effective Electrical Protection

Transcription

1 Harger Lightning & Grounding *Grounding and Bonding* The Foundation For Effective Electrical Protection

2 Objectives 1. Define the difference between grounding & bonding and to describe the roles they play in providing protection for personnel and equipment. 2. Define the role of grounding & bonding as the key element for an effective electrical protection or power quality system. 3. Present the different types of equipment/products used to implement an effective grounding and bonding system.

3 Outline I. Grounding I. What is Grounding II. Why Ground? I. Personnel Safety II. Equipment Protection III. Lightning Dissipation IV. Electrostatic Discharge V. Signal Reference Grounding III.How to Ground I. Products Used

4 Outline II. Bonding i. What is Bonding? ii. Why Bond? iii. How to Bond i. Products used III. Total Systems Protection Approach IV. Soils Resistivity/Ground Testing

5 Outline V. Grounding and Bonding Applications i. Lightning Protection ii. Computer Installations iii. Telecommunications

6 I. What is Grounding? Definition: (IEEE 100) A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the Earth, or to some conducting body of relatively large extent that serves in place of the Earth. Purpose: For establishing and maintaining the potential of the Earth or approximately that potential, on conductors connected to it, and for conducting ground current to and from the Earth.

7 II. Why Ground? Required by CODE (NFPA 70 - NEC Article 250) The National Electrical Code, NFPA-70, addresses proper electrical systems and equipment installation to protect people and property from hazards arising from the use of electricity in buildings and structures.

8 II. Why Ground? Personnel Safety Reduce potential differences between non- current carrying parts (enclosures) and between non-current carrying parts and Earth.

9 Personnel Safety Current Ranges 1 6 ma,, (often referred to as let-go currents) though unpleasant to sustain generally does not impair control of muscles ma currents may be painful difficult or impossible to release energized objects rasped by the hand ma ventricular fibrillation stoppage of the heart inhibition of respiration might occur

10 Equipment Grounding (Personnel Safety)

11 Personnel Safety ANSI/IEEE Std Step Potential: Difference in surface potential experienced by a person s s feet bridging a distance of 1m without contacting any other grounded surface. Touch Potential: Potential difference between GPR and the surface potential at the point where a person is standing, while at the same time having hands in contact with a grounded structure. MOST DANGEROUS

12 Personnel Safety Step Potential: Controlled by properly designed ground electrode system (grid) or the use of wire mesh. Flexible Braid Switch Handle Bond Touch Potential: Controlled by proper bonding and protective systems such as personnel safety mats. Bonding Conductor Safety Mat

13 II. Why Ground? Equipment Protection Operate over-current devices during a ground fault. Provide over-voltage voltage control.

14 System Grounding (Equipment Protection)

15 Earth as Equipment Grounding Electrode UNACCEPTABLE / CODE VIOLATION

16 II. Why Ground? Lightning Dissipation

17 II. Why Ground? ESD (Electrostatic Discharge)

18 II. Why Ground? Signal Reference Grounding - Noise Control

19 Summary: Why Ground? Human Safety Protect Equipment Lightning Protection Electrostatic Discharge Signal Reference Grounding

20 III. How to Ground Establish a ground Electrode sufficient enough to clear fault currents and provide effective dissipation of a lightning strike.

21 Grounding Electrode System NEC to to 54 Formed by bonding all of the following: Metal underground water pipe (soil contact at least 10 ) Building steel Concrete encased electrode (Ufer Ground) Ground ring Made and other electrodes Local underground systems or structures Rod & Pipe electrodes Plates electrodes

22 Resistance of Made Electrodes Single Electrode NEC : R R = 25 ohms or less R R > 25 ohm, a second electrode is installed Electrodes at least 6 feet apart

23 Components: Ground Electrode System Soil Ground Electrode Conductors Connectors Electrodes

24 Ground Conductor Considerations Sizing - Sufficient to withstand maximum fault current for the maximum clearing time. Inductance - Flat strap conductors have less inductance than their similarly sized round conductor counterparts. Strength/Durability - Round conductors whether solid or stranded are much stronger than a 24 or 26 gauge flat strap conductor. This should be a consideration when backfilling trenches. Exothermic Connections - The preferred type of connection for underground uses. Availability as well as ease of connection is better for the round conductors than the flat strap conductors. Cost Effectiveness - Although the inductance may be less for the flat strap conductors, their cost is much higher. It may be more cost effective to use multiple round conductors, thus lowering overall ground system impedance than single flat strap conductors.

25 Effect of Lead Inductance For 1 meter of conductor, Typically R=0.005 Ohm L=1.5µH V=L di + IR dt For typical strike Imax = 30,000 A in 1µsec V = (1.5 x 10-6 ) (30,000) + (30,000 x 0.005) (1 x 10-6 ) V = 45, Volts/meter of conductor Inductive Term Greatly Dominates Resistive Term R L Conductor Model

26 Ground Electrodes Electrodes must be of proper material and cross section to provide a low impedance path to fault current without fusing. Many Types of Electrodes are Available Driven Ground Rods Pole Butt Plates (Distribution Poles) Ground Plates Counterpoise Wires Foundations (UFER GROUNDS) Electrolytic (Enhanced) Ground Rods

27 Ground Electrode Considerations Soil Restivity - Some soils, (such as sandy soils), have such high resistivities that conventional ground rods or ground electrode systems may be unable to attain the desired ground resistance requirement. Enhanced ground electrodes or ground enhancement materials may be required to meet the grounding specification. Soil PH/type - PH a factor in choosing. Some ground rod types work better in different soils. Soil Characteristics - Some sites may have only a few inches of soil (or none) sitting on top of bedrock. In this case, ground mesh is the preferred electrode. (Never drill into bedrock).

28 Electrode Considerations Cont.. Ground Rod Diameter - Doubling diameter of ground rod reduces resistance only 10%. Using larger diameter ground rods is mainly a strength issue (ie. In rocky conditions, a larger diameter ground rod might be advantgageous). RESISTANCE, % 120% 100% 80% 60% 40% 20% 0% ROD DIAMETER, INCHES Ground Rod Length - Doubling length theoretically reduces resistance 40%, actual reduction depends on soil resistivities encountered in multilayered soils. Ground Rod Spacing - Approximately twice the length. (in good soil). RESISTANCE, OHMS ½ 1/2 1" ROD DEPTH, FT

29 Ground Rod Spacing ¾ x 10 Rod ρ = 100 Ω m One Ground Rod ( ρ ) R := ln 4Lr 2 πlr r R = 32 Ω Two Ground Rods Spacing = 20 K := n BB = 2 1 BB 1 R := 1 n ρ 2 π Lr ln 4 Lr r R = 17.4 Ω 1 + ρ π S K ( )

30 Special Electrodes Ufer Grounds - Concrete encased electrode. For example, tying into the tower footing rebar or building pad rebar provides a Ufer ground. Ufer grounds should never be used as the sole ground electrode. Copper Ground Mesh - Used to augment the grounding system. The mesh can be strategically placed to protect personnel against step and touch potentials.

31 Enhanced Grounding Material Should be > 95% pure carbon Should not contain concrete or bentonite fillers

32 Applications Vertical Application Horizontal Application

33 Enhanced Ground Rods Contains electrolytic salts that lower ground resistivity over time

34 Galvanized Ground Rod NFPA - NEGRP Pawnee Site 5/8 x 10 after 10 years in the ground

35 Copper Clad Steel Ground Rod NFPA - NEGRP Pawnee Site 5/8 x 8 after 10 years in the ground

36 Horizontal Cu-Clad Steel Ground Rod in GEM NFPA - NEGRP Pawnee Site Rod corroded away at Cadweld connection 8 of top of rod gone 10 years in the ground

37 II. Bonding i. What is Bonding Definition: (NEC 250) The permanent joining of metallic parts to form an electrically conductive path that will assure electrical continuity and the capacity to conduct safely any current likely to be imposed.

38 ii. Why Bond? No or Poor bonding is often the principle cause of many hazardous and noise- producing situations. Leading to: Unacceptable Voltage Drops Heat Generation Intermittent Operation Electrical Noise High Resistance Grounds Bonding provides near zero voltage difference during ground potential rise

39 iii. How to Bond Interconnect ALL Ground Electrode Systems Electrical Grounding System Lightning Grounding System Telecommunications Grounding System Cable Grounding System Connect all conductive objects together both internal and external to the facility

40 Connectors Connections must be of proper material and mass, and be able to resist corrosion to maintain original low resistance for the life of the system. Types Exothermic Mechanical Compression

41 Ultraweld What is an exothermic reaction? An exothermic reaction is a chemical reaction that liberates heat (thermal energy). The molten copper melts the objects being connected together forming the molecular bond.

42 Exothermic Connections Provides a Molecular Bond Ampacity exceeds that of conductors Connections will not loosen Connections never increase in resistance Does not deteriorate with age Maintenance free

43 Mechanical Connections Used when compression or exothermic connections are not practical/feasible Surface preparation essential Tighten to 75 Inch Pounds Example - Lug to Ground Bar

44 Mechanical Connections 1 2 Surface Preparation Antioxidant Applied 3 Mechanical Connection

45 Compression Connections More reliable than mechanical, less effective than exothermic (Not recommended for underground use) Examples - conductor to compression lug, C-taps

46 Connection Process 2 Crimp Minimum Make sure end of conductor remains at end of barrel; Make first crimp then repeat crimping process

47 Bonding Products Used Ground Bars Equipment Ground Plates Fence and Gate Jumpers Equipotential Mesh and Mats Signal Reference Grids Coaxial Ground Kits

48 III. Total Systems Approach Lightning Protection Surge Suppression Bonding Grounding

49 Why a Systems Approach? These subsystems are not independent. Lightning and Electrical Transient Protection Systems rely on a good grounding & proper bonding for effective performance. Grounding & Bonding, Surge and Lightning Protection are not always well understood in their application in industry. Improper Grounding & Bonding are commonly the root cause of Power Quality Problems.

50 Risks of Not Providing Protecting Human safety Equipment damage Downtime and loss of operations Customer dissatisfaction about reliability Loss of revenue and service

51 IV. Soils Resistivity & Ground Testing

52 How do we create an effective ground electrode system (GES)? What type of facility? Residence, Cellular Tower, Hospital What is the application? Power system, lightning protection, ESD, etc What are the ohmic requirements of the GES? <25 Ω,, <5 Ω, <1 Ω How much area is available? What are the soil conditions? Dry, wet, sandy, rocky, etc.

53 Resistivities of Different Soils

54 Soil Conditions Soil Resistivity Must Be Carefully Considered, Including Moisture Content and Temperature.

55 Soil Resistivity: Wenner Method Soil Resistivity ρ E = 2*π * a * R E

56 Ground Electrode Testing 62% Method (IEEE Std 81) C = Current Probe P = Potential Probe X = Electrode under test Uses Ohms Law to Determine Resistance: R = E/I Resistance = Voltage/Current

57 Probe Spacing P too close to X = False low P too close to C = False high C too close to X = inaccurate reading

58 Site Testing 1. Determine size of ground grid system and calculate length of test leads required. (Pythagorean theorem). Lead Length Critical. 2. Starting at 50, record readings every 50 to obtain a ground resistance curve 3. The point where curve flattens out is the system s ground resistance. (62%)

59 Test Format Distance in Feet Readings in Ohms, Easterly Direction Readings in Ohms, Northerly Direction East Direction North Direction

60 Clamp - On Ground Tester Check ground resistance without auxiliary stakes Resistance without disconnecting from other grounds Ideal for multigrounded systems Rapid continuity check Ground rod or leakage current measurements

61 Clamp-On Method Can be used on both single grounding electrode and multi-bonded/multi bonded/multi- grounding electrode systems. Can only be used on sites supplied by commercial power. Will not work in the case of a 3-phase 3 delta ungrounded system.

62 Clamp-On Method There cannot be any current on the neutral wire, (Less than 5 amps), or the results will be unreliable Proper location of the clamp-on meter is mandatory

63 Clamp-on Ground Tester

64 Clamp-on Ground Tester

65 V. Grounding & Bonding Applications I. Lightning Protection I. What is Lightning? Damaged Caused.. II. What is Lightning Protection? III. Risks of Direct/Indirect Strikes IV. Basic Components II. III. Computer Room Installations I. Ground Subsystems II. Sources of Transients III. Signal Reference Grids Telecomm Installations I. Premise Wiring II. Central Office III. Wireless

66 What is Lightning? Consider Lightning a Gigantic Electrical Spark traveling between Cloud to Cloud or Cloud to Earth containing an average Charge of 30 to 50 Million Volts and a Current of 30,000 Amps.

67 Lightning Characteristics Lightning - High frequency (approx. 1 megahertz) electrical discharge carrying on average 18,000 amps and 30 million volts. Time duration of event is measured in microseconds. Lightning Conductors - Multiple, parallel low impedance paths sufficient enough to carry lightning currents safely to ground terminal system. (IE Master labeled lightning protection system or structural steel). Minimum standard requirements set by UL96A & NFPA 780. * Due to its high frequency & voltage, lightning does not like to stay on one conductor. Therefore, multiple parallel paths are critical!!!

68 Electricity Characteristics Electricity - low frequency (60 Hz) low voltage (<600) and low amperage (<2000). Time duration of an electrical fault is approximately 1 second, possibly longer. (Electrical) Ground Conductor - Sufficient enough in size to provide a safe path for fault currents. Sizing set forth by NEC. Properly size, one conductor is enough. In fact parallel paths are against NEC code.

69 In other words A lightning ground does not equal a green wire ground!!!!!

70 What is Lightning Protection? NFPA 780 A Complete System of Air Terminals, Conductors, Ground Terminals, Interconnecting Conductors, Surge Suppression Devices, and other Connectors or Fittings required to complete the System. UL96A - Installation Standard Master Label Independent Third Party Testing UL96 - Manufacturing Standard

71 Lightning Damage Can Be Traced To: Inadequate direct strike protection Incorrect grounding which allows lightning to flow near sensitive electronics Insufficient transient protection & filtering of power lines Lack of protection on telephone, data and signaling circuits

72 Basic Principles of Lightning Protection Intercept the Lightning Discharge Safely Conduct the Lightning Currents Minimize the Effects of Lightning Currents Dissipate the Lightning Currents in the Earth

73 NFPA 780 Lightning Protection Standard Scope - This document shall cover traditional lightning protection system installation requirements for ordinary structures, misc. structures, special occupancies, etc.

74 Risks Posed from a Direct Strike

75 Risks Posed from an Indirect Strike

76 Basic LP Components Air Terminals Lightning Conductors Ground Terminals Connectors/Fittings Surge Suppression Devices

77 II. Computer Room Installations Grounding System Four Distinct Subsystems NEC Compliant Fault/Personnel Protection Power System Ground (including surge suppression) Lightning Protection subsystem (per NFPA 780) Telecom, data transmission, and signaling circuit surge protection grounding subsystem. Signal Reference Structure

78 Sources of Transient Over-voltages voltages Lightning Induced Surges Power Systems Operations Power System Faults Reactive Load Switching Harmonics Earth Potential Rise Nuclear Electromagnetic Pulse & Solar Flares

79 Transients May be Induced onto: Power Lines Telephone Lines Data Signaling Lines RF Feeders Building Structural Members (lightning) Differential Grounds

80 SIGNAL REFERENCE GRID (SRG) Function: Minimize voltage differences between interconnected equipment by providing a low impedance equipotential ground plane for high frequency low voltage noise. Function:

81 SIGNAL REFERENCE GRID (SRG) SRG Bonded to Building Steel PDU Processors Low Impedance Riser Pedestal Bond Prefabricated SRG

82 III. Telecom Installations

83 Telecom Bonding The sensitivity of the electronic equipment requires that the telecomm cabling and power be effectively equalized to prevent loops or transients that can damage the equipment. To ensure effective equalization, the telecomm ground should be directly attached to the electrical service ground

84 Premise Wiring Applications Applicable Codes/Standards NEC 70 Articles 250, 800 & 830 ANSI J-STDJ STD-607-A

85 NEC Article 800 Communications Circuits Scope - Covers telephone, telegraph (except radio),etc, and telephone systems not connected to a central station system but using similar types of equipment, methods of installation, and maintenance.

86 NEC Article 830 Network Powered Broadband Communications Systems Scope - Covers network-powered broadband communications systems that provide any combination of voice, audio, video, data, and interactive services through a network interface unit.

87 ANSI J-STD-607-A This standards specifies the requirements for a uniform telecommunications grounding and bonding infrastructure that shall be followed within commercial buildings where telecommunications equipment will be installed.

88 Main Elements of Telecomm Grounding and Bonding Structures Telecomm Bonding Conductor Telecomm Main Grounding Busbar (TMGB) Telecomm Bonding Backbone (TBB) Telecomm Grounding Busbar (TGB) Grounding Equalizer, (Formerly TBBIC)

89 Telecommunications Grounding Infrastructure

90 Telecomm Bonding Conductor (Bond to Electrical Service Ground)

91 Telecomm Main Grounding Busbar (TMGB)

92 Telecommunications Bonding Backbone (TBB) Is a conductor that connects all Telecomm Grounding Busbars with the Telecomm Main Ground Busbar. Equalize potential differences between the telecomm system to which it is bonded

93 Telecomm Grounding Busbar (TGB)

94 Grounding Equalizer Conductor that connects elements of the telecommunications grounding infrastructure. (Formerly TBBIBC). Interconnects multiple TGB s s on the top floor and every 3 rd floor in between

95 Conductor Sizing Applies to TBB, GE & TBC

96 Summary For over the past hour, we have presented a tremendous amount of information concerning grounding and bonding, hopefully some of it was informative. If there are other issues you would like addressed, please contact us.

97 Thank You