Grounding & Bonding

Grounding & Bonding The Foundation For Effective Electrical Protection January 24, 2006 Tuesday 2:15-3:00 PM Curtis R. Stidham Harger Lightning & Grounding

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

Outline • Grounding & Bonding – – – – –

What is Grounding & Bonding? Why do we Ground? Different types of Ground Systems Hazards & Safety Relevant Standards

• Ground Electrode System – What makes up the system? – Results from NFPA - NEGRP – Connections (Critical Element)

Outline • Grounding and Bonding Applications – Power Systems – Lightning Protection – Computer Installations

Foundation of Protection Lightning Protection Surge Suppression Bonding Grounding

Key to Systems Approach •

Protection systems are not independent.

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Lightning Protection and Surge Suppression Systems rely on a good grounding & proper bonding for effective performance.

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Grounding & bonding are not always well understood in their application.

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Improper grounding & bonding is commonly the root cause of Power Quality Problems.

Risks of Not Providing Protection • • • • •

Human safety Equipment damage Downtime and loss of operations Customer dissatisfaction about reliability Loss of revenue and service

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.

What is Grounding? • Purpose: – Used for establishing and maintaining the potential of the Earth or approximately that potential, on conductors connected to it. – Conduct ground current to and from the Earth.

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.

Why Ground?

Personnel Safety Reduce Potential Differences ƒ Between enclosures ƒ Between enclosures and Earth

Why Ground? Personnel Safety Current Ranges

• 1-6 mA, (often referred to as let-go currents) – unpleasant – Does not impair control of muscles

• 9-25 mA – may be painful – difficult or impossible to release energized objects grasped by the hand

• 60-100 mA – ventricular fibrillation – stoppage of the heart – inhibition of respiration might occur

Why Ground? Personnel Safety ANSI/IEEE Std. 80-2000

ER O U G D AN M O ST

• 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.

S

• Step Potential: Difference in surface potential experienced by a person’s feet bridging a distance of 1m without contacting any other grounded surface.

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

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

Why Ground? • Lightning Protection Systems (Dissipate Energy) NFPA 780

Why Ground? •

ESD (Electrostatic Discharge)

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Why Ground? Signal Reference Grounding – Noise Control

IEEE Std 1100 (Emerald Book) Powering and Grounding Electronic Equipment

Summary: Why Ground? • • • • •

Human Safety Protect Equipment Lightning Protection Electrostatic Discharge Signal Reference Grounding

Relevant Standards & Codes •

NFPA 70 National Electric Code

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IEEE Std C2 National Electric Safety Code

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NFPA 780 Standard for the Installation of Lightning Protection Systems

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ANSI/J-STD-607-A-2002 Commercial Building Grounding (Earthing) and Bonding Requirements for Telecommunications

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ANSI T1.313-2003 Electrical Protection for Telecommunications Central Offices and Similar Type Facilities

Relevant Standards & Codes • IEEE Std 80 Guide for Safety in AC Substation Grounding • IEEE Std 142 (Green Book): Recommended Practice for Grounding of Industrial and Commercial Power Systems • IEEE Std 602 (White Book): Recommended Practice for Electric Systems in Health Care Facilities • IEEE Std 1100 (Emerald Book): Recommended Practice for Powering and Grounding Electronic Equipment

Equipment Grounding • Purpose: • Provide Personnel Safety

• Means: • Interconnecting all non-current carrying metal components to eliminate potential differences between them. (raceways, cabinets, frames, cable armor, building steel, etc…) • Connecting the equipment grounding conductor to Earth will eliminate potential differences between metal components and Earth.

Equipment Grounding (Personnel Safety)

System Grounding • Purpose: • Provide equipment protection by operating over-current devices to clear fault current and providing a potential reference.

• Means: • An intentional connection to equipment ground from one of the current carrying conductors of an electrical distribution system.

System Grounding

(Equipment Protection)

Effective Grounding Path NEC 250

• The path to ground from circuits, equipment, and metal enclosures for conductors shall (1) be permanent and electrically continuous, (2) have capacity to conduct safely any fault current likely to be imposed on it, and (3) have sufficiently low impedance to limit voltage to ground and to facilitate the operation of the circuit protective devices. • The Earth shall not be used as the sole equipment grounding conductor.

Earth as Equipment Grounding Electrode Unacceptable / Code Violation

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.

Bonding • Interconnect ALL Ground Electrode Systems – – – –

Electrical Grounding System Lightning Grounding System Telecommunications Grounding System Cable Grounding System

• Interconnect ALL conductive objects together both internal and external to the facility • Provides near zero voltage difference during GPR

Bonding • 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 Components: • Conductors • Connectors/Clamps/Lugs • Ground Bars

Bonding Components: • Equipment Ground Plates • Fence Clamps and Gate Jumpers • Equipotential Mesh and Mats

Bonding Components: • Signal Reference Grids

• Coaxial Ground Kits

Ground Electrode System • Soil • Ground Electrode Conductors • Connectors • Electrodes

Soils •

Soil Resistivity - 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.

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Soil PH/type - PH a factor in choosing. Some ground rod types work better in different soils.

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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).

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

Grounding Electrode System NEC 250.52

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, piping, tanks, well casings • Rod & Pipe electrodes • Plates electrodes

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

NEC 250: Resistance of Made Electrode • Single Electrode – R = 25 ohms or less – R > 25 ohm, a second electrode is installed

• Electrodes at least 6 feet apart

Electrode Considerations Cont.. RESISTANCE VS ROD DIAMETER

diameter reduces resistance only 10%.

100%

RESISTANCE, %

Ground Rod Diameter - Doubling

120%

80% 60%

20% 0% 0.500

twice the length. (in good soil).

0.750

0.875

1.000

1.125

1.250

1.375

1.500

RESISTANCE VS ROD DEPTH

600 500

RESISTANCE, OHMS

Ground Rod Spacing - Approximately

0.625

ROD DIAMETER, INCHES

Ground Rod Length - Doubling length reduces resistance 40%, actual reduction depends on soil resistivities encountered in multi-layered soils.

RESISTANCE %

40%

1/2 ROD OHMS 1" ROD OHMS

400 300 200 100 0

5

10 15

20 25 30 35 40

45 50 55 60

ROD DEPTH, FT

65 70 75 80 85

90 95 100

Ground Rod Spacing ¾” x 10’ Rod ρ= 100 Ωm

• One Ground Rod

R = 32 Ω

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Two Ground Rods Spacing = 20’

R = 17.4 Ω

Electrode Considerations 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.

Conductive Backfill

Vertical Application

Horizontal Application

Enhanced Ground Rods Contain electrolytic salts that lower soil resistivity over time

Galvanized Ground Rod (5/8”x10’) NFPA - NEGRP Pawnee Site

10 years in the ground

Copper Clad Steel Ground Rod (5/8”x8’) NFPA - NEGRP Pawnee Site

10 years in the ground

Horizontal Cu-Clad Steel Ground Rod in GEM® NFPA - NEGRP Pawnee Site • •

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This electrode (5/8”x8’) and the connections were completely enclosed in GEM. The 1’ long bent portion of the ground rod was completely corroded away up to the exothermic connection. The remaining ½” long portion of the ground rod and the exothermic weld was connected with less than one strand of wire. The mechanical connection was also very corroded. Only small sections of the rod were found within the GEM due to significant corrosion of the ground rod.

Horizontal Cu-Clad Steel Ground Rod in GEM® NFPA - NEGRP Pawnee Site

Leads

Bent Portion of Rod Missing Only Copper Coating Remains at this Point Remaining Portion of Rod

10 years in the ground

Horizontal Cu-Clad Steel Ground Rod in GEM® NFPA - NEGRP Pawnee Site

Ground Conductor Considerations Sizing - 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 are much stronger than thin flat strap conductor. This should be a consideration when backfilling trenches. Exothermic Connections - Preferred type of connection.

Effect of Inductance For 1 meter of 4/0 conductor: L = 1.02 µH mW Voltage Rise:

R

L

R = 0.16 Conductor Model

dI V = (L × ) + (R × I ) dt

For a strike of Imax = 18,000 A in 1µsec 18000 −3 V = (1.02 × 10 × ) + ( 0 . 16 × 10 × 18000) −6 1 × 10 −6

= 18,360 + 3 Volts/meter of conductor Inductive Term Greatly Dominates Resistive Term

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

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 18,000 Amps.

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

Lightning Surface Arcing

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

Risks Posed from a Direct Strike

Risks Posed from an Indirect Strike

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

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. 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!!!

Characteristics of Electricity 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 - Sufficient enough in size to provide a safe path for fault currents. NEC requirement 25 ohms or less. Examples are metallic water pipes, ground rods and structural steel. * As long as the grounding conductor has sufficient voltage rating, the electrical fault current will stay on the conductor and pass safely to ground.

In other words…..

A lightning ground does not equal a “green wire” ground!!!!!

Lightning • Lightning Travels on the outside surface of a conductor, the so called “skin affect”. Therefore, the larger the surface area of a conductor and not necessarily the cross sectional area, the better path it makes. • Remember, multiple parallel paths are very important. The fewer paths you have the larger the surface area or diameter the conductor needs to have.

Lightning Conductor Types Solid conductors Difficult to work with Concentric strand - Next easiest to work with Rope Lay LP - Easiest to work with Flat strap - least inductance; hardest to work with

Lightning Conductor Routing & Placement General rules of Thumb for Routing: • Maintain downward sloping path to ground (equipotential bonds exception) • Do not run conductors uphill (1/4 rise acceptable to a point) • Maintain at least an 8” radius of bend

Ground From Antenna

• Uphill path to ground • Radius of bend less than 8” • Bonding issue

To Comm. Equipment Drain Pipe

• Water pipe?

Lightning Surface Arcing

Lightning Ground Systems • Provide multiple ground paths for lightning energy • • • •

Radials effectively lower impedance (R & XL) Divert lightning energy away from equipment shelter Maximum Radial Length of 90’ for Lightning Effectiveness Use Radials of different lengths

Computer Installations Grounding System – Four Distinct Subsystems •

NEC Compliant Fault/Personnel Protection Power System Ground (including surge suppression)

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Lightning Protection subsystem (per NFPA 780)

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Telecom, data transmission, and signaling circuit surge protection grounding subsystem.

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Signal Reference Structure

High Frequency Grounding System • Reduces or eliminates high frequency transients by achieving a common ground reference for all equipment within a contiguous area. • Consists of a Signal Reference Grid, low-impedance bonding straps, transient suppression plates and bare copper bonding conductors.

Why a High Frequency Grounding System? •

Most computer systems today run on roughly a 3 volt operating system. A transient of just one volt can cause serious data errors. (Transient OverVoltages).

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It is imperative to tie all equipment together with a low impedance “signal reference” bonding system to keep any voltage differences at a minimum.

Sources of Transient Over-voltages • Lightning Induced Surges • Power Systems Operations • Power System Faults • Reactive Load Switching • Harmonics • Ground Potential Rise

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

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.

SRG Types • Round Conductor – Easier to install when retrofitting an existing raised floor system.

• Flat Strip – Superior system. (Less impedance than round conductors; very important at high frequencies). – Less labor to install.

Grounding & Bonding The Foundation For Effective Electrical Protection January 24, 2006 Tuesday 2:15-3:00 PM Curtis R. Stidham Harger Lightning & Grounding