ML110960443

WB-DC-30-32, Rev. 3, Design Criteria for Grounding - (Unit 1/Unit 2).

ML110960443
Person / Time
Site:	Watts Bar
Issue date:	07/02/2008
From:	Pachigolla R Tennessee Valley Authority
To:	Office of Nuclear Reactor Regulation
References
WB-DC-30-32, Rev 3
Download: ML110960443 (22)

Text

Attachment 5 TVA Letter Dated March 31, 2011 Responses to Licensee Open Items to be Resolved for SER Approval TVA Design Criteria WB-DC-30-32, Revision 3,"Design Criteria For Grounding

-(Unit 1/Unit 2)," Dated July 2, 2008 I I TITLE DESIGN CRITERIA FOR GROUNDING

-(UNIT 1 / UNIT 2)WB-DC-30-32 Rev. 0003 Page 1 of 21 NPG Design Criteria Document Quality Related Effective Date Z Yes 0 No 07-02-2008 Prepared by: R. Pachigolla Reviewed by: R. F. Adkinson 06-25-2008 Date Approved by: M. L. Bali 07-01-2008 Date NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT 1 / UNIT 2) Rev. 0003 Document Page 2 of 21 Revision Log Revision or Affected Change Effective Page Number Date Numbers Description of Revision/Change 0 7-1-88 Initial Issue of WB-DC-30-32, Design Criteria For Grounding.

This revision incorporates applicable commitments and requirements through May 16, 1986. In addition, various editorial and administrative (Revised Reference Number) changes have been made throughout.

1 2-5-93 Incorporated DIM-WB-DC-30-32-1 (B26 890306 016) and general revision.Revised Pages: ii, iii, iv, v, vi, 3, 4, 5, 6, 7, 8, 9, 11, 12,14,15,16,17,19, 23, 24 Deleted Pages: 25 2 11-22-2000 Revised to convert scanned document to a Word document.Deleted Coordination Log, which is not required per NEDP-10.Revised references in Section 5.Renumbered entire document, which changed page numbers on the Table of Contents (page iii-iv).Pages Revised: All Total Pages: 27 (includes pages i-iv and 1-23)3 07-02-2008 All This design criteria was converted from Word 95 to Word 2003 Template using Rev. 2 This DCD has been reviewed and determined to be fully applicable to both Unit 1 and Unit 2.Outstanding WITEL Punchlist items are listed below:-- PL-08-0191, see Section 2.2.1 NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT I / UNIT 2) Rev. 0003 Document Page 3 of 21 Table of Contents 1.0 SCO PE ......................................................................................................................................

4 1 .1 S c o p e .........................................................................................................................................

4 1.2 System and Com ponent Description

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4 2.0 DESIG N REQ UIREM ENTS ...................................................................................................

4 2.1 System Grounding

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4 2.1.1 6900-Volt and 480-Volt Power System s ................................................................

4 2.1.2 Instrum ent Transform ers .......................................................................................

4 2.2 Equipm ent Grounding

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5 2.2.1 Ground G rid System ...............................................................................................

5 2.2.2 Grounding Connection M ethods ............................................................................

9 2.2.3 Equipm ent and Structure Grounding

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10 2.2.4 Cathodic Protection

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20 3.0 TESTING AND INSPECTIO N REQ UIREM ENTS .................................................................

20 3.1 Soil Resistivity

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20 3.2 G round Resistance

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20 4.0 EXCEPTIO NS ..........................................................................................................................

20

5.0 REFERENCES

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20 5.1 TVA Standard Drawing ........................................................................................................

20 5.2 TVA Docum ents .......................................................................................................................

20 5.3 Codes and Standards

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21 NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT I / UNIT 2) Rev. 0003 Document Page 4 of 21 1.0 SCOPE 1.1 Scope This document defines the design requirements for the ground grid system. It also defines the connecting of the electrical system and nonelectrical conductive material which either encloses or is adjacent to energized conductors to ground.The electrical grounding system is a non-safety related system and consequently does not require quality assurance documentation.

1.2 System

and Component Description The purpose of grounding the neutral of the three-phase power system to ground is to stabilize or control the voltages between the phase conductors and ground and to provide a return path for ground current required to operate protective devices during line-to-ground faults.The purpose of the ground grid system is to provide safe "touch" and "step" potentials throughout the plant area and a low resistance to earth for grounding the electrical system.It also provides a means of connecting non-electrical conductive material which either encloses or is adjacent to energized conductors to ground for personnel protection.

2.0 DESIGN

REQUIREMENTS

2.1 System

Grounding 2.1.1 6900-Volt and 480-Volt Power Systems The 6900-volt and 480-volt power systems shall be grounded in accordance with Design Criteria WB-DC-30-28, "Low and Medium Voltage Power Systems" (Reference 5.2.1).The diesel generators are 6900-volt, 3 phase, wye-connected with the neutral grounded through a resistor.

Ground faults will be detected by a voltage relay across the neutral grounding resistor.

Grounds cause an alarm but do not cause any breaker operation.

2.1.2 Instrument

Transformers The following establishes grounding requirements for instrument transformers (for additional criteria on instrument transformers, see Reference 5.2.5): A. All instrument transformer secondaries shall be grounded. (Delta connected instrument transformers are effectively grounded through relay coils).B. Each instrument transformer secondary shall be grounded at one point only.C. The common neutral connection for individual neutrals, or wye points of two or more instrument transformers, shall be grounded at one point only using an insulated conductor.

NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT I / UNIT 2) Rev. 0003 Document Page 5 of 21 2.1.2 Instrument Transformers (continued)

D. When the current transformer secondaries on two or more power circuit breakers are paralleled, a six-pole, PK-12 test block shall be installed in each power circuit breaker mechanism housing. A local ground shall be connected to pole 6 of the test block so that the current transformer will be shorted to local ground when the cover is removed.Phases A, B, C, and the neutral will be connected to poles 1, 2, 3, and 4 respectively.

E. Installed current transformers which are not in service shall have their secondaries shorted and connected to the local grounding system.F. The secondary of instrument transformers located within the main generating plant complex shall be grounded at the first electrical control board reached by the transformer secondary circuits with the following exception:

Potential transformers used for main generator voltage regulation, metering, and relaying shall be grounded locally. This applies if the transformer secondaries are connected to different boards with more than one cable, e.g., one cable connecting the transformer to the exciter board, another cable connecting the transformer to the control board, etc.G. The preferred circuit grounding point of instrument transformers located in the switchyards, transformer yard, and outlying facilities is the station ground bus of the first electrical control board reached by the secondary circuit. If connection to this ground bus is not possible because the circuit does not run to an electrical control board, then a ground connection should be made remote from any likely high-tension fault points such as power circuit breakers, power transformers, or main switchyard support structures.

No circuit will be designed which will require current to flow through the station ground or through a switchyard ground bus with the exception of the synchronizing circuit associated with the instruments located on the synchronizing panels.2.2 Equipment Grounding 2.2.1 Ground Grid System The ground grid system shall consist of a main ground mat and a surface ground mat. The following objectives shall be considered during the design and layout of the ground grid system: a. Protect personnel against dangerous touch and step potentials under all operating conditions.

b. Prevent the development of high transient voltages on electrical equipment casings and frames.c. Provide a path to ground for switching surges, protective equipment and lightning discharges as well as for neutral currents of electrical systems.d. Facilitate relaying in clearing ground faults.e. Stabilize the circuit potential with respect to ground.

NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT 1 / UNIT 2) Rev. 0003 Document Page 6 of 21 2.2.1 Ground Grid System (continued)

Maximum utilization should be made of ground grid layout designs from previous projects having similar features.The maximum allowable voltage rise between the ground grid system and a remote ground shall not exceed 4000 volts during a maximum symmetrical phase-to-ground fault.Allowances shall also be made for asymmetrical decrement and future system growth.Access points to the ground grid system should be installed at strategic locations for future use. Where there is more than one at a particular location, access points should be spaced at least 3 feet apart. Establish an access to bleed and discharge the lines with a permanent connection on the transformers.

To enhance the ground grid system, the extension embedded in the structure foundation should be attached to the reinforcing bar (i.e., rebar) system in each column footing;however, the rebar system shall not be used as a fault current return path. If the Cadweld process of brazing is being specified, the connection should be made near the end of the rebar.NOTE: APPROVAL SHALL BE OBTAINED FROM THE APPROPRIATE CIVIL ENGINEERING ORGANIZATION BEFORE EITHER OF THESE CONNECTING PROCESSES IS PERMITTED.

Ground test stations are not required; ground impedance tests are being made at the transformer neutrals.

All ground cables crossing contraction/expansion joints shall be installed per details as shown on Drawing 15N800-2.

[PL-08-0191]

When entering a structure below finished grade, ground cables shall be sealed to prevent water from seeping between the strands.A minimum clearance of 3 feet shall be provided between main ground mat and surface ground mat cables and buried metallic pipe which cross or run parallel to the cables. When this clearance cannot be maintained, the surface of the cable shall be coated with a bituminous insulating compound, such as Kopper Bitumastic Jet-Set Primer or equivalent, as long as the distance between the cable and the pipe is less than 3 feet.A. Main Ground Mat Design and Layout The first step in the design of a ground mat is the investigation of soil characteristics in the area where the layout is planned. This investigation shall be performed in accordance with Electrical Design Standard DS-E16.1.1, (Reference 5.2.3).The main ground mat shall be located beneath the switchyard and transformer yard areas, with extensions into the main generating plant complex and to most buildings and structures in outlying areas. It shall be laid out in "checkerboard fashion" using bare, stranded, soft drawn copper conductors interconnected by one of the methods described in IEEE Standard 80, "IEEE Guide for Safety in AC Substation Grounding" (Reference 5.3.1).

NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT 1 / UNIT 2) Rev. 0003 Document Page 7 of 21 2.2.1 Ground Grid System (continued)

To ensure that the overall resistance to ground remain relatively constant, these conductors shall be buried in a trench not less than 5 feet below finished grade in close-grained clay or soil. Although use of soil around main ground mat conductors is acceptable, clay is preferred due to its lower resistivity.

The soil or clay shall be free of rock, gravel, and shale.The size of the main ground mat conductors shall be selected based on the following criteria: a. Main ground mat conductors shall be adequately sized to resist fusing under worst case conditions of maximum fault current and maximum fault duration. (Fault current contribution from anticipated switchyard growth shall be included.)

b. Main ground mat conductors shall be sized to have mechanical strength adequate to withstand the physical abuse expected in the area of installation.

c. Conductivity of the conductor selected shall be sufficient to prevent substantial contribution to hazardous potential differences. (This criteria is generally assumed to be satisfied when criteria (a) and (b) have been met.)d. Thermal capabilities of the conductors.

e. The maximum temperature rise for the ground conductors carrying fault current.The main ground mat shall be extended into all structures of the plant. For outlying facilities located away from the main ground mat, a local grounding system may be used as long as it meets the objectives of this design criteria.

Extensions shall be embedded in the outer perimeter foundation of each major structure or grouping of structures.

After entering the concrete foundation, an extension shall be routed around the outer perimeter to a convenient exit point for tying back into the main ground mat. Entry and exit points shall be separated to form at least two independent ground paths back to the main ground mat.Where applicable, the following equipment shall be connected directly to the main ground mat: a. Main generator housing.b. Steel reactor primary containment vessels.c. Power transformer tanks and neutrals.d. Neutral resistors and/or neutral reactors.

NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT 1 / UNIT 2) Rev. 0003 Document Page 8 of 21 2.2.1 Ground Grid System (continued)

e. Power circuit breaker bases.f. Potential transformer cases.g. Capacitor potential devices.h. Lightning arresters.

i. Air terminals (lightning rods).j. Grounding switches.k. SF 6 containment vessels.I. Isolated-phase bus enclosure shorting plates.m. 161-KV potheads.These direct connections to the main ground mat shall be accomplished by means of risers connected to the equipment located in the transformer yards and switchyards.

For equipment located in structures, this requirement shall be satisfied by connecting the equipment to the extensions embedded in the structure's outer perimeter foundation.

To prevent possible damage, conductors associated with the lightning protection system (e.g., down-conductors from air terminals) should not be embedded in concrete structures or foundations; lightning protection system cables should be exposed, routed externally, and connected to extensions placed outside the concrete structure and its foundation.

To prevent the occurrence of side flashes, interconnections shall be made between conductors of the lightning protection system and large metal objects located within 6 feet of the lightning protection system components.

Cables from the main ground mat shall be extended into all structures.

All ground cables entering buildings through concrete below finished grade shall be sealed (or otherwise protected) to prevent water from flowing between the strands.B. Surface Ground Mat Design and Layout The surface ground mat is the main grounding system for equipment and structures within the transformer yards and switchyards.

Consequently, layout of the surface ground mat conductors will be primarily dependent upon the location and arrangement of equipment in these areas. The surface ground mat shall be composed of 4/0 AWG (see Reference 5.3.1, Section 7.0) copper cable, placed approximately 6 inches below the finished surface. Surfacing material in the switchyards and transformer yards is crushed stone usually of sufficient depth to allow the surface ground mat conductors to be laid directly on top of the finished subgrade.

NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT I / UNIT 2) Rev. 0003 Document Page 9 of 21 2.2.1 Ground Grid System (continued)

C. Ground Grid System Design Verification The main ground mat shall be designed to limit the maximum voltage rise to 4000 volts during a maximum symmetrical phase-to-ground fault. Design verification shall be performed in accordance with DS-E16.1.1, Section 4.3 (Reference 5.2.3).D. Step and Touch Potentials The ground grid system shall be designed to protect personnel from hazardous step and touch potentials.

The system shall be designed to limit step and touch potentials to "tolerable" levels. The tolerable step and touch potentials can be calculated by obtaining the tolerable body current as defined in IEEE Standard 80 (Reference 5.3.1).The ground grid periphery should be designed to eliminate the possibility of"touch contacts".

These touch contacts at the ground grid periphery can be eliminated by extending the switchyard/transformer yard surfacing material 2 or 3 feet (see Reference 5.3.1) beyond the fence line.The entire main ground mat shall be designed to ensure against dangerous step and touch potentials.

This shall include any modifications to the main ground mats and any special danger points that may exist in certain areas of the proposed ground mat layout.To determine the adequacy of the ground grid system, a sample calculation in Appendix II of IEEE Standard 80 (Reference 5.3.1) should be used as a guide.E. Testing After the installation of the station ground grid has been completed, a field measurement of the ground resistance should be made. It is desirable, in measuring the resistance of the completed system, to allow some time to elapse before measurements are made, so that the earth around the ground rods will be consolidated (see Section 3.2 for field measurements of ground resistance).

Measurements should be made periodically after the original installation test, so that it can be determined whether the resistance is remaining constant or is increasing.

If later tests show that the resistance is increasing to an undesirable value, steps should be taken to reduce the resistance.

2.2.2 Grounding

Connection Methods Grounding connections and splices shall be made by one or a combination of the following three processes:

a. Compression terminations and splices b. Brazing c. Cadweld process NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT I / UNIT 2) Rev. 0003 Document Page 10 of 21 2.2.2 Grounding Connection Methods (continued)

When performed according to the manufacturer's procedures and recommendations, any one or a combination of the three will provide adequate grounding connections and splices.The connections and splices shall, as a minimum, be visually inspected to verify the integrity of the installation.

2.2.3 Equipment

and Structure Grounding A. Switchyard, Transformer Yard, and Outlying Facilities (Refer to grounding details, reference 5.1.1)1. Main Power Transformers Connection of risers from the surface and main ground mats to the main power transformers shall be made.2. Instrument Transformers The case or tank of all metal encased instrument transformers shall be grounded.3. Power Circuit Breakers Power Circuit Breakers should be grounded.

The ground terminal points of the power circuit breakers shall be connected to risers which are connected to the main ground mat.4. Lightning Arresters Lightning arrester support structures shall be grounded in two locations.

A riser from the main ground mat shall be connected to one corner of the support structure and a riser from the surface ground mat shall be connected to the diagonally opposite corner. Outdoor equipment should be shielded from direct lightning strokes.5. Air Terminals (Lightning Rods)Air terminals located on structures shall be connected to the main ground mat.Materials procured for the lightning protection system shall meet the requirements of NFPA 78 (Reference 5.3.4).6. Railroad Rails Railroad sidings entering the switchyard/transformer yard areas shall be solidly grounded.When danger exists due to a possible difference of potential between rails and surrounding metal structures outside the switchyard/transformer yard areas (off-loading facilities, docks, etc.), grounding of railroad sidings shall be handled in the same manner as for ground mat areas, except insulating joints are not required.

Grounding connections shall be made to the local grounding system.

NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT 1 / UNIT 2) Rev. 0003 Document Page 11 of 21 2.2.3 Equipment and Structure Grounding (continued)

7. Vehicles for Handling Flammable Liquids and Gases To prevent the occurrence of static-caused fires and explosions, vehicles being used to transport flammable liquids and gases shall be connected into the local grounding system before the materials are transferred.

Typical locations where such a grounding system may be required are at hydrogen tanks or trailers, railroad fuel oil transfer pumps, gasoline tanks, etc.For additional information concerning the proper utilization of the bonding cables on the ground cable reel, see TVA Occupational Health and Safety Standard 1910.106, (see reference 5.2.2).8. Grounding Switches For the conventional switchyard layout, main ground mat risers shall be connected directly to the ground terminal point of grounding switches.

In a switchyard layout composed of SF6 equipment, grounding switches are an integral part of the SF 6 system which is totally enclosed and electrically continuous.

Consequently, direct connections between the ground grid system and the grounding switch are not necessary; connections to ground are directly achieved by means of ground grid extensions connected to other equipment in the SF 6 system.9. Hand-Operated Switches Switches within the switchyard/transformer yard areas that require an operator to handle "steel" in operating the switch shall be grounded.

The ground cable connecting the ground plate and the switch handle shall be installed such that it is visible for inspection throughout its length.10. Conduit and Conduit Boxes Metallic conduit systems, whether embedded or exposed, shall be installed in accordance with those positions of the National Electric Code, Article 250 (Reference 5.3.5) which ensure that the systems will be adequately grounded and electrically continuous to function as the equipment grounding conductor.

11. Potheads Pothead support structures shall be connected into the ground grid system in two locations by means of surface ground mat risers.12. Fences All gate posts and every other line post of permanent fences around the switchyard and transformer yard areas shall be connected into the ground grid system.Plant perimeter fences and fences external to the switchyard and transformer yard areas shall be grounded.

NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT 1 / UNIT 2) Rev. 0003 Document Page 12 of 21 2.2.3 Equipment and Structure Grounding (continued)

In order to decrease the length of fence that could become energized under certain conditions and simultaneously decrease the safety hazard to the number of plant personnel and general public, the fence frame and fabric in long runs (e.g., around the plant perimeter) shall be broken at approximately 1000-foot intervals.

This "break" may be accomplished by locating two posts side by side such that a 1 inch open space exists between the first 1000-foot section and the beginning of the second 1000-foot section or part thereof.Any special recommendations by the manufacturer concerning grounding shall be followed in addition to the requirements herein.Additional grounding requirements for fencing are described in General Engineering Specification G-47 (Reference 5.2.4).13. Isolated Phase Bus The isolated phase bus shall be grounded by means of a main ground mat riser.14. Equipment Support Structures Equipment support structures located outside the switchyard and transformer yard areas shall be connected into the local ground system. Surface ground mat risers shall be used for grounding support structures in the switchyard and transformer yard areas.When a support structure consist of four or more "legs", two diagonally opposite legs shall be connected to ground.15. Tanks Tanks shall be grounded.

The cable(s) used for grounding tanks located within the switchyard/transformer yard area shall be connected to the surface ground mat. When corrosion resistance of tanks is necessary; coatings, paintings or cathodic protection shall be used, see Section 2.2.4.16. Sprinkler System Support Posts The surface ground mat extension shall be connected to the sprinkler system support posts to provide protection to equipment (transformer, shunt reactors, etc.) against fire hazards.17. Cabinets and Racks Electrically conductive instrumentation features, such as racks, panels, cabinets and locally mounted instruments having the potential to become a path to ground under fault conditions shall be grounded in accordance with one of the following two methods: a. Attach the cabinets and racks to a grounded structural member (e.g., embedded plates, steel platforms, etc.)

NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT 1 I UNIT 2) Rev. 0003 Document Page 13 of 21 2.2.3 Equipment and Structure Grounding (continued)

b. Cabinets and racks mounted on concrete slabs shall be grounded by connecting into the surface ground mat in the switchyard and transformer yard areas and to the local grounding system in other locations.

18. Cable Trays Cable trays in manholes and cable tunnels shall be connected into the ground grid system. This grounding may be accomplished directly by attaching a ground cable to the cable tray system itself or indirectly by grounding the cable tray supports.

Where the direct grounding method is utilized, a ground post connector shall be used to connect one of the ground cables in the incoming conduit bank to the cable tray system. The same ground connection shall be made on the opposite side of the manhole or cable tunnel.B. Main Generating Plant Complex 1. General All electrically conductive items, including miscellaneous steel, which could become a hazard to operating and maintenance personnel during ground fault conditions shall be grounded.

Grounding requirements for specific pieces of equipment are outlined below.2. Tanks, Motors, Packaged Equipment, Battery Racks, Heat Exchangers Etc.The equipment listed above shall be procured with ground terminals provided by the manufacturer.

3. Cable Trays Cable tray grounding shall be the same as for cable tray systems in cable tunnels defined in Section 2.2.3A.18.

4. Control Boards Control boards are equipped with a manufacturer-supplied ground bus as an integral part of the unit. This ground bus extends the length of the control board and is in the form of either an uninsulated metal bar (copper or aluminum) or an insulated stranded conductor (green in color). The ground bus shall be connected into the ground grid system. Ground cables generally exit through the floor near one or both ends of the control board for the purpose of making this connection.

5. Switchgear Switchgear is equipped with a manufacturer-supplied ground bus (rigid) as an integral part of the equipment.

Ground grid connections and any required circuit ground connections to this bus shall be made in the same manner as for control boards.

NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT 1 / UNIT 2) Rev. 0003 Document Page 14 of 21 2.2.3 Equipment and Structure Grounding (continued)

6. Instrument Transformers Instrument transformers neutrals shall be grounded in accordance with requirements defined in Section 2.1.2.7. Main Generators A minimum of two direct paths to ground shall be made between the main generator housing and the main ground mat.8. Data Logging and Computer Equipment Equipment (such as cabinets and consoles) are to be connected to station ground. Additionally, input signal cables require grounding with method depending on the computer input philosophy and the type of multiplexing used in the applicable system house. In general, input signals will be grounded at the computer end with shields grounded at the computer end.Metal cable trays and conduits shall be bonded together with low resistance joints. They should be connected to station ground.9. Communication Room Equipment The communication room equipment shall be connected into the ground grid system. Equipment racks shall be connected to station ground, and grounding shall be extended to equipment as required.10. Conduit and Conduit Boxes Conduit and conduit boxes should be grounded in the same manner as defined in Section 2.2.3A.10.

11. Cranes Cranes shall be grounded in accordance with National Electrical Code Article 610-61 (Reference 5.3.5).12. Step-Type Voltage Regulators Manufacturers' recommendations shall be followed for each regulator installation.

Specifically, most manufacturers state that a voltage regulator neutral must be grounded if the system neutral is grounded and must be left ungrounded if the system neutral is ungrounded.

NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT I / UNIT 2) Rev. 0003 Document Page 15 of 21 2.2.3 Equipment and Structure Grounding (continued)

13. Cable Shield Grounding a. Power Cable Shield Grounding Cable shields should be solidly grounded at both ends of the cable whenever possible as defined in IEEE Standard 80, Section 16.4 (Reference 5.3.1). If the shield is grounded at only one end, the length of cable run will be limited by the acceptable voltage rise of 25 volts maximum.The following are the maximum shield lengths acceptable for shielded cable with only one ground.One Cable Size Conductor Per Duct (Ft.)1/0 AWG 1250 4/0 AWG 865 350 MCM 710 500 MCM 580 750 MCM 510 1000 MCM 450 2000 MCM 340 b. Instrumentation Cable Shield Grounding Three Cables Per Duct (Ft.)4500 3000 2260 1870 1500 The requirement for instrumentation cable shield grounding and the shielding system in general are addressed in Section 2.2.3C.1.C. Instrumentation and Control (I&C) Grounding System and Multiplexed (Time Shared)Systems Grounding 1. I&C Grounding System The I&C grounding system for the plant shall be designed to minimize noise voltages in the signal cables.The following ground buses shall be provided in each control panel, as applicable, and as specified in the, control panel ordering data: 1. Panel equipment ground bus 2.3.Analog signal system ground bus Digital signal system ground bus NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT I / UNIT 2) Rev. 0003 Document Page 16 of 21 2.2.3 Equipment and Structure Grounding (continued)

All signal circuits shall be electrically insulated from the control panel chassis. All instrument chassis and cases shall be solidly connected to the cabinet frame or to the panel equipment ground bus. The analog and digital signal ground buses, mounted vertically in each rack, shall be insulated from each other and from all other grounds except where they are connected to a single main instrument ground bus.The main instrument ground bus shall be connected to the plant ground grid or mat via a removable link for checking ground bus isolation.

This connection shall be provided by an insulated copper cable.a. Grounding Signal Circuits and Shields The following procedure should be followed for grounding signal circuits and shields: (1) Digital event contact signals shall not be grounded at any point external to the power supply.(2) All shields shall be grounded, and grounding shall be done at the same point as the signal circuit.(3) The shields for all thermowell installed thermocouples, both grounded and ungrounded types, shall be connected to ground at the wellhead (station ground) only and shall not be continuous to the instrument ground.(4) Each Resistance Temperature Detector (RTD) system consisting of one power supply and one or more RTD's shall be grounded at only one point. However, shields shall be continuous to the sensor but not grounded to or at it.(5) In case of common power supply, such as a group of milliampere transmitters, both the low side of the signal pair and the shield shall be tied to ground at the power supply.(6) The RTD's embedded in windings of transformers and rotating machines, shall be grounded at the frame of the respective equipment, except for RTD's at the equipment frame where other devices are used in the circuit to prevent dangerous overvoltages from building up on the RTD circuits.

NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT 1 / UNIT 2) Rev. 0003 Document Page 17 of 21 2.2.3 Equipment and Structure Grounding (continued)

b. Panel Equipment Ground Bus and Grounding A panel equipment ground bus of 1/4" X 1" minimum cross sectional area bare copper bar shall be provided in each equipment rack. It shall be run lengthwise through the rack at a convenient location just above the mounting base. If shipping section breaks are required, means shall be provided so that the ground bus can be made continuous in the field. All adjoining steel panel sections comprising a rack shall be solidly connected to the respective panel equipment ground bus. A solderless clamp terminal shall be provided at each end of the panel equipment ground bus, for connection of a bare copper ground cable from the station ground grid.The exposed front panel of any component chassis of an instrumentation rack shall be solidly grounded to the chassis to which it is attached.

All equipment chassis in an instrumentation rack shall be connected by wiring to the ground bus provided for the rack.The minimum cable size for connecting each panel or cabinet instrumentation copper grounding bus to the station ground grid should be in accordance with the equipment manufacturers recommendations.

c. Analog Signal System Ground Bus and Grounding The analog signal system ground bus shall be a 1/4" X 1" minimum cross sectional area insulated copper bar, and shall run the entire height of each rack within a group of racks comprising a control panel. Within each control panel, the individual analog signal ground buses shall be interconnected with insulated copper cable.The analog signal system ground bus shill be provided with a single point ground for analog signal circuits.

All analog signal circuits shall be electrically insulated from the rack, panel, or console chassis. The instrument common shall be floated within the instrument chassis (not connected to chassis) except where it is desired to ground the instrument signal circuit. The instrument common shall be brought out to the insulated terminal or connector on the instrument chassis for connection to the analog signal ground bus.All control panel analog signal ground buses in a given area (panel) shall be connected at one point (analog signal ground bus in one of the panels)which in turn shall be connected to the main instrument ground bus. All interconnections between analog signal ground buses shall be made with insulated copper cable large enough to provide a low resistance path.The interconnecting cable and analog signal ground buses shall be electrically insulated from all ground points between the instrument ground connection and the main instrument ground bus.

NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT 1 / UNIT 2) Rev. 0003 Document Page 18 of 21 2.2.3 Equipment and Structure Grounding (continued)

d. Digital Signal System Ground Bus and Grounding Because high frequency pulse signals are generated in a digital system circuitry, and the ground wires cannot be considered to be at zero potential until they reach the common ground point, each digital signal system ground circuit shall be wired such that it will have an individual routing to the main instrument ground bus.Each digital system rack shall contain a digital signal system ground bus, of 1/4" X 1" minimum cross sectional area insulated copper bar, which shall run the entire height of each rack within a group of racks comprising a control panel. The ground wires from each digital circuit shall be individually attached to it. The individual digital system ground wired shall be physically separated as much as possible to minimize electromagnetic induction between them. Digital system ground cables shall not be bundled into cable groups. The digital signal ground bus of each rack shall be connected by means of a single insulated copper wire to the nearest common digital signal ground bus.All digital signal system ground buses shall be electrically insulated from racks, floor grids, or any other potential ground point. The common digital signal ground bus in the local area shall be connected to the main instrument ground bus with a large insulated copper cable. Where skin effect phenomenon peculiar to high frequency signals tends to cause most of the current to flow through a thin outer layer of a conductor, thus creating a path of high impedance, the common digital signal ground bus and the main instrument ground bus shall be connected with an insulated conductor having a large surface area.e. Instrument Power System Ground Isolation The power to each instrument in a control panel shall be provided via an isolation transformer to preclude formation of ground loops between electrical power ground points and instrument ground points.These isolation transformers shall utilize a Faraday shield between the primary and secondary windings to preclude electrostatic noise pickup on the power transmission lines from entering the I&C systems. The case of the isolation transformer shall be bonded to the conduit supplying power to the control panel. The Faraday shield shall be connected to the station ground grid via the conduit.f. Instrument and Control Channel Grounding In the current loop systems (i.e., 4 to 20 milliamperes signal transmission circuits), the circuit shall be grounded at the loop power supply common which is the same as the signal circuit common. The I&C circuits shall be grounded at only one point including those I&C channels requiring two grounds as described hereunder:

NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT I / UNIT 2) Rev. 0003 Document Page 19 of 21 2.2.3 Equipment and Structure Grounding (continued)

(1) Channels Requiring One Ground For channels where the sensor is not intentionally grounded or where a physical parameter (such as pressure), drives the transmitter directly, only one ground in the transmitter output circuit (current loop) shall be provided.

This ground shall be connected at the loop power supply common terminal which is the same as the loop common signal line (transmitter common output terminal).

(2) Channel Requiring Two Ground Points The channels like the grounded thermocouples, where the transducer is intentionally connected to station ground, require a transmitter or another in-line device which has good electrical isolation between its input and output circuits.

The channels shall be treated as two isolated circuits, each grounded at a single point, precluding loop currents from circulating in the signal lines between the two grounds.g. Ground Loop Prevention The single-ended equipment input amplifiers and recording equipment are sources of ground loops because they lack ground isolation.

The ground loops shall be prevented from occurring by using the following procedure:

(1) Ground the cable shield at only one point.(2) If a floating transducer or signal-output is used, ground the low side of the signal pair at only one point.(3) If a grounded transducer is being used, the amplifier output must be floating and ground the cable as close to transducer ground as possible.2. Multiplexed (Time Shared) Systems Grounding For an acceptable overall performance of a multiplexed (time shared) system, the signal cable installation system shall be relatively noise-free.

The shields, computer ground bus and signal circuits of multiplexed (time shared) systems shall be connected to the main instrument ground bus in accordance with the computer manufacturer's instructions.

For computer equipment grounding, see Section 2.2.3B.8.3. Use of Insulated and Isolated Ground Bus The Watts Bar Nuclear Plant main complex shall contain an uninsulated common facility ground bus (i.e., station ground grid or mat) and one or more insulated and isolated instrument ground buses.

NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT I / UNIT 2) Rev. 0003 Document Page 20 of 21 2.2.3 Equipment and Structure Grounding (continued)

The isolated instrument ground bus shall be a 1/4" X 1" minimum cross-sectional area insulated copper bar to which instrument and control grounds shall be attached.

This bus shall be connected directly to the station ground grid or mat at one point. A removable link between the instrument ground bus and the station ground mat shall be provided to check instrument ground bus isolation.

2.2.4 Cathodic

Protection Provisions for cathodic protection shall not be provided unless necessary to resolve a specific problem, part of a complete systems design package recommended by the supplier, or directed otherwise on design drawings.

In most applications, adequate protective measures against corrosion are provided by coating, painting, and/or proper selection of construction materials.

3.0 TESTING

AND INSPECTION REQUIREMENTS 3.1 Soil Resistivity Soil resistivity measuring techniques shall be in accordance with IEEE Standard 81,"Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System" (Reference 5.3.2).3.2 Ground Resistance Ground resistance measuring techniques shall be in accordance with IEEE Standard 81 (Reference 5.3.2).4.0 EXCEPTIONS It is recognized that exceptions to these design criteria may be required.

An exception to a specific requirement may be requested and approved as described in NEDP-1, Section 7.0, and Appendix C (Reference 5.2.7).

5.0 REFERENCES

The following references have been identified in or otherwise form the basis of this design criteria.

As applicable, the latest revision of these references in effect at the time of design should be consulted by the engineer or designer when using this criteria.5.1 TVA Standard Drawing 5.1.1 SD-E16 Series -Electrical Standard Drawings -Grounding Details 5.2 TVA Documents 5.2.1 WB-DC-30-28

-Low and Medium Voltage Power Systems NPG Design DESIGN CRITERIA FOR GROUNDING

-WB-DC-30-32 Criteria (UNIT 1 / UNIT 2) Rev. 0003 Document Page 21 of 21 5.2 TVA Documents (continued) 5.2.2 1910.106 -TVA Occupational Health and Safety Standard, Flammable and Combustible Liquids 5.2.3 DS-E16.1.1

-Grounding Main Ground System 5.2.4 General Engineering Specification G-47, Installation, Modification, and Maintenance of Electrical Grounding Systems and Lightning Protection Systems 5.2.5 DS-E1 6.2.3 -Grounding, Instrument Transformers

5.2.6 Deleted

5.2.7 NEDP-1 -Design Basis and Design Input Control, latest revision 5.3 Codes and Standards 5.3.1 IEEE 80 -IEEE Guide for Safety in AC Substation Grounding 5.3.2 IEEE 81 -IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of Ground System 5.3.3 IEEE 142 -IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems 5.3.4 NFPA 78, Lightning Protection Code 5.3.5 National Electrical Code Articles 250 and 610-61