The U.S. Nuclear Regulatory Commission (NRC) issues regulatory guides to describe and make available to the public methods that the NRC staff considers acceptable

for use in implementing specific parts of the agency’s regulations, techniques that the staff uses in evaluating specific problems or postulated accidents, and data that the

staff need in reviewing applications for permits and licenses. Regulatory guides are not substitutes for regulations, and compliance with them is not required. Methods and

solutions that differ from those set forth in regulatory guides will be deemed acceptable if they provide a basis for the findings required for the issuance or continuance of

a permit or license by the Commission.

This guide was issued after consideration of comments received from the public. The NRC staff encourages and welcomes comments and suggestions in connection with

improvements to published regulatory guides, as well as items for inclusion in regulatory guides that are currently being developed. The NRC staff will revise existing guides,

as appropriate, to accommodate comments and to reflect new information or experience. Written comments may be submitted to the Rules and Directives Branch,

Office of Administration, U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001.

Regulatory guides are issued in 10 broad divisions: 1, Power Reactors; 2, Research and Test Reactors; 3, Fuels and Materials Facilities; 4, Environmental and Siting;

5, Materials and Plant Protection; 6, Products; 7, Transportation; 8, Occupational Health; 9, Antitrust and Financial Review; and 10, General.

Requests for single copies of draft or active regulatory guides (which may be reproduced) should be made to the U.S. Nuclear Regulatory Commission, Washington, DC

20555, Attention: Reproduction and Distribution Services Section, or by fax to (301) 415-2289; or by email to Distribution@nrc.gov. Electronic copies of this guide and other

recently issued guides are available through the NRC’s public Web site under the Regulatory Guides document collection of the NRC’s Electronic Reading Room

at http://www.nrc.gov/reading-rm/doc-collections/ and through the NRC’s Agencywide Documents Access and Management System (ADAMS)

at http://www.nrc.gov/reading-rm/adams.html, under Accession No. ML052290422.

U.S. NUCLEAR REGULATORY COMMISSION November 2005

REGULATORY GUIDE

OFFICE OF NUCLEAR REGULATORY RESEARCH

REGULATORY GUIDE 1.204

(Draft was issued as DG-1137, dated February 2005)

GUIDELINES FOR LIGHTNING PROTECTION

OF NUCLEAR POWER PLANTS

A. INTRODUCTION

Title 10, Part 50, of the Code of Federal Regulations (10 CFR Part 50), “Domestic Licensing of Production

and Utilization Facilities,” delineates the design- and qualification-related regulations that the U.S. Nuclear Regulatory

Commission (NRC) has established for commercial nuclear power plants. In particular, General Design Criterion 2,

“Design Bases for Protection Against Natural Phenomena,” set forth in Appendix A to 10 CFR Part 50, requires,

in part, that structures, systems, and components (SSCs) that are important to safety in nuclear power plants

must be designed to withstand natural phenomena. The design bases for these SSCs must reflect (1) appropriate

consideration of the most severe of the natural phenomena that have been historically reported for the site

and surrounding area, with sufficient margin for limited data, quantity, and period of time in which the historical data

have been accumulated; (2) appropriate combinations of the effects of normal and accident conditions with the effects

of the natural phenomena; and (3) the importance of the safety functions to be performed.

Fast transient overvoltages generated by lightning discharges can cause equipment damage, system

malfunctions, or power interruptions at nuclear power generating plants if the plants are not adequately protected

against such conditions. However, adequate equipment and system design can greatly reduce or alleviate

the adverse consequences of abnormal voltage disturbances. Confirmatory research in support of this observation

can be found in NUREG/CR-6866, “Technical Basis for Regulatory Guidance on Lightning Protection in Nuclear

Power Plants,” dated July 2005.
RG 1.204, Page 2

This regulatory guide offers guidance for NRC licensees and applicants to use in developing

and implementing practices that the NRC staff finds acceptable for complying with the agency’s

regulatory requirements. Specifically, this guidance applies to the design and installation of lightning

protection systems (LPSs) to ensure that electrical transients resulting from lightning phenomena

do not render safety-related systems inoperable or cause spurious operation of such systems. The scope

of the guidance includes protection of (1) the power plant and relevant ancillary facilities, with the boundary

beginning at the service entrance of buildings; (2) the plant switchyard; (3) the electrical distribution system,

safety-related instrumentation and control (I&C) systems, communications, and personnel within

the power plant; and (4) other important equipment in remote ancillary facilities that could impact safety. 

The scope includes signal lines, communication lines, and power lines, as well as testing and maintenance. 

The scope does not cover testing and design practices that are specifically intended to protect

safety-related I&C systems against the secondary effects of lightning discharges [i.e., low-level power surges

and electromagnetic and radio-frequency interference (EMI/RFI)]. These practices are covered in

Regulatory Guide 1.180, “Guidelines for Evaluating Electromagnetic and Radio-Frequency Interference

in Safety-Related Instrumentation and Control Systems.” Regulatory Guide 1.180, which the NRC

issued in January 2000 and revised in October 2003, addresses design, installation, and testing practices

for dealing with the effects of EMI/RFI and power surges on safety-related I&C systems.

The NRC issues regulatory guides to describe to the public methods that the staff considers

acceptable for use in implementing specific parts of the agency’s regulations, to explain techniques

that the staff uses in evaluating specific problems or postulated accidents, and to provide guidance

to applicants. Regulatory guides are not substitutes for regulations, and compliance with regulatory guides

is not required.

In general, information provided in regulatory guides is reflected in the NRC’s “Standard Review

Plan for the Review of Safety Analysis Reports for Nuclear Power Plants” (NUREG-0800). The NRC’s

Office of Nuclear Reactor Regulation uses the Standard Review Plan as guidance in reviewing applications

to construct and operate nuclear power plants. This regulatory guide corresponds to the revised Chapter 7,

“Instrumentation and Controls,” of the Standard Review Plan.

This regulatory guide contains information collections that are covered by the requirements

of 10 CFR Part 50 which the Office of Management and Budget (OMB) approved under OMB control

number 3150-0011. The NRC may neither conduct nor sponsor, and a person is not required to respond to,

an information collection request or requirement unless the requesting document displays a currently valid

OMB control number.
RG 1.204, Page 3

B. DISCUSSION

Experience shows that lightning can pose operational threats to nuclear power generating plants.

Therefore, protection is essential to avoid malfunctions and upsets that, in turn, can lead to reactor trips. 

Hence, nuclear power plants should have a well-designed and properly installed LPS to safeguard their SSCs

from lightning strikes and the resulting secondary effects. To protect against the effects of lightning strikes,

the LPS should incorporate strike termination devices, discharge down conductors, and a grounding system. 

To protect against the secondary effects of lightning strikes, such as potentially disruptive surges

and EMI/RFI that can propagate to internal structures and cause damage to safety-related systems,

the LPS should also incorporate appropriate individual equipment grounding systems and surge protection

devices (SPDs).

Surge protection measures should include protection of the power plant, ancillary facilities

that could impact safety, the switchyard, the electrical distribution system, safety-related I&C systems,

and communication systems from both direct lightning strikes and the resulting power surges. SPDs should

be applied at the entry and egress points for signal-, communication-, and power-line conductors. 

They should also be applied to any equipment that is thought to be vulnerable to high-energy surges. 

The selection of SPDs typically depends on the location of the devices and the size(s) needed to prevent

the energy from a lightning strike from impinging a facility or piece of equipment.

Details about LPS design guidelines are available in numerous documents, but the best-known

source in use today is the Standard for the Installation of Lightning Protection Systems, which the

National Fire Protection Association (NFPA) promulgates as NFPA 780-2004. This NFPA standard

is the foundational document for use in protecting facilities from direct lightning strikes. Almost all standards

that discuss lightning protection reference NFPA 780. Although NFPA 780 gives well-founded guidance

on lightning protection, it has a disclaimer concerning electric power generation facilities:

Electric generating facilities whose primary purpose is to generate electric power are excluded

from this standard with regard to generation, transmission, and distribution of power. Most

electrical utilities have standards covering the protection of their facilities and equipment.

Installations not directly related to those areas and structures housing such installations can be

protected against lightning by the provisions of this standard.

A second source of well-founded information about LPS installation practices is Installation

Requirements for Lightning Protection Systems, which Underwriters Laboratories (UL) promulgates

as UL 96A-2001.

 UL 96A contains requirements that cover the installation of LPSs on all types of structures

and applies to LPSs that are complete and cover all parts of the structure. However, like NFPA 780,

UL 96A has a disclaimer for electric power facilities:

These requirements do not cover the installation of lightning protection systems for electrical

generating, distribution, or transmission systems.

Because of the exclusion of electric power generation facilities by NFPA 780 and UL 96A,

the NRC staff has chosen to endorse other equivalent standards that directly apply to nuclear power plants. 

However, the staff has used NFPA 780 and UL 96A as a basis to guide the identification of those other

standards. As a result, the NRC staff has selected for endorsement a total of four standards issued by

the Institute of Electrical and Electronics Engineers (IEEE), which taken together, provide comprehensive

lightning protection guidance for nuclear power plants. Specifically, the four standards are IEEE

Std. 665-1995 (reaffirmed 2001), IEEE Guide for Generating Station Grounding, IEEE Std. 666-1991

(reaffirmed 1996), IEEE Design Guide for Electrical Power Service Systems for Generating Stations,

IEEE Std. 1050-1996, IEEE Guide for Instrumentation and Control Equipment Grounding in Generating

Stations, and IEEE Std. C62.23-1995 (reaffirmed 2001), IEEE Application Guide for Surge Protection

of Electric Generating Plants.
RG 1.204, Page 4

IEEE Std. 665-1995 (reaffirmed 2001) describes facility grounding practices and serves as

the primary source of guidance on lightning protection for structures at power generating stations. As such,

this standard identifies the grounding practices that the electric utility industry has generally accepted

as contributing to effective grounding systems for personnel safety and equipment protection in generating

stations. This standard also provides guidance for the design of generating station grounding systems

and grounding practices applied to generating station indoor and outdoor structures and equipment.

IEEE Std. 666-1991 (reaffirmed 1996) describes grounding practices for neutral grounding

and grounding methods for medium-voltage equipment. As such, this standard is a design guide intended

for application to generating station service systems that supply electric power to auxiliary equipment. 

This design guide applies to all types of generating stations that produce electric power, and is

particularly applicable to stations in which the electric power service system is required to perform

continuously. Such a service system consists of a main auxiliary power distribution network that might

supply many subsystems (including direct current systems and Class 1E power systems), much of which

is medium-voltage (2.4–13.8 kV) equipment. This standard addresses recommendations for neutral

grounding, as well as the grounding of generating station auxiliaries. It also covers grounding methods

for both low-voltage (120–480 V) and medium-voltage power service systems. (The low-voltage

grounding methods parallel similar guidance in IEEE Std. 665.) In addition, this standard covers surge

protection of transformers, switchgear, and motors, paralleling similar guidance in IEEE Std. C62.23.

 IEEE Std. 1050-1996 describes design and installation practices regarding grounding methods

for generating station I&C equipment. As such, this standard recommends grounding methods for I&C

equipment to achieve a suitable level of protection for personnel and equipment, as well as suitable

noise immunity for signal-ground references in generating stations. IEEE Std. 1050-1996 is comprehensive,

in that it covers both theoretical and practical aspects of grounding and noise minimization.

IEEE Std. C62.23-1995 (reaffirmed 2001) describes surge protection application practices

applicable to power generating stations. As such, this standard consolidates many electric utility industry

practices, accepted theories, existing standards/guides, definitions, and technical references as they

specifically pertain to surge protection in electric power generating plants. This standard also provides

information on proper surge protection techniques and interference reduction practices for communication,

control, and protection circuits.

IEEE Std. C62.23 also covers the protection of transmission lines from direct lightning strikes

(using overhead ground wires, tower footing resistance, counterpoise wires, and surge arresters

on transmission lines); protection of distribution lines from direct lightning strikes, switching surges,

and ferroresonance; and the selection of arrestors for distribution lines. In addition, this standard

covers the protection of switchyard equipment from direct lightning strikes (using overhead wires or masts),

protection from incoming surges on the transmission line, protection of directly connected switchyard

equipment (using surge arrestors), protection from internally generated switching surges, and protection

of control and communication circuits in the switchyard.

IEEE Std. C62.23 also covers the protection of both indoor and outdoor equipment (including

transformers, motors, switchgear, etc.) from direct lightning strikes, incoming surges, internally generated

surges, and ground potential rises. It also covers the protection of control and communication circuits

and discusses the beneficial effects of shielding, grounding, and cable routing in the power plant buildings. 

In addition, the standard covers the protection of remote ancillary facilities, dealing primarily with

protection from direct lighting strikes and the surges induced on underground cables.
RG 1.204, Page 5

The four primary standards selected for endorsement contain numerous references to other

secondary standards that can clarify their information. As a result, the applicable portions of those

secondary standards are included in the NRC’s endorsement of the four primary standards. Table 1

lists the secondary standards referenced in the recommended primary standards.

Table 1. Secondary Standards Referenced in the Primary Standards Endorsed

Standard Number Standard Title

IEEE Std. 80-2000 IEEE Guide for Safety in AC Substation Grounding (ANSI)

IEEE Std. 81-1983 IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface

Potentials of a Ground System (ANSI)

IEEE Std. 81.2-1991 IEEE Guide for Measurement of Impedance and Safety Characteristics of Large,

Extended or Interconnected Grounding Systems

IEEE Std. 142-1991 IEEE Recommended Practice for Grounding of Industrial and Commercial

Power Systems (IEEE Green Book)

IEEE Std. 367-1996 IEEE Recommended Practice for Determining the Electric Power Station Ground

Potential Rise and Induced Voltage from a Power Fault (ANSI)

IEEE Std. 487-2000 IEEE Recommended Practice for the Protection of Wire-Line Communication

Facilities Serving Electric Supply Locations (ANSI)

IEEE Std. 1100-1999 IEEE Recommended Practice for Powering and Grounding Electronic Equipment

(IEEE Emerald Book) (ANSI)

IEEE Std. C37.101-1993 IEEE Guide for Generator Ground Protection (ANSI)

IEEE Std. C57.13.3-1983

(reaffirmed 1990)

IEEE Guide for the Grounding of Instrument Transformer Secondary Circuits

and Cases (ANSI)

IEEE Std. C62.92.1-2000 IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems,

Part I-Introduction (ANSI)

IEEE Std. C62.92.2-1989

(reaffirmed 2001)

IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems,

Part II-Grounding of Synchronous Generator Systems (ANSI)

IEEE Std. C62.92.3-1993

(reaffirmed 2000)

IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems,

Part III-Generator Auxiliary Systems (ANSI)

IEEE Std. C62.41.1-2002 IEEE Guide on the Surge Environment in Low-Voltage (1000 V and Less)

AC Power Circuits (ANSI)

IEEE Std. C62.41.2-2002 IEEE Recommended Practice on Characterization of Surges in Low-Voltage

(1000 V and Less) AC Power Circuits (ANSI)

IEEE Std. C62.45-2002 IEEE Recommended Practice on Surge Testing for Equipment Connected to

Low-Voltage (1000 V and Less) AC Power Circuits (ANSI)
RG 1.204, Page 6

The only omission in the principal lightning protection standards is concise coverage of testing

and maintenance practices. The IEEE standards selected for endorsement refer to other standards that routinely

cite specific practices, but none of those standards solely provide comprehensive guidance for testing

and maintenance. Annex D, “Inspection and Maintenance of Lightning Protection Systems,” and Section B.4

of Annex B, “Principles of Lightning Protection,” to NFPA 780 do discuss a compilation of reasonable

practices; however, they are not part of the NFPA 780 requirements and are included in the document

for informational purposes only. Hence, although the NRC staff has not selected NFPA 780 for endorsement

in this guide, the staff acknowledges that Annexes B and D to NFPA 780 can serve as good references

for comprehensive testing and maintenance practices.

Section C of this regulatory guide (below) provides specific regulatory positions based on the four

endorsed IEEE standards and suggested testing and maintenance practices. The scope of the NRC staff’s

regulatory positions includes protection of (1) the power plant, plant switchyard, and relevant ancillary

facilities; (2) the electrical distribution system, safety-related I&C systems, communications, and personnel

within the power plant; and (3) other important equipment in remote ancillary facilities that could impact

safety. The scope also includes the protection of signal lines, communication lines, and power lines

interfacing with the power plant systems. In addition, the scope includes the testing and maintenance of LPSs.

The scope does not include the protection of high-voltage power lines. In addition, the scope

does not cover the testing practices intended to protect safety-related I&C systems against the secondary effects

of lightning discharges (i.e., low-level power surges and EMI/RFI), as those practices are covered

in Regulatory Guide 1.180, “Guidelines for Evaluating Electromagnetic and Radio-Frequency Interference

in Safety-Related Instrumentation and Control Systems.”

In general, non-safety-related equipment does not fall under the guidelines presented in this

regulatory guide, but non-safety-related equipment is included if its failure can impact the function

and performance of safety-related equipment. Lightning protection is also recommended for this type

of non-safety-related equipment.

Alternative lightning protection methods — including lightning rods with radioactive tips,

early streamer emission lightning rods, and lightning prevention devices — have been considered

during the periodic review and revision of the endorsed industry standards. However, to date, none of

the endorsed standards have adopted the alternative methods. The perception is that the claimed performance

for such methods has yet to be validated because the methods are proprietary and detailed design information

is not publicly available. Flexibility is built into the regulatory process and, to the extent that alternative

methods are incorporated into the endorsed industry standards in the future, they will be reviewed

and considered for acceptance by the NRC staff.
RG 1.204, Page 7

C. REGULATORY POSITION

1. Lightning Protection System Design and Installation

The NRC staff has selected for endorsement a total of four IEEE standards, including IEEE

Std. 665-1995 (reaffirmed 2001), IEEE Guide for Generating Station Grounding, IEEE Std. 666-1991

(reaffirmed 1996), IEEE Design Guide for Electrical Power Service Systems for Generating Stations,

IEEE Std. 1050-1996, IEEE Guide for Instrumentation and Control Equipment Grounding in Generating

Stations, and IEEE Std. C62.23-1995 (reaffirmed 2001), IEEE Application Guide for Surge Protection

of Electric Generating Plants. The staff finds that the practices in these four standards are acceptable

for the design and installation of LPSs in nuclear power plants.

The selected IEEE standards are endorsed in their entirety by the NRC staff, with one exception

to IEEE Std. 665-1995. Specifically, Section 5.7.4 misquotes subclause 4.2.4 of IEEE Std. 142-1991,

IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems, as stating

“concrete below ground level is a semiconducting medium of about 30 Ω-cm resistivity.” However,

the proper reference is subclause 4.2.3, and the resistivity of concrete under the stated conditions should be

listed as about 3000 Ω-cm.

The endorsed IEEE standards reference numerous other standards that contain complementary

and supplementary information. Thus, as specified in Table 1, the NRC staff also endorses the portions

of the referenced standards that are cited in the endorsed standards; those cited portions are to be used

in a manner that is consistent with the practices in the endorsed standards. Taken together, the combination

of standards provides comprehensive and acceptable guidance for the design and installation of LPSs

at nuclear power generating stations.

2. Testing and Maintenance of Lightning Protection Systems

The following practices form the basis for an adequate and acceptable testing and maintenance

approach to confirm the proper installation of an LPS and ensure its continued ability to provide the level

of protection for which it was designed:

• All new LPSs should be inspected following installation, and the systems should be inspected

on a regular, periodic basis throughout their lifetime. In particular, an LPS should be inspected

whenever any alterations or repairs are made to a protected structure, as well as following any

known lightning transient to the system. An LPS should be visually inspected at least annually. 

In areas where severe climatic changes occur, it is advisable to inspect the LPS semiannually

or following extreme changes in ambient temperature. The NRC staff also recommends

completing an in-depth inspection of the LPS every 3–5 years.

• Testing and maintenance procedures should be established for each LPS. The frequency of testing

and maintenance will depend on weather-related degradation of protective features, frequency

and severity of damage attributable to lightning transients, and required protection level. Also,

an LPS testing and maintenance program should include (1) inspection of all conductors

and system components, (2) tightening of all clamps and splicers, (3) measurement of the earth

grounding resistance, (4) measurement of the resistance of ground terminals, (5) inspection or testing

(or both) of SPDs to assess their effectiveness, (6) periodic testing and maintenance of earth

grounding systems, (7) refastening and tightening of components and conductors as required,

(8) inspection and testing when the LPS has been altered by additions to, or changes in,

the structure, and (9) complete records.
RG 1.204, Page 8

D. IMPLEMENTATION

The purpose of this section is to provide information to applicants and licensees regarding the NRC

staff’s plans for using this regulatory guide. This guide applies to new plants and no backfitting is intended

or approved in connection with its issuance. Except in those cases in which an applicant or licensee proposes

or has previously established an acceptable alternative method for complying with specified portions

of the NRC’s regulations, the methods described in this guide will be used in evaluating (1) submittals

in connection with applications for construction permits, design certifications, operating licenses, and

combined licenses, and (2) submittals from operating reactor licensees who voluntarily propose to initiate

system modifications that have a clear nexus with this guidance. An LPS can be a built-in design feature

associated with a component or system, or a separate dedicated protection system, as discussed in this guide.
1 Copies are available at current rates from the U.S. Government Printing Office, P.O. Box 37082, Washington, DC

20402-9328 [telephone (202) 512-1800], or from the National Technical Information Service (NTIS), 5285 Port Royal

Road, Springfield, Virginia 22161 [http://www.ntis.gov, telephone (703) 487-4650]. Copies are available for

inspection or copying for a fee from the NRC’s Public Document Room (PDR), which is located at 11555 Rockville

Pike, Rockville, Maryland; the PDR’s mailing address is USNRC PDR, Washington, DC 20555-0001. The PDR can

also be reached by telephone at (301) 415-4737 or (800) 397-4205, by fax at (301) 415-3548, and by email

to PDR@nrc.gov. 

2 NFPA documents may be purchased from the National Fire Protection Association, which is located at 1 Batterymarch

Park, PO Box 9101, Quincy, MA 02269-9101.

3 UL documents may be purchased from Underwriters Laboratories, which is located at 320 37th Avenue, St. Charles, IL 

60174.

4 IEEE publications may be purchased from the IEEE Service Center, which is located at 445 Hoes Lane, Piscataway, NJ 

08855.

RG 1.204, Page 9

BIBLIOGRAPHY

NUREG/CR-6866, P.D. Ewing, et al., “Technical Basis for Regulatory Guidance on Lightning Protection

in Nuclear Power Plants,” USNRC, July 2005.1

Regulatory Guide 1.180, “Guidelines for Evaluating Electromagnetic and Radio-Frequency Interference

in Safety-Related Instrumentation and Control Systems,” USNRC, January 2000, revised October 2003.1

NUREG-0800, “Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants,”

Revision 4, USNRC, June 1997.1

NFPA 780-2004, Standard for the Installation of Lightning Protection Systems, National Fire Protection

Association, 2004.2

UL 96A-2001, Installation Requirements for Lightning Protection Systems, Underwriters Laboratories,

2001.3

IEEE Std. 665-1995 (reaffirmed 2001), IEEE Guide for Generating Station Grounding,

Institute of Electrical and Electronics Engineers, 1995.4

IEEE Std. 142-1991, IEEE Recommended Practice for Grounding of Industrial and Commercial

Power Systems, Institute of Electrical and Electronics Engineers, 1991.4

IEEE Std. 666-1991(reaffirmed 1996), IEEE Design Guide for Electrical Power Service Systems

for Generating Stations, Institute of Electrical and Electronics Engineers, 1991.4

IEEE Std. 1050-1996, IEEE Guide for Instrumentation and Control Equipment Grounding

in Generating Stations, Institute of Electrical and Electronics Engineers, 1996.4

IEEE Std. C62.23-1995 (reaffirmed 2001), IEEE Application Guide for Surge Protection

of Electric Generating Plants, Institute of Electrical and Electronics Engineers, 1995.4
RG 1.204, Page 10

REGULATORY ANALYSIS

1. Problem

Title 10, Part 50, of the Code of Federal Regulations (10 CFR Part 50), “Domestic Licensing

of Production and Utilization Facilities,” delineates the design- and qualification-related regulations that

the U.S. Nuclear Regulatory Commission (NRC) has established for commercial nuclear power plants. 

In particular, General Design Criterion 2, “Design Bases for Protection Against Natural Phenomena,”

set forth in Appendix A to 10 CFR Part 50, requires, in part, that structures, systems, and components (SSCs)

that are important to safety in nuclear power plants must be designed to withstand natural phenomena. 

The design bases for these SSCs must reflect (1) appropriate consideration of the most severe of the natural

phenomena that have been historically reported for the site and surrounding area, with sufficient margin

for the limited accuracy, quantity, and period of time in which the historical data have been accumulated;

(2) appropriate combinations of the effects of normal and accident conditions with the effects of the natural

phenomena; and (3) the importance of the safety functions to be performed.

While these regulations address lightning protection for safety-related electrical equipment, they

do not explicitly provide guidance concerning the design and installation of lightning protection systems

(LPSs) to ensure that electrical transients resulting from lightning phenomena do not render safety-related

systems inoperable or cause spurious operation of such systems. The best-known source in use today

is the Standard for the Installation of Lightning Protection Systems, which the National Fire Protection

Association (NFPA) promulgates as NFPA 780-2004. This NFPA standard is the foundational document

for use in protecting facilities from direct lightning strikes. Almost all standards that discuss lightning

protection reference NFPA 780. However, although NFPA 780 gives well-founded guidance on lightning

protection, it contains a disclaimer that electric power generation facilities are excluded from its guidance. 

Hence, there is a need to develop regulatory guidance on lightning protection specifically for nuclear power

generating plants.

2. Alternative Approaches

The NRC staff considered two alternative approaches to address the problem identified above:

(1) Take no action.

(2) Develop guidance with a sound technical basis.

The first alternative, “take no action,” requires no additional cost (compared to current conditions)

for either the staff or applicants, because the current process will not change. However, the process

of establishing lightning protection for safety-related systems may involve significant effort on the part

of applicants and licensees to anticipate the type and level of evidence that is acceptable to the staff

to demonstrate compatibility of equipment in response to these phenomena. In addition, the NRC staff’s

review may require considerable effort to evaluate submitted approaches, and the staff will have to handle

reviews on a case-by-case basis.

The second alternative is to “develop guidance with a sound technical basis.” This alternative

would reduce the level of effort associated with each application for both the staff and applicants

(compared to that associated with Alternative 1). This is because systematic staff review and endorsement

of current standards and up-front resolution of open issues allow a more effective use of resources

than an ad hoc, case-by-case method of handling lightning protection issues. The result of this alternative

would be a complete guide on acceptable lightning protection practices.
RG 1.204, Page 11

3. Values and Impacts

The NRC staff currently performs case-by-case license reviews related to lightning protection

of safety-related systems. Consequently, the staff developed a regulatory guide to endorse acceptable

engineering practices for complying with the NRC’s related regulatory requirements. The value of this

approach is that a regulatory guide offers clear guidance on practices that enhance the safety of the nuclear

power plants.

This guidance is consistent with current established practices applied throughout the commercial

power industry. Therefore, costs associated with implementing this guidance are expected to be minimal. 

This guidance would apply to new nuclear power plants.