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99

Information Notice No. 93­99: Undervoltage Relay and Thermal

Overload Setpoint Problems

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

December 21, 1993

NRC INFORMATION NOTICE 93‐99: UNDERVOLTAGE RELAY AND THERMAL OVERLOAD

SETPOINT PROBLEMS

Addressees

All holders of operating licenses or construction permits for nuclear power

plants.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees to continuing discoveries of undervoltage relay and

thermal overload setpoints that are set too low. It is expected that

recipients will review the information for applicability to their facilities

and consider actions, as appropriate, to avoid similar problems. However,

suggestions contained in this information notice are not NRC requirements;

therefore, no specific action or written response is required.

Background

In August 1976, the NRC wrote generic letters to all LWR licensees regarding

the need for licensees to evaluate any generic implications of several events

at the Millstone site involving episodes of sustained degraded grid voltage.

On June 2, 1977, as part of multiplant action MPA‐23, the staff stated its

position that all licensees must have a second level of undervoltage

protection with a time delay. (During degraded grid voltage episodes,

undervoltage relays and thermal overload protective relays protect the safety

equipment.) In a generic letter dated August 8, 1979, the NRC requested all

licensees to determine the capability of the offsite power system to operate

all required loads within their voltage ratings under all conditions within

their design basis. The licensees reviewed their undervoltage relay setpoints

on each of these occasions. In 1981, the NRC issued Branch Technical

Position, Power System Branch‐1, "Adequacy of Station Electric Distribution

System Voltages," which discussed, among other things, the addition of a

second undervoltage relay with an associated time delay. Between 1981 and

1992, the NRC issued two information notices, in which it discussed

undervoltage relay setpoints and other concerns:

1. IN 84‐02, "Operating a Nuclear Power Plant at Voltage

Levels Lower than Analyzed," issued January 10, 1984.
2. IN 91‐29, "Deficiencies Identified During Electrical Distribution

System Functional Inspections," issued April 15, 1991.

9312150073.

IN 93‐99

December 21, 1993

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3. IN 91‐29, Supplement 1, "Deficiencies Identified During

Electrical Distribution System Functional Inspections," issued

September 14, 1992.

In addition, recognizing that certain degraded grid voltage problems resulted

from inadequate control over the design process, on September 12, 1988, the

NRC issued Generic Letter 88‐15, entitled, "Electric Power Systems ‐

Inadequate Control Over Design Processes."

Description of Circumstances

Since 1989, the NRC has performed electrical distribution system functional

inspections at nuclear power plants. The NRC has found design weaknesses in

several electrical distribution system areas including undervoltage relay

setpoints for degraded grid voltage. These are addressed in IN 91‐29 and in

IN 91‐29, Supplement 1. From May 1990 to January 1993, over 30 licensees

wrote to the NRC about inadequate setpoints. Licensees found that the

undervoltage relay setpoints were set so low that safety equipment would not

have been protected if degraded grid voltage had occurred. Three licensees

discovered deficiencies in which the thermal overload protective relay

setpoints were set too low.

Discussion

In the last few years, licensees began extensive efforts to improve the

adequacy and completeness of the set of design bases, design analyses, and

final design output documents that define the design of their facilities. The

licensees began these initiatives primarily because, during inspections such

as safety system functional inspections and safety system outage modification

inspections, the NRC consistently found that some licensees have made plant

modifications which have affected the functionality of safety systems without

making the appropriate setpoint change. The NRC inspection findings prompted

many licensees to review and reconstitute their design bases.

These reviews and the generic communication documents discussed above prompted

licensees to submit technical specification changes for undervoltage relay

setpoints that were discovered to be incorrect after the problem was created

for one of several reasons shown (see Attachment 1). Attachment 1 was

developed from 50.72 reports as a representative summary of the types and

number of problems encountered. Subsequent LERs may provide additional

information. Of the reports where the cause could be determined based on the

event report, design error was the predominant cause of the problem.

Licensees have generally found setpoint problems when the setpoints were

examined as a result of a special inspection or design basis reconstitution..

IN 93‐99

December 21, 1993

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This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.
/S/'D BY BKGRIMES

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contact: C. Morris, NRR

(301) 504‐2778

Attachments:

1. Representative List of Undervoltage

Relay Setpoint Revisions 1988‐1993

2. List of Recently Issued NRC Information Notices

.

Attachment 1

IN 93‐99

December 21, 1993

Page 1 of 2

Representative List of Undervoltage Relay Set Point Revision Notifications and

Technical Specification Change Proposals for 1988‐1993

Event Notification Licensee Event Notification Date Class

Number

11374 Pilgrim 1 January 30, 1988 1

14043 Pilgrim 1 November 18, 1988 5

00000 Pilgrim 1 June 30, 1988 1

14780 Cooper 1 February 17, 1989 5

16540 Crystal River 3 September 8, 1989 1

17121 Robinson 2 November 16, 1989 1

18322 Oconee 1, 2 and 3 April 24, 1990 5

18466 McGuire 1 and 2 May 14, 1990 1

18892 St. Lucie 2 July 14, 1990 2

19023 Haddam Neck 1 August 2, 1990 5

20021 Calvert Cliff 1 and 2 December 6, 1990 1

20435 Ft. Calhoun February 12, 1991 1

20503 Kewaunee 1 February 20, 1991 5

20542 Salem 2 February 27, 1991 2

21691 Dresden 2 and 3 August 23, 1991 5

22281 Dresden 2 November 20, 1991 1

22498 Zion 1 and 2 December 19, 1991 5

22580 Indian Point 3 January 9, 1992 5

22658 Dresden 3 January 22, 1992 5

22847 (TOL) Washington Nuclear 2 February 19, 1992 5

22918 Washington Nuclear 2 March 1, 1992 1

23148 Vermont Yankee April 1, 1992 4

23191 Quad Cities 1 and 2 April 7, 1992 5

23338 LaSalle 1 and 2 April 27, 1992 1

23365 LaSalle 2 April 29, 1992 1

23385 Crystal River 3 May 1, 1992 5

23439 (TOL) Grand Gulf May 11, 1992 3

23452 Comanche Peak 2 May 13, 1992 1

23576 Crystal River 3 June 4, 1992 1

23784 Zion 1 and 2 July 2, 1992 5
23932 Byron 1 and 2 July 24, 1992 1

00000 Callaway July 23, 1992 1

24229 Crystal River 3 September 14, 1992 1

24384 Dresden 2 and 3 October 6, 1992 5

*TOL = Thermal Overload.

Attachment 1

IN 93‐99

December 21, 1993

Page 2 of 2

Event Notification Licensee Event Notification Date Class

Number

00000 Prairie Island 1 & 2 November 6, 1992 1

00000 St. Lucie 1 November 30, 1992 1

24757 Maine Yankee December 15, 1992 5

00000 Diablo Canyon 1 and 2 December 22, 1992 1

24845 Point Beach 1 and 2 January 7, 1993 1

25248 (TOL) Pilgrim March 29, 1993 5

25362 South Texas April 6, 1993 5

Events Notification Number‐00000 Licensees whose undervoltage relay setpoint

notifications to the NRC were reported by LERs, licensee letter, etc., and

event notification numbers if any, were not known.

Class Definitions:

1. Undervoltage Relay setpoints were found to be incorrect because of

design errors.

2. Undervoltage Relay setpoints, as found, were not the same as required by

the technical specification.

3. Safety load circuit breaker thermal overloads or other trip setpoints

were too low because of design errors.

4. Safety load circuit breaker thermal overload, or other trip setpoints,

as found, were not the same as required by the technical specification.

5. Membership in one of the preceding four classes could not be definitely

established because of incomplete information in the event notification.

.

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