Information Notice 1994-80, Inadequate DC Ground Detection in Direct Current Distribution Systems

Inadequate DC Ground Detection in Direct Current Distribution Systems

ML031060424
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant
Issue date:	11/25/1994
From:	Grimes B Office of Nuclear Reactor Regulation
To:
References
IN-94-080, NUDOCS 9411170279
Download: ML031060424 (14)

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UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 November 25, 1994 NRC INFORMATION NOTICE 94-80: INADEQUATE DC GROUND DETECTION IN DIRECT

CURRENT DISTRIBUTION SYSTEMS

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

Purpose

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

notice to alert addressees to the potential for operating with undetectable

grounds in vital direct current (dc) distribution systems due to inadequate

ground-detection equipment or inadequate ground-alarm setpoints or both. 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 do not

constitute NRC requirements; therefore, no specific action or written response

is required.

Background

NRC Information Notice 88-86 and its supplement, "Operating with Multiple

Grounds in Direct Current Distribution Systems," alerted all addressees to

potential problems and failure modes caused by grounds on vital dc

distribution systems, which are normally ungrounded systems. Specifically, several plants had operated for extended periods with grounds on their dc

systems. The information notice also listed licensee event reports (LERs)

related to dc system grounds. Supplement 1 described a specific failure mode

whereby grounds on a dc system could cause equipment to remain energized

during harsh environments instead of assuming the de-energized state needed to

comply with protection system requirements.

Description of Circumstances

Discussions between the staff and several licensees indicated that ground

detectors may not be capable of detecting certain grounds and that ground

detector alarm resistance-to-ground values may be inadequate. In addition, the discussions indicated that some licensee reviews and actions associated

with Information Notice 88-86 and Supplement 1 were limited to specific

examples cited, such as qualification of ASCO solenoids. Other specific

examples follow.

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, A_, IN-94-80

November 25, 1994 Point Beach

During a monthly surveillance test on June 4, 1990, the licensee noted a solid

ground on the dc distribution system. Although the ground detector was

equipped with an alarm, the control room annunciator had not alarmed. The

licensee determined that as a result of its design, the ground detector could

become magnetized when a ground of one polarity occurred. Consequently, if a

solid ground occurred of the opposite polarity, the hysteresis or

magnetization of the ground relay coil would prevent actuation of the ground

detector. The licensee concluded that the ground detector was insensitive and

incapable of detecting even solid grounds. As corrective action, a new, more- sensitive ground detector was installed with an alarm value set at

20,000 ohms.

Palisades

On August 8, 1991, the licensee discovered a solid ground in a dc system. The

associated control room annunciator had not alarmed. The ground-detector- alarm setpoint was based on a 5-milliampere (ma) current; however, the maximum

current that could flow in the circuit was determined to be only 4.2 ma.

Also, if a ground occurred gradually, it would not be detected because of the

mechanical properties of the detector-alarm mechanism. For corrective action, the licensee is monitoring the battery voltage on the positive and negative

buses once each shift.

Fermi

In September 1991, an NRC inspection determined that the licensee 260-volt

battery ground detector could not consistently detect grounds since the

relative brightness of lights was being used subjectively to indicate the

presence of a ground. To improve sensitivity, the licensee replaced the

existing 25-watt light bulbs with 10-watt bulbs. This change raised the

threshold for detecting grounds from about 92 ohms to about 1000 ohms.

Davis-Besse

During a November 15-19, 1993 inspection, the NRC determined that the ground

detector on the new alternate station blackout diesel generator battery was

only capable of detecting solid grounds and that the detector manufacturer

could not quantify the setpoint. In response, the licensee proposed to review

the adequacy of the present ground detector and investigate alternative

methods to detect the presence of grounds in the dc system.

Grand Gulf

On November 1, 1994, a reactor scram occurred due to a ground in the "A"

backup scram valve control circuitry while a half scram was already present

due to surveillance testing of the reactor protection system. The hard

ground, in conjunction with the use of a newly upgraded 125-volt dc ground

A

u_ IN-94-80

November 25, 1994 detector, enabled enough current to flow to energize the backup scram valve's

solenoid and open the backup scram valve. The modified detector circuitry

provided a 1000-ohm path through an annunciating relay (control room alarm) to

ground. This upgraded detector circuitry replaced a circuit which originally

had a 100,000-ohm path to ground. For corrective action the licensee has

removed the hard ground, will disable the 1000-ohm path and has de-energized

the detector circuit used for local ground indication except when periodic

ground checks are performed.

Discussion

In Information Notice 88-86, Supplement 1, the staff described the potential

for ASCO solenoid valves to inadvertently actuate or fail to drop out due to

grounds of approximately 4000 ohms or less at the Summer station. The Summer

licensee has also removed the on-line ground detector because its inherent

1500-ohm resistance-to-ground value was below the plant threshold value.

After Information Notice 88-86 Supplement 1 was issued, the staff met with

Commonwealth Edison on March 1, 1989, to discuss corrective action for the

ground problem at Quad Cities (discussed in Information Notice 88-86). The

Commonwealth Edison review revealed the potential for inadvertent relay

operation at a much higher ground-resistance value for General Electric HFA,

HMA, and HGA 125-volt dc relays. All Commonwealth Edison nuclear plants now

have ground-clearing action with priorities based on two ground-threshold

values, 125,000 and 20,000 ohms. The 125,000-ohm value is based on the

minimum ground current required to keep an HGA relay from dropping out when

its normal power is removed and an additional ground exists between the relay

and its actuating switch contact (see Figure 1). The 20,000-ohm threshold is

based on the minimum current needed to pick up the most sensitive relay

(General Electric HMA) under the same conditions. (For further information on

determining a ground threshold for a dc system, see Annex C of IEEE Std 946-

1992, "IEEE Recommended Practice for the Design of DC Auxiliary Power Systems

for Generating Stations.")

Nuclear power plant dc distribution systems are typically two-wire ungrounded

battery/charger systems equipped with ground-detection/alarm circuitry

including features such as annunciation in the control room, local indication, and recording. Ground detectors are incorporated in the dc systems so that if

a single ground does occur, personnel are aware of the ground and can take

immediate steps to clear the ground fault from the system.

Failure to promptly eliminate a single ground could mask subsequent additional

grounds. Multiple grounds could lead to unpredictable spurious operation of

equipment, inoperable equipment, unanalyzed loads on batteries, or unanalyzed

equipment failure modes that could be expected to occur during harsh

environments attendant to accidents. In addition, installed ground detectors

and portable ground-locating equipment themselves may create a ground on the

dc system and may not maintain a minimum threshold resistance-to-ground value

above which predictable system/component operation can be assured.

'> <IN-94-80

November 25, 1994 This information notice requires no specific action or written response. If

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

one of the technical contacts listed below or the appropriate NRR project

manager.

NBf an K. G imes, Director

Division of Project Support

Office of Nuclear Reactor Regulation

Technical contacts: Roger Mendez, RIII

(708) 829-9745 Fred Burrows, NRR

(301) 504-2901 Attachments:

1. Figure 1: Ground-Threshold Valves

2. List of Recently Issued NRC Information Notices

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Attachment 1 IN 94-80

November 25, 1994 1oJ!M W"" A&=

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Note: A complete circut through the rely (or olnold), ground buf And

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Figure 1

" Iachment 2 IN 94-80

November 25, 1994 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Notice No. Subject Issuance Issued to

94-79 Microbiologically 11/23/94 All holders of OLs or CPs

Influenced Corrosion for nuclear power reactors.

of Emergency Diesel

Generator Service

Water Piping

94-78 Electrical Component 11/21/94 All holders of OLs or CPs

Failure due to Degrada- for nuclear power reactors.

tion of Polyvinyl Chloride

Wire Insulation

94-77 Malfunction in Main Gen- 11/17/94 All holders of OLs or CPs

erator Voltage Regulator for nuclear power reactors.

Causing Overvoltage at

Safety-Related Electrical

Equipment

94-76 Recent Failures of Charging/ 10/26/94 All holders of OLs or CPs

Safety Injection Pump Shafts for pressurized water

reactors.

93-60, Reporting Fuel Cycle 10/20/94 All 10 CFR Part 70

Supp. 1 and Materials Events to fuel cycle licensees.

the NRC Operations Center

94-75 Minimum Temperature 10/14/94 All holders of OLs or CPs

for Criticality pressurized-water reactors

(PWRs).

94-74 Facility Management 10/13/94 All U.S. Nuclear Regulatory

Responsibilities for Commission Medical

Purchased or Contracted Licensees.

Services for Radiation

Therapy Programs

94-73 Clarification of Critical- 10/12/94 All fuel fabrication

ity Reporting Criteria facilities.

94-72 Increased Control Rod 10/05/94 All holders of OLs or CPs

Drop Time from Crud for pressurized water

Buildup reactors.

OL = Operating License

CP = Construction Permit

\K>, VIN-94-80

November 25, 1994 This information notice requires no specific action or written response. If

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

one of the technical contacts listed below or the appropriate NRR project

manager.

Original signed by B.D. Liaw for

Brian K. Grimes, Director

Division of Project Support

Office of Nuclear Reactor Regulation

Technical contacts: Roger Mendez, RIII

(708) 829-9745 Fred Burrows, NRR

(301) 504-2901 Attachments:

1. Figure 1: Ground-Threshold Valves

2. List of Recently Issued NRC Information Notices

DOCUMENT NAME: 94-80.IN

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IN-94-XX

October XX, 1994 This information notice requires no specific action or written response. If

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

one of the technical contacts listed below or the appropriate NRR project

manager.

Brian K. Grimes, Director

Division of Project Support

Office of Nuclear Reactor Regulation

Technical contacts: Roger Mendez, RIII

(708) 829-9745 Fred Burrows, NRR

(301) 504-2901 Attachments:

1. Figure 1: Ground-Threshold Valves

2. List of Recently Issued NRC Information Notices

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IN-94-XX

October XX, 1994 This information notice requires no specific action or written response. If

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

one of the technical contacts listed below or the appropriate NRR project

manager.

Brian K. Grimes, Director

Division of Project Support

Office of Nuclear Reactor Regulation

Technical contacts: Roger Mendez, RIII

(708) 829-9745 Fred Burrows, NRR

(301) 504-2901 Attachments:

1. Figure 1: Ground-Threshold Valves

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October XX, 1994 This information notice requires no specific action or written response. If

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

one of the technical contacts listed below or the appropriate NRR project

manager.

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Roger Mendez, RIII

(708) 829-9745 Fred Burrows, NRR

(301) 504-2901 Attachments:

1. Figure 1: Ground-Threshold Valves

2. List of Recently Issued NRC Information Notices

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IN-94-XX

September XX, 1994 This information notice requires no specific action or written response. If

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

one of the technical contacts listed below or the appropriate NRR project

manager.

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Roger Mendez, RIII

(708) 829-9745 Fred Burrows, NRR

(301) 504-2901 Attachments:

1. Figure 1: Ground-Threhold Valves

2. List of Recently Issued NRC Information Notices

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IN-94-XX

September XX, 1994 This information notice requires no specific action or written response. If

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

one of the technical contacts listed below or the appropriate NRR project

manager.

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Roger Mendez, RIII

(708) 829-9745 Fred Burrows, NRR

(301) 504-2901 Attachment:

List of Recently Issued NRC Information Notices

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IN-94-XX

August XX, 1994 This information notice requires no specific action or written response. If

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

one of the technical contacts listed below or the appropriate NRR project

manager.

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Roger Mendez, RIII

(708) 829-9745 Fred Burrows, NRR

(301) 504-2901 Attachment:

List of Recently Issued NRC Information Notices

DOCUMENT NAME: G:\SHARED/INGR2 *See Previous Concurrence

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IN-94-XX

August XX, 1994 This information notice requires no specific action or written response. If

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

one of the technical contacts listed below or the appropriate NRR project

manager.

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Roger Mendez, RIII

(708) 829-9745 Fred Burrows, NRR

(301) 504-2901 Attachment:

List of Recently Issued NRC Information Notices

DOCUMENT NAME: G:\SHARED/INGR2 To receive a copy of this document, indicate in the box C=Copy wlo attachment/enclosure E=Copy with attachment/encLosure N KNo copy

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OFFICIAL RECORD COPY