Information Notice 1992-29, Potential Breaker Miscoordination Caused by Instantaneous Trip Circuitry

Potential Breaker Miscoordination Caused by Instantaneous Trip Circuitry

ML031200401
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, 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, Crane
Issue date:	04/17/1992
From:	Rossi C Office of Nuclear Reactor Regulation
To:
References
IN-92-029, NUDOCS 9204100124
Download: ML031200401 (3)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C.

20555

April 17, 1992

NRC INFORMATION NOTICE 92-29:

POTENTIAL BREAKER MISCOORDINATION CAUSED BY

INSTANTANEOUS TRIP CIRCUITRY

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 potential breaker miscoordination involving instantane- ous trip circuitry installed by the manufacturer in certain solid state trip

units. It is expected that recipients will review the information for applica- bility 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.

Description of Circumstances

On November 6, 1991, personnel at the Sequoyah Nuclear Plant (Sequoyah) cali- brated a solid state trip unit in a 480 Volt power circuit breaker.

The

licensee had recently revised the procedure used for the calibration and had

increased the test current required for measuring the response time of the

short time delay trip element from 10 to 13.5 times the rating of the current

sensor. When plant personnel applied the test current, the current caused the

breaker to trip instantly rather than after the expected short time delay.

The

instantaneous trip indicated that breaker miscoordination existed between the

480 Volt power circuit breaker and any downstream load breaker.

Discussion

The trip unit was an "Amptector Solid State Trip Device" (Amptector) which was

installed in a type DS power circuit breaker. Both the circuit breaker and the

trip unit were manufactured by the Westinghouse Electric Corporation (Westing- house). The licensee knew that the trip unit was equipped with a long and a

short time delay element.

However, the licensee did not know that the Amptector

trip unit included a circuit which provided an instantaneous trip feature.

After the unexpected instantaneous trip, the licensee reviewed the vendor's

manual, "Westinghouse Instructions for Low-Voltage Power Circuit Breakers Types

DS and DSL." In Section 8.3 of the manual the licensee found that Westinghouse

had included a "making current release" feature in certain models of Amptector

trip units not equipped with an instantaneous trip element. The manual

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IN 92-Z-9 April 17, 1992 referred to this feature as a "Discriminator." The Discriminator is a circuit

in the trip unit which determines at the time of a fault whether or not there

was any current flow in the primary circuit previous to the fault. The trip

unit would trip instantaneously if (1) there was "no measurable current" flow

previous to the fault, indicating that the circuit is just being closed or that

another switching device upstream of the breaker has been closed, and (2) the

current in the primary circuit exceeded approximately 12 times the sensor's

rating. Otherwise, the short time delay element would function to delay the

breaker tripping. Westinghouse stated that, in the event of a fault, this

feature allowed the breaker to provide selectivity and continuity in unfaulted

sections of a system so that service to those sections could be maintained but

if there was no previously operating load in the faulted circuit the trip unit

would function instantaneously to limit damage.

Based on the information found in Section 8.3, the licensee determined that the

instantaneous trip that occurred during calibration of the above circuit

breaker resulted from the Discriminator circuit sensing that there was no

measurable current flow previous to the test and that the test current was

greater than 12 times the sensor's rating.

Previous calibrations of similar

circuit breakers had not resulted in instantaneous trips because the test

current was less than 12 times the sensor's rating.

The licensee noted that, for the Discriminator feature, "no measurable current" is current flow that is

less than approximately 10 percent of the sensor's rating.

The licensee determined that the Discriminator feature could cause breaker

miscoordination in other electrical systems.

This problem could occur in those

systems in which a motor control center (MCC) serves both safety-related and

nonsafety-related loads and current flow in the nonsafety-related portion of

the circuit could be less than 10 percent of the sensor's rating.

In these

circuits, a fault on a nonsafety-related portion of the circuit could cause the

safety-related MCC to become deenergized and result in the loss of the

safety-related loads.

After consulting with Westinghouse, the licensee issued a design change to

disable the Discriminator feature on 41 circuit breakers. Westinghouse stated

that its DS type circuit breakers receive interrupting tests with the short and

long time delay elements only (i.e. no instantaneous element) and disabling the

Discriminator feature would not affect the interrupting ratings of the

breakers. However, Westinghouse also indicated that disabling the Discrimina- tor feature could increase the potential for equipment damage and personnel

injury in downstream portions of the circuit.

Westinghouse informed the licensee that the Discriminator circuit had been a

part of the Amptector trip unit since its first installation in DS type circuit

breakers.

However, Westinghouse did not address the Discriminator feature in

the vendor's manual from the time of the manual's original issue in

January 1971 until Revision C was issued in August 1976.

Licensees may have designed circuits without considering the effect of the

Discriminator feature on breaker coordination because-this feature was not

IN 92-29

April17, 1992

Page 3 of3 addressed in early revisions of the vendor's manual and because testing of the

time delay elements with current less than approximately 12 times the sensor's

rating may not reveal the presence of the Discriminator circuit.

The staff previously discussed the importance of proper breaker coordination in

NRC Information Notice 88-45, "Problems In Protective Relay and Circuit Breaker

Coordination," July 7, 1988.

This information notice requires

you have any questions about the

of the technical contacts listed

Reactor Regulation (NRR) project

no specific action or written response.

If

information in this notice, please contact one

below or the appropriate Office of Nuclear

manager.

charles E. Rossi, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical contacts:

Paul J. Fillion, RII

(404) 331-7288

Joseph Birmingham, NRR

(301) 504-2829 S-u4 v

s---

Attachment:

List of Recently Issued NRC Information Notices

LIST OF RECENTLY

ISSUED

NRC INFORMATION

NOTICES

Information

Date of

Notice No.

Subject

Issuance

Issued to

92-28 Inadequate Fire Suppres-

04/08/92

All holders of OLs or CPs

slon System Testing

for nuclear power reactors.

92-27 Thermally Induced Acceler-

04/03/92

All holders of OLs or CPs

ated Aging and Failure of

for nuclear power reactors.

ITE/GOULD A.C. Relays Used

in Safety-Related Applic-

• tions

92-26 Pressure Locking of Motor-

04/02/92

All holders of OLs or CPs

Operated Flexible Wedge

for nuclear power reactors.

Gate Valves

92-25 Potential Weakness in

03/31/92

All holders of OLs or CPs

Licensee Procedures for A

for nuclear power reactors.

Loss of the Refueling

Cavity Water

92-24 Distributor Modification to

03/30/92

All holders of OLs or CPs

Certain Comnercial-Grade

for nuclear power reactors.

Agastat Electrical Relays

92-23 Results of Validation

Test-

03/27/92

All holders of OLs or CPs

ing of Motor-Operated Valve

for nuclear power reactors

Diagnostic

Equipment

and all vendors of notor- operated valve (NOV) diag- nostic equipment.

92-22 Criminal

Prosecution and

03/24/92

All holders of OLs or CPs

Conviction of Wrongdoing

for nuclear power reactors.

Committed by A Commercial-

Grade Valve Supplier

92-21 Spent Fuel Pool Reactivity

03/24/92

All holders of OLs or CPs

Calculations

for nuclear power reactors.

92-20

Inadequate Local Leak Rate

03/03/92

All holders of OLs or CPs

Testing

for nuclear power reactors.

OL

• Operating License

CP - Construction Permit

Attachment

IN 92-29

April 17, 1992

Page I of I