Information Notice 1998-03, Inadequate Verification of Overcurrent Trip Setpoints in Metal-Clad, Low-Voltage Circuit Breakers

Inadequate Verification of Overcurrent Trip Setpoints in Metal-Clad, Low-Voltage Circuit Breakers

ML031130094
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:	01/21/1998
From:	Roe J Office of Nuclear Reactor Regulation
To:
References
IN-98-003, NUDOCS 9801200035
Download: ML031130094 (8)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001 January21, 1998 NRC INFORMATION NOTICE 98-03: INADEQUATE VERIFICATION OF OVERCURRENT TRIP

SETPOINTS IN METAL-CLAD, LOW-VOLTAGE CIRCUIT

BREAKERS

Addressees

All holders of operating licenses for nuclear power reactors.

Purpose

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

addressees that inadequate verification of overcurrent trip setpoints for metal-clad, low-voltage

circuit breakers may result in the loss of multiple safety functions. 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.

Description of Circumstances

On September 16, 1996, an Asea Brown Boveri (ABB) Company type K600S 480-Vac circuit

breaker prematurely tripped on overcurrent at Centerior Energy Company's Perry Nuclear Power

Plant and de-energized a safety-related motor control center (MCC). The circuit breakers SS-5 solid-state trip device (Power Shield) sensed an overcurrent even though actual MCC current at

the time of the trip was well below the Power Shield's expected trip setpoint. Subsequent

inspection of the circuit breaker revealed that the leads from one of the three current

transformers (CTs) used to sense fault currents were reversed. The reversed CT leads caused

the Power Shield's long time overcurrent trip setpoint to shift downwards from 660 amps of

primary current to approximately 330 amps and caused the breaker to trip inadvertently on

overcurrent.

Discussion

Three-phase solid-state and microprocessor trip units are sensitive to the polarity (or phase) of

the signal from the phase sensor CTs. The signal can be effectively shifted 1800 out of phase if

the CT leads are reversed (or connections between the lower CT terminals and the Power Shield

unit terminals are reversed) or if the CT coil is installed upside down. Normally, the signals from

the three phase sensor CTs are separated by 1200 of phase. However, when a single 1800

phase shifted signal from an incorrectly wired or installed CT is combined in the processor with

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IN 98-03 January 21, 1998 the other two, the sensed peak amplitude of the combined signal can be effectively

doubled. In

at least one instance, as described above, this condition has caused the solid-state

trip unit to

actuate at a primary current level significantly below the expected level.

However, such an unexpected phase shift in CT secondary output current is not

detected

during single-phase primary current calibration and testing. Therefore, additional

inspections or

tests may be necessary to adequately verify proper operation of circuit breakers

equipped with

solid-state trip units. ABB's procedure to check phase sensor polarity for solid-state

trip

devices, Types SS-3, -4, -5, -13, -14, and -15 is as follows:

1. On the static trip (Power Shield) unit terminal strip, connect an analog meter (i.e.,

conventional electromechanical d'Arsonval meter movement), set to measure

current

(100 mA scale) as follows to check the phase sensors for each pole: Negative

test lead

at Terminal 5, Positive test lead successively for each test at Terminals 6, 7, and

8 for

the left, center, and right poles respectively.

2. Connect the NEGATIVE (-) terminal of a 1.5-volt battery to the finger assemblies

of the

breaker's LOWER primary disconnects for each of the above corresponding

poles for

each test. Then while observing the meter, touch a test lead from the POSITIVE

(+)

battery terminal to the corresponding pole's UPPER primary disconnect successively

for

each test.

3. Check that there is a momentary deflection of the meter needle in the positive

direction

as the positive battery lead is touched to each upper primary disconnect. A positive

deflection indicates that the polarity of the signal from the CT as read at the static

trip

unit is correct.

Normally, this test will confirm that both the wiring and the Phase Sensor CT orientation

are

correct. However, it should be noted that if both the wiring and the CT orientation

on the same

pole are reversed, this will also produce a signal of the correct polarity or correct

phase

relationship to the other two poles, but this is not the preferred condition. The

factory uses this

test as a troubleshooting procedure, not a substitute for good visual inspection

of the assembly.

In addition, the test will confirm correct CT winding, internal lead-to-winding connections

cover marking, which would not otherwise be readily apparent if incorrect on new and

CTs from the

CT manufacturing facility.

The circuit breaker that tripped prematurely on overcurrent at Perry was a new

circuit breaker

supplied by ABB Power T&D Co., Inc. On March 26, 1997, ABB notified the

NRC in a 10 CFR

Part 21 report that a potential exists for new low-voltage, K-line circuit breakers, equipped with

the Power Shield solid-state trip device, to trip well below the device's trip setting

if the circuit

breakers overcurrent sensing CTs are incorrectly connected to the Power Shield

trip device.

Although the circuit breaker that tripped prematurely at Perry was a new circuit

breaker, the

I

IN 9803 January 21, 1998 NRC is aware of

potential exists for refurbished breakers to have a similar wiring defect. The

service centers for

instances in which ABB K-line circuit breakers have been sent to ABB

breakers to the licensee with the CTs

refurbishment, and the service center returned the circuit

incorrectly wired.

these six CTs are referred

A typical metal-clad ABB K-line circuit breaker has six CTs. Three of

remaining three CTs are

to as "phase sensors," and are used to detect fault currents. The within the Power

signal

referred to as "power sensors," and are used.to develop a trip reference

on each phase of

Shield trip unit. Two CTs, a phase sensor and a power sensor, are installed

terminal blocks attached

the circuit breaker. The leads from all six CTs are terminated on three

of the circuit breaker. These

to the front of the moulded CT mounting board near the bottom

trip unit by a

three terminal blocks, in turn, are connected to the Power Shield solid-state

ABB Wiring Diagram

multiconductor wiring harness. Attachment I to this notice is a copy of of the CT

709551, Revision 16, for reference. Attachment 2 to this notice is a photograph Typical

the drawing.

mounting board at the bottom of a K-line breaker to aid in interpreting include the

breakers

wiring and assembly errors that have been discovered in ABB K-line circuit

following:

blocks.

• CT leads were terminated on the wrong terminals of the lower terminal

of the lower

• Multiconductor cable conductors were terminated on the wrong terminals

terminal blocks, or on the wrong terminals of the Power Shield Trip Unit.

terminal

• CTs were installed upside down but were correctly terminated on the lower

blocks (same effect as reversing the leads).

lifted to perform

• After the wires on terminals 6, 7, and 8 of the Power Shield trip unit are

11, 12, 13, and 14 are

continuity checks of the phase sensors, or the wires on terminals

lifted to conduct calibration tests, the wires are relanded incorrectly.

equipment

Circuit breaker wiring or assembly errors can be introduced by the original

services, or utilities during

manufacturer, companies performing circuit breaker refurbishment

in service or stored in

testing. Incorrectly wired or assembled circuit breakers may be currently cannot detect

testing

a warehouse for service at a later date. Because single-phase calibration

for incorrectly wired or

the CT wiring or installation errors described herein, the potential exists

but to trip load

assembled circuit breakers to pass normal single-phase calibration testing

safety-related load

currents less than the desired trip setpoint (e.g., on startup of the breaker's

during a design-basis accident) resulting in a loss of safety functions.

IN 98-03 January 21, 1998 This information notice establishes no new NRC requirements; therefore, no specific action or

written response is required by this notice. However, recipients are reminded that they are

required by 10 CFR 50.65 to take industry-wide operating experience (including information

presented in NRC information notices) into consideration, where practical, when setting goals

and performing periodic evaluations. 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.

fa Jack W. Roe, Acting Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical contacts: Donald Kosloff, RIII Stephen Alexander, NRR

440-259-3610 301-415-2995 E-mail: dck@nrc.gov E-mail: kmenrc.gov

Virgil Beaston, NRR David Skeen, NRR

301-415-5774 301-415-1174 E-mail: vlbenrc.gov E-mail: dls@nrc.gov

Attachments:

1. ABB K-Line Power Shield Auxiliary Wiring Diagram 709551, Revision 16

2. Photograph of the CT Mounting Board at the Bottom of a K-Line Breaker

. List of Recently Issued NRC Information Notices

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Attachment 2 IN 98-03 January 21. 1998 PHOTOGRAPH OF CT MOUNTING BOARD AT BOTTOM OF K-LINE BREAKER

4 Attachment 3 IN 98-03 January 21, 1998 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Notice No. Subject Issuance Issued to

98-02 Nuclear Power Plant Cold 1/21/98 All holders of operating licenses

Weather Problems and for nuclear power reactors

Protective Measures

98-01 Thefts of Portable Gauges 1/15/98 All portable gauge licensees

97-91 Recent Failures of Control 12/31/97 All industrial radiography

Cables Used on Amersham licensees

Model 660 Posilock Radiography

Systems

97-90 Use of Nonconservative 12/30/97 All holders of OLs for nuclear

Acceptance Criteria in power reactors except those

Safety-Related Pump who have ceased operations

Surveillance Tests and have certified that fuel has

been permanently removed from

the vessel

97-89 Distribution of Sources and 12/29/97 All sealed source and device

Devices Without Authorization manufacturers and distributors

97-88 Experiences During Recent 12/16/97 All holders of OLs for pressurized- Steam Generator Inspections water reactors except those who

have permanently ceased

operations and have certified that

fuel has been permanently

removed from the reactor

97-87 Second Retrofit to 12/12/97 All industrial radiography

Industrial Nuclear Company licensees

IR 100 Radiography Camera, to Correct Inconsistency in

10 CFR Part 34 Compatibility

OL = Operating License

CP = Construction Permit