- About NRC           - The Commission
      - Statutory Authority
      - Strategic Plan
      - Budget &  Performance
      - Organization & Functions
      - Locations
      - History
      - Values
      - Policymaking
      - Radiation Protection
      - Fire Protection
      - How We Regulate
      - Grants
      - Civil Rights
      -     
    - NuclearReactors           - Operating Reactor Locations
      - Reactor Quick Links
      - Power Reactors
      - Research and Test Reactors
      - Operating Reactors
      - Operator Licensing
      - New Reactors
      - Operator Licensing for New Reactors
      - Reactor Safety Research
      - Advisory Comm (ACRS)
      - Decommissioning of Nuclear Facilities
      -     
    - NuclearMaterials           - Fuel Cycle Facility Locations
      - Uranium Recovery Locations
      - Materials Quick Links
      - Special Nuclear Material
      - Source Material
      - Byproduct Material
      - Med, Academic & Ind Uses
      - Source Materials Facilities
      - Uranium Recovery
      - Fuel Cycle Facilities
      - Materials Transportation
      - Materials Research
      - Advisory Comm (ACMUI)
      - Decommissioning of Nuclear Facilities
      - Materials Environmental Reviews
      -     
    - RadioactiveWaste           - Waste Quick Links
      - Low-Level Waste
      - Waste Incidental to Reprocessing
      - Incidental Waste Criteria
      - High-Level Waste
      - Uranium Mill Tailings
      - LLW Disposal
      - HLW Disposal
      - Spent Fuel Storage
      - Spent Fuel Transport
      - Waste Research
      -     
    - NuclearSecurity           - Security Spotlight
      - NRC Responsibilities
      - Domestic Safeguards
      - Radioactive Material Security
      - Facility Protection
      - Nuclear Material Control
      - International Safeguards
      - Information Security
      - Emergency Preparedness
      - &;   
    - Public Meetings& Involvement           - Public Meeting Schedule
      - Commission	    Schedule
      - Conferences & Symposia
      - Documents for Comment
      - Brochures & Fact Sheets
      - Involving Stakeholders
      - Info Quality Guidelines
      - Rulemaking Process
      - Licensing
      - Enforcement
      - Hearing Process
      - Hearing Applications
      -     

**UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555-0001**

January 21, 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; IN 98-03

**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 breaker's 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 180 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 120 of phase. However, when a single 180 phaseshifted signal from an incorrectly wired or installed CT is combined in the processor with 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:

- 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.
- 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.
- 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 and cover marking, which would not otherwise bereadily apparent if incorrect on new 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 breaker's 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 potential exists for refurbished breakersto have a similar wiring defect. The NRC is aware of instances in which ABB K-line circuit breakers have been sent toABB service centers for refurbishment, and the service center returned the circuit breakers to the licensee with the CTsincorrectly wired.

A typical metal-clad ABB K-line circuit breaker has six CTs. Three of these six CTs are referred to as "phase sensors," andare used to detect fault currents. The remaining three CTs are referred to as "power sensors," and are used to develop a tripreference signal within the Power Shield trip unit. Two CTs, a phase sensor and a power sensor, are installed on each phaseof the circuit breaker. The leads from all six CTs are terminated on three terminal blocks attached to the front of themoulded CT mounting board near the bottom of the circuit breaker. These three terminal blocks, in turn, are connected tothe Power Shield solid-state trip unit by a multiconductor wiring harness. Attachment 1 to this notice is a copy of ABBWiring Diagram 709551, Revision 16, for reference. Attachment 2 to this notice is a photograph of the CT mounting boardat the bottom of a K-line breaker to aid in interpreting the drawing. Typical wiring and assembly errors that have beendiscovered in ABB K-line circuit breakers include the following:

- CT leads were terminated on the wrong terminals of the lower terminal blocks.
- Multiconductor cable conductors were terminated on the wrong terminals of the lower terminal blocks, or on the wrongterminals of the Power Shield Trip Unit.
- CTs were installed upside down but were correctly terminated on the lower terminal blocks (same effect as reversing theleads).
- After the wires on terminals 6, 7, and 8 of the Power Shield trip unit are lifted to perform continuity checks of the phasesensors, or the wires on terminals 11, 12, 13, and 14 are lifted to conduct calibration tests, the wires are relandedincorrectly.

Circuit breaker wiring or assembly errors can be introduced by the original equipment manufacturer, companies performingcircuit breaker refurbishment services, or utilities during testing. Incorrectly wired or assembled circuit breakers may becurrently in service or stored in a warehouse for service at a later date. Because single-phase calibration testing cannotdetect the CT wiring or installation errors described herein, the potential exists for incorrectly wired or assembled circuitbreakers to pass normal single-phase calibration testing but to trip load currents less than the desired trip setpoint (e.g., onstartup of the breaker's safety-related load during a design-basis accident) resulting in a loss of safety functions.

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.

----