ML092230554 

UNITED STATES 

NUCLEAR REGULATORY COMMISSION 

OFFICE OF NUCLEAR REACTOR REGULATION 

OFFICE OF NEW REACTORS 

WASHINGTON, DC 20555-0001 

September 15, 2009 

NRC INFORMATION NOTICE 2009-16: SPURIOUS RELAY ACTUATIONS RESULT IN 

LOSS OF POWER TO SAFEGUARDS BUSES 

ADDRESSEES

All holders of operating licenses or construction permits for nuclear power reactors under the 

provisions of Title 10 of the Code of Federal Regulations (10 CFR) Part 50, “Domestic Licensing 

of Production and Utilization Facilities,” except those that have permanently ceased operations 

and have certified that fuel has been permanently removed from the reactor vessels. All holders 

of and applicants for combined licenses under the provisions of 10 CFR Part 52, “Licenses, 

Certifications, and Approvals for Nuclear Power Plants.” 

PURPOSE

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

addressees of an event at Point Beach Nuclear Plant (PBNP) in which spurious relay actuation 

resulted in the loss of offsite power to a safety-related 480 Volts alternating current (Vac) 

safeguards bus for more than 6 hours. The NRC expects that recipients to review the 

information for applicability to their facilities and consider actions, as appropriate, to avoid 

similar problems. Suggestions contained in this IN are not NRC requirements; therefore, no 

specific action or written response is required. 

DESCRIPTION OF CIRCUMSTANCES 

On January 15, 2008, PBNP Unit 1 experienced a loss of power to the safety-related train B 

480 Vac safeguards bus which was followed by a lockout and loss of the 13.8 kilovolt alternating 

current (kV)/4.16 kV transformer that supplies offsite power to both safety-related 4.16 kV 

safeguards buses. This electrical isolation resulted in the licensee declaring an Unusual Event, 

and a plant shutdown required by technical specifications (TSs). 

The event was caused by a spurious relay actuation when deterioration of insulation on a 

submerged electrical supply cable from the 13.8 kV/4.16 kV transformer to a non-safety-related 

bus resulted in repeated cable groundings. The repeated groundings caused high-frequency 

current transients to propagate through circuits on the low side of the transformer which 

spuriously actuated a ground fault protection relay (50G) tripping the supply breaker to the 

train B 480 V safeguards bus. Twenty-four seconds later, a lockout and loss of the 

13.8 kV/4.16 kV transformer occurred. 

The licensee concluded in its root cause evaluation that the length of the cables (5,010 feet) 

associated with the supply breaker for the train B 480 Vac safeguards bus was a significant 
IN 2009-16 

Page 2 of 3 

factor in the spurious actuation of the 50G relay. Specifically, the potential for high-frequency 

currents to adversely affect the 50G relay increases as the length of the conductors increase. 

Based on its analysis of cable lengths, numbers of conductors per phase, and pickup setpoint, 

the licensee found that the 50G relay of this breaker was particularly susceptible to this potential 

actuation mechanism. 

In 1995, the licensee for PBNP completed a significant modification to the PBNP 4.16 kV 

electrical distribution system that included the installation of six Asea Brown Boveri (ABB) 50G 

relays. The other 4.16 kV breaker ground fault relays at PBNP are Westinghouse 50G relays 

with an average of 364 feet of supply cables. As such, the licensee concluded that the 

likelihood of developing similar undesired high frequency currents in the Westinghouse 50G 

relays was very low. The Westinghouse 50G relays are also considered immune to highfrequency signal interference due to basic internal design differences. 

The 50G relay that actuated during this event was removed and sent to a laboratory for 

additional testing. This testing confirmed that the relay was particularly responsive to highfrequency primary currents. The relay was found to actuate on a primary current spike that 

lasted less than the time delay setting. Therefore, if the high frequency current pulse was of 

sufficient magnitude, the relay would spuriously actuate before the time delay setpoint. 

As a result of the evaluation, the licensee implemented corrective actions to increase the 

primary current pickup setpoint of the subject 50G relay from 10 Amperes (Amps) to 30 Amps in 

order to provide additional margin to prevent premature tripping. Additionally, the licensee 

changed the time delay setpoint from 0.1 seconds to 0.3 seconds to also decrease the 

probability of spurious relay actuation. The licensee planned to change the setpoint of all six 

ABB 50G relays in the same manner. 

Additional information is available in NRC Special Inspection Report 05000266/2008007 and 

05000301/2008007, dated April 21, 2008, available on the NRC’s public Web site in the 

Agencywide Documents Access and Management System (ADAMS), under Accession 

No. ML081130194. This event is also discussed in PBNP Licensee Event Report 50-266/2008-

001, dated March 14, 2008, which can be found in ADAMS under Accession No. ML080770186. 

BACKGROUND

This IN focuses on the safety issues related to relay setpoints and actuations. The NRC 

addressed safety issues related to submerged electrical cables in Generic Letter 2007-01, 

“Inaccessible or Underground Power Cable Failures that Disable Accident Mitigation Systems or 

Cause Plant Transients,” dated February 7, 2007, available on the NRC’s public Web site at 

http://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/2007/gl200701.pdf. 

Previously issued INs related to relays include the following: 

• NRC IN 2008-12, “Braidwood Unit 1 Reactor Trip due to Offsite Power Fluctuation,” dated 

July 7, 2008 
IN 2009-16 

Page 3 of 3 

• NRC IN 2007-14, “Loss of Offsite Power and Dual-Unit Trip at Catawba Nuclear Generating 

Station,” dated March 30, 2007 

• NRC IN 2005-15, “Three-Unit Trip and Loss of Offsite Power at Palo Verde Nuclear 

Generating Station,” dated June 1, 2005 

DISCUSSION

Licensees rely on electrical circuit breakers and their associated relays operating at the correct 

setpoints to satisfy TSs related to electrical power and the operability of TS-required equipment. 

Licensees must consider the design of circuits when determining setpoints at which the relays 

will actuate their associated circuit breakers to ensure that power is not unnecessarily 

interrupted to safety-related equipment. Because licensees often use relays and circuit 

breakers of the same type and manufacturers in redundant trains of safety systems, certain 

relay and breaker problems raise the possibility of a common-mode failure. 

CONTACT

This IN requires no specific action or written response. Please direct any questions about this 

matter to the technical contacts listed below or to the appropriate Office of Nuclear Reactor 

Regulation (NRR) project manager. 

/RA/ /RA/ 

Glenn Tracy, Director Timothy J. McGinty, Director 

Division of Construction Inspection Division of Policy and Rulemaking 

 and Operational Programs Office of Nuclear Reactor Regulation 

Office of New Reactors 

Technical Contacts: Kenn Miller, NRR/DE Eric Thomas, NRR/DIRS 

 301-415-3152 301-415-6772 

E-mail: kenn.miller@nrc.gov E-mail: eric.thomas@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, 

http://www.nrc.gov, under Electronic Reading Room/Document Collections. 
IN 2009-16 

Page 3 of 3 

• NRC IN 2007-14, “Loss of Offsite Power and Dual-Unit Trip at Catawba Nuclear Generating 

Station,” dated March 30, 2007 

• NRC IN 2005-15, “Three-Unit Trip and Loss of Offsite Power at Palo Verde Nuclear 

Generating Station,” dated June 1, 2005 

DISCUSSION

Licensees rely on electrical circuit breakers and their associated relays operating at the correct 

setpoints to satisfy TSs related to electrical power and the operability of TS-required equipment. 

Licensees must consider the design of circuits when determining setpoints at which the relays 

will actuate their associated circuit breakers to ensure that power is not unnecessarily 

interrupted to safety-related equipment. Because licensees often use relays and circuit 

breakers of the same type and manufacturers in redundant trains of safety systems, certain 

relay and breaker problems raise the possibility of a common mode failure. 

CONTACT

This IN requires no specific action or written response. Please direct any questions about this 

matter to the technical contacts listed below or to the appropriate Office of Nuclear Reactor 

Regulation (NRR) project manager. 

/RA/ /RA/ 

Glenn Tracy, Director Timothy J. McGinty, Director 

Division of Construction Inspection Division of Policy and Rulemaking 

 and Operational Programs Office of Nuclear Reactor Regulation 

Office of New Reactors 

Technical Contacts: Kenn Miller, NRR/DE Eric Thomas, NRR/DIRS 

 301-415-3152 301-415-6772 

E-mail: kenn.miller@nrc.gov E-mail: eric.thomas@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, 

http://www.nrc.gov, under Electronic Reading Room/Document Collections. 

ADAMS Accession Number: ML092230554 ME0904 

OFFICE EEEB:DE EEEB:DE TECH EDITOR BC:EEEB:DE D:DE BC:EEB:DE 

NAME KMiller MMcConnell KAzariah-Kribbs GWilson PHiland RJenkins 

DATE 08/26/09 08/27/09 8/17/09 email 08/27/09 08/27/09 08/28/09 email

OFFICE PGCB:DPR PGCB:DPR BC:PGCB:DPR D:DPR:NRR D:DCIP:NRO 

NAME DBeaulieu CHawes MMurphy TMcGinty GTracy 

DATE 08/28/09 09/02/09 09/03/09 09/03/09 09/15/09 

OFFICIAL RECORD COPY