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LER-2017-001, Unit 3 Reactor Protection System Actuation - Reactor Trip due to Turbine Trip from Generator Lockout
Oconee Nuclear Station Unit 3
Event date: 07-24-2017
Report date: 09-20-2017
Reporting criterion: 10 CFR 50.73(a)(2)(iv)(A), System Actuation
2872017001R00 - NRC Website
LER 17-001-00 for Oconee Nuclear Station, Unit 3, Regarding Reactor Protection System Actuation - Reactor Trip due to Turbine Trip from Generator Lockout
Person / Time
Site: Oconee Nuclear Station Unit 3, Oconee Duke energy icon.png
Issue date: 09/20/2017
From: Dunton C T
Duke Energy Carolinas
Document Control Desk, Office of Nuclear Reactor Regulation
Download: ML17271A134 (5)



The sketch on the right depicts the 525kV power circuit breaker (PCB) arrangement.

The Red Bus[EA] and the Yellow Bus[EA] are commercial, non-safety buses.

Transmission crews are not part of the nuclear plant's staff, but are responsible for performing work in the 525kV switchyard[FK]. The coordination, planning. and execution of Transmission work activities are controlled by several approved processes which establish communication and approval protocols between the Nuclear and Transmission organizations.


On 7/24/17, with ONS Unit 3 operating at 100% power.

Transmission Department Relay personnel in the ONS Switchyard Relay House were performing preventive maintenance on a relay device associated with PCB-57. This is a non-Unit PCB that isolates a commercial transmission line from the Red Bus in the 525kV switchyard.

The maintenance procedure first opens and isolates PCB-57, then triggers the protective relaying while ensuring that the PCB trip function actuates properly. In this case the Transmission crew inadvertently connected the test equipment to the incorrect relay device (several identical relay devices are mounted in one cabinet but actuate separate PCBs). The crew connected test equipment to the adjacent relay device for PCB-58 instead of the relay device for PCB-57. PCB-58 is a Unit breaker that connects the Unit 3 generator output[EL] to the switchyard buses. Activating a test signal on the PCB-58 relay device resulted in Unit 3 generator "lockout- and a loss of electrical load by tripping open PCB-58 and PCB-59. The lockout generates a turbine trip which in turn trips the reactor via the Reactor Protection System (RPS)[JC].

The reactor trip was uncomplicated, with all systems responding normally.


One cause is attributed to a lack of rigor by the Transmission technicians in utilizing appropriate Human Performance tools to ensure their actions were performed on the intended component.

A second cause is attributed to a lack of coordination between the Transmission and Nuclear organizations for implementation of interface processes regarding the risk of work performed in close proximity of the PCB-58 relay device.

Transmission personnel are accountable for the use of human performance tools to ensure component identification is correct and maintained during maintenance. These tools include component markers and physical barriers to ensure maintenance is only performed on the assigned component. These types of tools were not adequately applied during performance of this task.

The interface guidelines call for Transmission to have work plan communications with the appropriate nuclear site groups so a risk assessment for the need of nuclear site oversight can be made. A breakdown in this communication led to a misunderstanding of the risk associated with this work (close proximity to the PCB-58 relay device) and thus the coordination and oversight of this work was inadequate.


This event was entered into the Corrective Action Program (NCR 02138958) which included a cause analysis. The cause analysis corrective actions are summarized as follows:

  • In order to improve the application of human performance rigor and the recognition of operational risks when working in the switchyard relay cabinets; 1. Training, applicable to this event, will be provided to the necessary groups, 2. Cabinet/relay labeling will be enhanced, and 3. Interface guidance applicable to this event will be improved.
  • The site work planning and approval processes will be improved for work within the site switchyards that require coordination with the Transmission department.


Loss of electrical load from 100 percent power is an analyzed event described in the ONS Updated Final Safety Analysis Report, Section 15.8. All onsite safe shutdown equipment performed as required with no complications. Therefore this event did not present a risk to the health and safety of the plant or the public.


Energy Industry Identification System (EIIS) codes are identified in the text as [XX].

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