Information Notice No. 93-15: Failure to Verify the Continuity of Shunt Trip Attachment Contacts in Manual Safety Injection and Reactor Trip Switches
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
February 18, 1993
NRC INFORMATION NOTICE 93-15: FAILURE TO VERIFY THE CONTINUITY OF SHUNT TRIP
ATTACHMENT CONTACTS IN MANUAL SAFETY INJECTION
AND REACTOR TRIP SWITCHES
All holders of operating licenses or construction permits for nuclear power
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a surveillance test method that is not adequate
to verify the operability of the contacts in manually operated safety
injection and reactor trip switches. 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 May 19, 1992, the Houston Lighting and Power Company, the licensee for the
South Texas Project, issued a notification of an unusual event and began a
technical specification-required shutdown of both units. The licensee had
discovered that it was not conducting adequate testing of the switch contacts
that directly actuate the shunt trip attachment of the reactor trip circuit
breakers. These initiation contacts are actuated by either the manual safety
injection or reactor trip switches and are the means by which operator action
directly actuates the shunt trip, one of two means by which the breaker is
tripped, the other being the undervoltage trip.
The licensee had been performing surveillance tests of the safety injection
switches and the reactor trip switches without opening the associated block
switch (see attached simplified diagram of the manual reactor trip circuitry).
If not opened, the block switch allows the same manual switch contacts that
actuate the undervoltage attachment to indirectly actuate the shunt trip
attachment. Therefore, the manual switch contacts which directly actuate the
shunt trip attachment are not necessary for actuation of the shunt trip and,
therefore, not independently verified as operable. This surveillance
inadequacy has also been identified at the Callaway Plant, the Seabrook
Nuclear Station, and the Wolf Creek Generating Station. The above mentioned
licensees have since properly verified the operability of the manual switch
February 18, 1993
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There are two manually operated safety injection switches and two manually
operated reactor trip switches. Each switch contains circuitry which is
designed to initiate a reactor trip. The attached diagram illustrates the
manner by which a reactor trip breaker is opened when any of the manual
switches is engaged. When a switch is operated, moveable contacts in that
switch change state. For example, when safety injection switch designated SI1
is engaged, the SI1 contacts in the 48 Vdc circuit open and SI1 contacts in
the 125 Vdc circuit close. The opened 48 Vdc SI1 contacts interrupt power to
the undervoltage trip coil (UV) of the undervoltage trip attachment.
Concurrently, the closed SI1 contacts in the 125 Vdc circuit allow the shunt
trip coil (STC) of the shunt trip attachment to become energized.
De-energizing the UV causes the armature of the undervoltage trip attachment
to make contact with and lift the trip lever of the associated reactor trip
breaker. Energizing the STC causes the shunt trip attachment armature to also
make contact with and lift the trip bar of the associated reactor trip
breaker. As a result of either of these actions, the circuit breaker opens to
initiate a reactor trip.
Opening of the SI1 contacts in the 48 Vdc circuit also de-energizes the auto
shunt trip relay (STA) causing the associated STA contacts in the 125 Vdc
circuit to close. The closed STA contacts provide an electrical path by which
the STC is energized, thus actuating the shunt trip attachment. This path is
in parallel to the electrical path provided by the closed 125 Vdc SI1 contacts
when the switches are actuated.
The licensee corrected their procedure as follows:
During continuity tests of the 125 Vdc SI1 contacts, the block switch
that is shown in series with the STA contacts is held in the open
position. This action assures that the voltage measured at the terminal
block of the reactor trip breaker results solely from the closure of the
125 Vdc SI1 contacts and not from the closed STA contacts. If the "block
switch" is not held in the open position, it is not possible to
independently verify the continuity of the 125 Vdc SI1 contacts.
Continuity checks of the 125 Vdc contacts in the other manually operated
switches are made in the same way.
Related Generic Communication
On May 23, 1985, the NRC issued Generic Letter 85-09 which described an
appropriate methodology for verifying the operability of moveable contacts in
the manual switches.
February 18, 1993
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This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.
ORIGINAL SIGNED BY
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contact: N. Fields, NRR
1. Auto/Manual Reactor Trip Circuit Typical Train
2. List of Recently Issued NRC Information Notices
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