United States Nuclear Regulatory Commission - Protecting People and the Environment

ACCESSION #:  9605240007

FOXBORO  The Foxboro Company

Foxboro, MA U.S.A. 02035-2099 Telephone: 508-543-8750 Telex: MCI-6817560

May 1, 1996

U S Nuclear Regulatory Commission
Events Assessment
Mail Stop 0-11A1
1155 Rockville Heights
Rockville, MD 02852

Attention: Dr. C. Vernon Hodge

Subject: Foxboro Model N-2AX+M_N Automatic-Manual Output Station

Dear Dr. Hodge,

Per our telephone conversation on April 30, 1996 with Mr. R. Wilson,
enclosed is a copy of the letter which will be mailed to all nuclear
power plants in the United States and to the appropriate nuclear plants
outside the United States.

Very truly yours,

Truman King
Director, Corporate Quality and
Product Safety

A Siebe Company

May 1, 1996

*City*, *State* *ZIP*

Attention: *Title* *FirstName* *Last Name*

Subject: Foxboro Model N-2AX+M_N Automatic-Manual Output Station


In accordance with 10 CFR Part 21.21 C, this letter is to inform you of a
potential failure of the Foxboro Model N-2AX+M_N Automatic-Manual Output
Station.  Two nuclear plants have reported a problem with the
transferring of the N-2AX+M_N series Automatic-Manual Output Station.
The station consists of two push buttons with LED lights, an
increase/decrease switch and an analog output meter.  The purpose of the
station is to allow operators to place the control loop in the automatic
operating mode by depressing the Auto push button; or to take direct
control of the output with the increase/decrease switch and indicator by
depressing the Manual push button.  The selected mode of control is
indicated by illumination of LEDs on the station.

The two nuclear plants have found that a few stations would not reliably
transfer from Auto to Manual and/or from Manual to Auto when the proper
push button was depressed.  The fact that the station does not switch is
apparent from the illuminated LEDs.  It was noted that external switching
of the control loop from sources other than the N-2AX+M_N series
Automatic-Manual Output Station performed properly.

Design analysis and testing of faulty modules indicate that the cause is
the switch assembly consisting of two Auto/Manual switches, Part Number
N0280RM.  These switches are purchased parts, adapted from the
manufacturer's standard product for use in the N-2AX+M_N series
Automatic-Manual Output Station.  The modification of the manufacturers
standard product is gold plating of the switch used in the station.  This
is expected to improve performance of the switch in the N-2AX+M_N.

The switch assembly is a spring return to normal, momentary push button.
In the non-depressed state, normally closed contacts complete the common
path for the opposite push button to ensure only one state can be
actuated at a time.  These normally closed contacts occasionally fail to
make properly upon spring return, opening the common for the opposite
switch, thus preventing transfer.

When the switch does not transfer on the first attempt, depressing the
active (lighted) push button will often reset the normally closed
contact.  The opposite push button then operates normally.

At these two nuclear plants there are approximately 22 stations installed
of which there are ten to twelve reported failures.  All of these
stations were manufactured in the same time frame.  Foxboro has shipped
1160 N-2AX+M_N series Automatic-Manual Output Stations to nuclear plants
over twenty years.  In addition, thousands of the same stations have been
shipped to other industries.  There have been no other reports of switch
failures to Foxboro.  The repair records of the stations and the orders
for replacement switches have been less than ten per year.  Discussions
with the switch manufacturer indicate that they have had no reports of
similar failures.

Based on the few reported failures, Foxboro cannot determine if there is
a design problem, a batch problem or an aging problem with the switch.
If the N-2AX+M_N series Automatic-Manual Output Station has not
experienced failure in your application, it is our recommendation that no
action be taken at this time.  If the station fails to transfer,
replacing the defective switch with a new switch or a redesigned switch
is recommended.

Recommended action:

Short term: Inform operators that there is some potential that the
Auto/Manual switch on the N-2AX+M_N series Automatic-Manual Output
Station may not switch properly when depressed.  If the station fails to
transfer, the operator should depress the active (lighted) push button
and then depress the opposite push button for the desired mode.  If
repeated cycling does not correct the problem, fully depress the desired
mode push button and "jiggle" the active mode push button.

Medium term: If the station fails to transfer following alternating
operations, the switch should be replaced with the current part number,
N0280RM.  The experience at the two utilities indicates that new switches
perform adequately for at least one year.

Long term: The switch manufacturer has removed this model from sale.
Foxboro is developing a replacement switch assembly that is a different
design and will be nuclear certified.  Failed switches can be replaced
with this new design as they become available.

If you have any questions concerning this issue please contact James
Keiper at (508) 549-6332.

Very truly yours,

Truman King
Director, Corporate Quality and
Product Safety

Page Last Reviewed/Updated Wednesday, February 12, 2020