Power Supply Obsolescence
Replacement/Refurbishment Process Guideline
(Prepared for review by NUOG Members)
Prepared by:
Power Supply Task Team
February 06, 2003 Draft
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Acknowledgements
This guideline has been developed through NUOG members, whose support is
gratefully acknowledged. The following task team members assisted in
development of this guide:
Allen Davidson
Benjie Beck
Craig Irish
Mark C. Simmons, Sr.
Stan Zabaglo
Fred Constance
Vijay Shertukde
Ujjal Mondal
Scientech, Inc
Entergy Nuclear, South
Nuclear Logistics, Inc
Progress Energy
Modumend, Inc.
Constellation Generation
Entergy Nuclear, North
CANDU Owners Group
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Table of Contents
Power Supply Obsolescence Replacement/Refurbishment Process Guideline
1.0
PURPOSE:............................................................................................................. 5
2.0
DEFINITIONS ...................................................................................................... 5
3.0
MAINTAINING CONFIGURATION CONTROL: ......................................... 7
4.0
CRITICAL CHARACTERISTICS AND AGING MECHANISMS: .............. 8
5.0
EQUIPMENT RELIABILITY AND MAINTENANCE ACTIVITIES: ......... 9
6.0
REPLACEMENT ALTERNATIVES:.............................................................. 11
6.1
6.2
6.3
6.4
6.5
REPAIR:.............................................................................................................. 12
REFURBISHMENT: .............................................................................................. 13
SIMPLE EQUIVALENCY EVALUATION: ................................................................ 13
REVERSE ENGINEERING ..................................................................................... 14
DESIGN MODIFICATION: .................................................................................... 14
7.0
DEDICATION OF POWER SUPPLIES AND SUBCOMPONENTS: ......... 15
8.0
SHELF LIFE AND IN STORAGE MAINTENANCE REQUIREMENTS .. 16
9.0
RECOMMENDATIONS.................................................................................... 17
10.0
REFERENCES .................................................................................................... 18
ATTACHMENT 1 .......................................................................................................... 19
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Executive Summary:
Many nuclear plants are experiencing increasing problems associated with
the aging and obsolescence of Power Supplies. In the decades since the
plants’ original design, it is becoming more essential to use engineering
resources to support old and disappearing critical components. Corporate
acquisitions and takeovers, as well as changes in the manufacturing base in
the last twenty years has left the nuclear utilities with substantial and
increasingly more complex operational problems concerning the
replacement of obsolete power supplies.
Power Supply obsolescence is an on-going concern in the power industry.
In the past, many companies built power supplies for GE and Westinghouse
and in turn they were qualified as part of the equipment where they were
installed. Since then, many of these companies do not manufacture these
power supplies or they are no longer in business. Other companies that
manufactured and provided power supplies in the 70’s and 80’s are no
longer in business. These power supplies are aging and require
replacement. Utilities are faced with the problem of replacing or repairing
these obsolete power supplies.
This guideline provides recommendations for addressing power supply
obsolescence issues in the industry today.
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1.0
Purpose:
Industry guidance exists for many areas associated with maintenance and
replacement of power supplies. This document assembles much of this
guidance to provide a basis for the development and implementation of
programs to assist in minimizing the risks associated with aging and
obsolete power supplies. The following areas are addressed through this
guide:
Understanding Power Supply Critical Characteristics and Aging
Mechanisms: This section discusses the function characteristics of Power
Supplies and the subcomponents that are subject to age related failures.
Maintenance Activities to Assure Power Supply Reliability: This
section of the guide addresses proactive methods for the identification of
power supplies in the plant and risk based assessments for the prioritization
of power supply maintenance activities.
Repair and/or Replacement: A discussion is provided on the various
options to address the aging and obsolescence of power supplies. These
options include: Refurbishment; Repair; Reverse Engineering; and/or
Replacement (cases where the power supply is no longer available as
safety related, and an equivalent replacement is available but requires
dedication activities for acceptability). A flowchart is provided to assist in
the decision process.
Shelf Life and In Storage Maintenance: Recommendations are provided
to ensure power supply inventory is maintained and operability is verified.
2.0
Definitions
Power Supply Obsolescence:
A power supply is considered obsolete when it is no longer supported to the
component level by the original manufacturer (OEM), or a third party
supplier in its original form.
Black Box Approach:
Black Box approach constitutes the replacement of subcomponents (such
as capacitors, resistors, transistors, etc.) without in-depth equivalency
evaluation or alternate replacement evaluation. In a Black Box approach,
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the critical characteristics of the power supply (form, fit and function) are
maintained and used as a basis for determining acceptability of the
replaced subcomponents.
Component/Subcomponent:
For the purpose of this guide, component is used to refer to the Power
Supply itself and the term subcomponent, is used to refer the pieces/parts
that make up the component.
Obsolescence Inventory Replacement Database (OIRD):
The NUOG database used for documenting obsolete items and potential
replacement solutions.
LINEAR POWER SUPPLY
This is the simplest form of power supply. The AC voltage (usually 60 hertz)
is stepped-down from the 110 or 220 input line voltage to the desired output
level. The AC Voltage is passed through a bridge rectifier to create the DC
voltage. Filters and regulators are used to achieve the desired
characteristics of the supply line/load regulation, output ripple, and noise.
Linear supplies are very stable. The outputs are easily regulated and
filtered resulting in good regulation and low noise.
The drawback of a linear supply is the physical limitation and weight of the
transformer required to achieve higher power outputs (high weight to power
output ratio). The efficiency of the linear is usually less than 50% requiring
twice the input power to develop the desired output power.
SWITCHING POWER SUPPLY:
The purpose of a switching supply is to generate higher output power in a
smaller lighter physical package. The supply takes the AC line input and
rectifies it to a 300 volt DC level. This voltage is pulse modulated (switched)
at a higher frequency than the 60 hertz line voltage, usually around 50
kilohertz creating a square wave. This signal is stepped-down through a
transformer to the desired output level and rectified again to create the DC
voltage. Filters and regulators are used to achieve the desired
characteristics of the supply line/load regulation, output ripple, and noise.
The high frequency of the square wave requires a lot less iron in the
transformer to develop higher power outputs. Since the transformer is
smaller, multiple windings on the same core are used to develop multiple
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outputs in a smaller physical package (lower weight to power output ratio in
comparison to linear power supply). The switching supply is significantly
more efficient than the linear usually approaching 90% so less input power
is required to develop the desired result.
The switching power supply’s drawback is the output is usually not as tightly
regulated as the linear. Since it is more difficult to filter a square wave than
a sine wave, the ripple and noise on the output voltage is also generally
greater. Tighter regulation and lower ripple/noise characteristics can be
achieved, but it is usually not required or cost effective for most
applications.
3.0
Maintaining Configuration Control:
While approaching solutions for Power Supply obsolescence one must
consider plant requirements for configuration management. “Creep” is
used in this discussion to describe the change of a power supply due to
design changes of the power supply subcomponents over time.
Throughout this guideline, recommendations for repair, refurbishment
and/or reverse engineering of power supplies are based, in some cases, on
“as found” conditions. The recommendation may involve the replacement
of power supply subcomponent, such as a capacitor or board. In some
cases, where unidentified in design documentation, the subcomponent is
identified through visual inspection. The major concern with this approach
revolves around the potential “creep” of the subcomponent specifications.
For example, an evaluation to replace an obsolete capacitor may select a
higher rated capacitor as a replacement and the acceptability of the
replacement is based on the higher rating, thus, de-rating the new
capacitor. While the new capacitor may restore the power supply to
acceptable ratings, future evaluations should consider the original capacitor
and the original specifications instead of the “as found” configuration with
the replacement capacitor.
For these reasons, when replacing power supply subcomponents with
currently available parts, it is essential the change is accurately
documented and the power supply’s configuration is maintained to its initial
specifications.
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4.0
Critical Characteristics and Aging Mechanisms:
The critical characteristics of the power supply for our present purposes are:




Dimensions
Mechanical Characteristics, e.g., seismic qualification
requirements
Electrical Characteristics:
o Output Voltage
o Output Current
o Output Voltage ripple
o Output Voltage regulation
o Response time
o Operating Temperature range
o Over-Voltage and Over-Current capability
o Input Voltage Quality
Environmental Qualification Requirements
When evaluating a power supply’s performance for a specific application,
the a review of the above characteristics to
The life of the power supply is based on the performance of subcomponents
associated with the power supply. Overtime, age sensitive subcomponents
go through changes that result in variations in the subcomponents
performance. These changes are the results of various factors, such as
operating temperatures, ambient temperatures, radiation exposure,
vibration, etc. Also, aging of one subcomponent, could lead to additional
stresses for another subcomponent in the power supply.
Historically, the solution to power supply aging and failures has been to
replace the capacitors without regard for the other associated
subcomponents. Some of the common subcomponents that have age
related failure mechanisms are:

Resistors are used in power supplies to drop the voltage to a level that
can be used by other subcomponents in the power supply. Over time
the resistor can age causing variations to the voltage and current
supplied to other parts of the power supply, resulting in further stresses
to the electronics that use the voltage.
Given the simplicity of the resistors design, age related failures are not
anticipated over the life of most plants.
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
Transformers constructed of windings and a core. The heat generated
from the current flow through the windings, induces age related failures
to the core and/or windings. Transformers have performed well over
the years and there is no data to suggest aging of transformers can be
used as a predictive measure for determining the life power supply.

Semi-conductors (diodes, transistors, etc.) exhibit age related failure
through excessive leakage current, open circuits and/or short circuits.
While the semi-conductors construction is such that the life of the item is
anticipated to exceed the life of the plant, slight variations in the
performance of the semi-conductor due to aging mechanisms will go
unnoticed unless the performance of the power supply is monitored over
time.

Capacitors are more prone to age related failures than any other power
supply subcomponent. As discussed in the shelf life section of this
guide, Aluminum Electrolytic Capacitors (AECs) have a design which is
more sensitive to age related failures. For this reason, the AECs are
generally held as the problem when a power supply fails.

Cable and wiring has been qualified to exceed the life of the plant and
does not pose an age related failure mechanism for power supplies.

Melamine fuses are used in some power supplies. The life of melamine
is generally established at less than 40 years.
Given the preceding, most power supply maintenance activities will
continue to revolve around the life of the Aluminum Electrolytic Capacitors.
When establishing maintenance intervals, shelf life or refurbishment criteria,
each power supply should be evaluated to determine the age sensitive
components. When refurbishing the power supply all age sensitive
materials should be considered.
5.0
Equipment Reliability and Maintenance Activities:
With power plants aging, Power Supply aging and obsolescence concerns
increase every day. The industry continues to struggle to find the right
balance between equipment reliability and the resources to dedicate to
preventative maintenance activities/programs. The following provides a
discussion of issues related to establishing a maintenance program for
power supplies.
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Understanding power supply degradation mechanisms and critical
characteristics provide a better understanding and the basis for selecting a
maintenance option. For the utility engineer, the decision making process
is based on various factors:





Interface, application, and function of the power supply
Availability of the equivalent replacement power supply
Availability of the subcomponents for the power supply that needs to be
replaced
Availability of technical skills and test equipment in-house
Availability of the approved qualified supplier who can perform
repair/refurbish and dedication of the unit for the utility company
Each plant is faced with the task of determining the level of resources to
expend on maintenance programs to ensure equipment reliability and
continued successful operation. The following factors should be
considered when devising a preventative maintenance plan:

Power Supply Identification:
Each plant should determine all power supplies installed in the plant. This
may be done using existing Bill Of Materials data, Component Data and
material issue history. For some plants, these sources may produce a
relevant list, however many power supplies were supplied as a
subcomponent of another piece of equipment, system or skid. System
Engineering involvement is usually required to identify these “hidden” power
supplies.
The list should, as a minimum, identify the power supply nameplate data
and the age of the power supply. If walkdowns are used to obtain this data,
the walkdowns should identify the date codes of any Aluminum Electrolytic
Capacitor contained on the power supply.

Criticality of the Power Supply:
Once the power supplies have been identified, the program should then
review the applications to establish how critical the power supplies are to
the safe operation of the plant. The program should categorize the power
supplies, based on the criticality. An example of categories may use
maintenance rule classifications:
o Not in scope of the Maintenance Rule
o Low Risk Significant
o High Risk Significant
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
Ensuring operability:
Once established, the criticality of the power supply should dictate the level
of preventative maintenance and inventory levels to ensure operating
spares are available and operability of the installed unit is monitored.
An example for High Risk Significant Power Supplies might include:

Monitoring power supply performance at specified intervals in an
attempt to predict age related failures. NOTE: Ref. 1 suggests that
these monitoring activities may not produce the desired affect and, in
some cases, have been determined to be ineffective.

Establish a replacement frequency for the power supplies based on the
risk associated with failure of the power supply. This would involve:
o Maintain sufficient spares in stock and ensure operability through
an aggressive In Storage Maintenance program.
o Start with a conservative replacement interval
o Monitoring the “as found” condition of the power supply removed
from the plant.
o Refurbish the power supply removed from the plant for
re-installation at the end of the next interval.
o Once “as found” data is established, it may be determined that
the replacement interval can be increased or decreased.
6.0
Replacement Alternatives:
The primary goal of this guidance is to provide potential solutions that
address the obsolescence of the power supply. Since obsolescence is only
an issue when a replacement is desired, this section of the guide discusses
the following alternatives for replacing a power supply:
1) Repair – The first line of defense against replacing a power supply.
2) Refurbishment – Replace the age sensitive subcomponents that are
close to the end of their useful life, potentially degrading the power
supply performance.
3) Simple Equivalency Evaluation – Locate a similar power supply that
meets the original design criteria and perform an evaluation to allow use
of the replacement. This may include dedication, as well.
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4) Reverse Engineer – Re-engineer the power supply based on the
information available for the existing power supply.
5) Design Modification – Redesign the system to use a power supply with
different performance characteristics or remove the need for the power
supply.
Determining the method to be used is contingent upon several factors
including: budget, time constraints, and the criticality of the need. These
options should be considered in conjunction with other factors discussed in
this guide to develop a long-term strategy for power supply obsolescence.
6.1
Repair:
Repair is generally considered a maintenance activity but should be
considered as an option in programs designed to manage power
supply obsolescence. Power supplies will fail regardless of our best
preventative maintenance efforts. In many cases, depending on the
resources available at a specific plant, the most economical option
would normally be to replace the failed unit. The following are
several ways the repair of a power supply may be factored into a
proactive obsolescence program:

A program may determine a power supply used in the plant is not
critical to plant operations and established no a “run to failure”
criteria for maintenance of the item. Upon failure, repair of the
unit should be the first option.

Another power supply, may fail prior to its established
preventative maintenance interval. In cases where a
replacement is unavailable, repair should be the first
consideration.

Prior to dispositioning a failed power supply that has been
replaced, it should be evaluated to determine whether the unit is
obsolete. If obsolete, the unit should be returned to inventory for
Procurement Engineering (PE) disposition. The PE disposition
may be to simply repair the unit and return it to inventory.
The following are other factors that should be considered when planning
whether to repair a power supply:

Ensure your facility has the available skills and test equipment to
properly troubleshoot and repair the problem.
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6.2

Qualified replacement parts may be unavailable. The cost and
effort required to locate and dedicate these parts may outweigh
the costs of other options presented within this guide.

Consider third hand supplier capabilities as a potential resource
for the repair.

Ensure the quality level of the repair program meets the quality
program of the original power supply manufacturer.
Refurbishment:
Refurbishment involves replacing the age sensitive subcomponents
of the power supply that have been determined to be near their end
of life. This, in many cases, can be the most economical path for
replacing an obsolete power supply, provided an existing unit is
available for refurbishment. The consideration to refurbish is based
on the availability of the power supplies subcomponents (or
equivalents) and the condition of the unit.
When refurbishing a power supply by replacing internal
subcomponents (such as a capacitors, resistors, etc.) that are
equivalent but not identical because identical replacements are not
available, an evaluation is necessary to, as a minimum, document
the any changes to the power supply configuration as a result of the
refurbishment. Operability must be established to the power
supply’s original specifications.
Industry guidance suggests the selection of “Premium” or Mil Spec
parts should be used to increase the reliability and life of the
refurbished unit.
When replacing subcomponents on a safety-related power supplies,
dedication activities must be established to provide reasonable
assurance the power supply will perform its safety related design
function. Dedication activities may be performed for each
subcomponent, however the dedication should include verification of
operability of the refurbished power supply.
6.3
Simple Equivalency Evaluation:
In many cases, replacement power supplies, built to today’s
technology, are available that can result in a simple equivalency
evaluation. In searching for an “equivalent” replacement, the
engineer should, at a minimum:
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
Search OIRD to determine if another plant has already
established an equivalent or a third party supplier has established
an equivalent unit.

Contact the OEM and OES to determine whether they have a
recommended replacement
When evaluating whether this option is the most economical, the
engineer should screen the replacement’s specifications against the
application’s critical characteristics to ensure all costs are
considered (i.e. dedication costs, mounting configuration change
costs, document updates, etc.)
This can be the most economical choice for a utility when it is
determined a replacement power supply is available that only
requires a “simple” equivalency.
6.4
Reverse Engineering
If the specification for the Power Supply being replaced is not
available, it will be necessary to develop the specifications and meet
the critical characteristics of the system where the power supply is to
be installed.
Reverse engineering of a power supply unit requires lot of
information about the existing units. If a power supply is to be
re-engineered, one needs to know the manufacturer’s data on
performance (beyond just the voltage and current out), interfaces
and interaction, function, schematics, and parts list. Reverse
Engineering can become labor intensive to do the prototyping and
circuit analysis on the actual hardware. It can be both costly and
time consuming, and may require several iterations to provide a unit
that performs to specifications.
For these reasons, Reverse Engineering is usually regarded as a
last resort.
6.5
Design Modification:
The Procurement Engineer (PE) is generally responsible for
providing a success path when a replacement power supply is
needed. In cases where the replacement is required for a minimal
number of applications, the PE should consult the system
engineer(s) for the application(s) where the power supply is needed.
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It may be found that the application(s) that requires the power supply
is obsolete. In these cases, it may be more economical to replace
the portion of the system that requires the power supply than
performing a modification to replace the power supply.
In cases where the power supply replacement is required and simple
solution is unavailable, a design modification may be the last resort.
In some cases, however, these modifications may be cheaper in the
long run for the utility. The costs of maintaining the current power
supply configuration verses the cost of changing he configuration
should be considered prior to making this choice.
7.0
Dedication of Power Supplies and subcomponents:
Upon initial meetings of the team preparing this guideline, it was felt that
guidance should be provided that established the most efficient
methodology for replacing/refurbishing obsolete Safety Related power
supplies. Throughout this guideline, dedication of the component and/or
subcomponent is referenced. The purpose of the following is to provide
guidance in establishing subcomponent equivalency evaluations and
acceptance criteria for the repaired and/or refurbished power supplies.
Dedication activities at Nuclear Power plants are performed per guidance
contained in NP-5652 and associated guidance. The requirements for
determining whether an item is suitable for dedication are:

Is the item subject to design or specification requirements that are
unique to facilities or regulated by the US NRC?

Is the item used in applications other than facilities or activities
licensed by the US NRC?

Is the item ordered from the manufacturer/supplier on the basis of
specifications set forth in the manufacturer’s published product
description?
In the case where a power supply is repaired and/or refurbished where the
subcomponent parts are replaced, the power supply no longer meets the
above definitions. In particular, the power supply is no longer
ordered/supplied from the manufacturer per the published product
description. In these cases the replacement subcomponent must be
established as equivalent subcomponents and dedicated individually.
NOTE: This section is under revision -
Page 15 of 19
Dedication of the subcomponents can a time consuming when functional
tests are performed at the subcomponent level. This guideline advocates
the use of the “Black Box” approach for functional testing of the
subcomponents.
The ‘Black Box” approach merely treats the power supply as a “black box”
where, upon application of a specified input, the specified output is verified
to determine operability. Dedication of the subcomponents is verified
based on this methodology.
For example: after being established as an equivalent, dedication activities
for a replacement circuit card may be as simple as verification of the part
number and dimensions with the operability of the card verified through the
functional testing of the power supply as a whole. This methodology can
greatly reduce the evaluation time when dedicating subcomponents by
eliminating the need to establish test/acceptance criteria and a test setup to
verify each input and output function of the subcomponent.
Many third part suppliers provide dedication services to the industry. This
should be considered when determining the costs of the dedication.
8.0
Shelf Life and In Storage Maintenance Requirements
Capacitors have been identified as the only subcomponent used in power
supplies with an established life expectancy in EPRI technical Report
“Power Supply Maintenance and Application Guide” (Ref. 1). From EPRI
Technical Report “Capacitor Application and Maintenance Guide”,
Aluminum Electrolytic Capacitors (AECs) are the only type capacitor that
exhibits any degree of age sensitivity. Given the limiting life subcomponent
of the power supply is the Aluminum Electrolytic Capacitor, establishing the
shelf life and In Storage Maintenance Requirements of power supplies
should focus on the life of the Aluminum Electrolytic Capacitors.
8.1
Shelf life:
AEC Shelf life recommendations vary from manufacturer to manufacturer.
These recommendations range from 2 years to 10 years, depending on the
quality of the subcomponent. Some manufacturers do not define a shelf life
but recommend checking the DC Leakage Current after prolonged storage.
MIL-HDBK-1131 (Ref. 3) suggests a 10 year shelf life for “military grade”
AECs when stored at 40ºC. Most of these recommendations are based on
storage in temperatures less than 40ºC with the assumption that the AEC
has been in storage without being powered.
Page 16 of 19
Further research, based on industry experience and limited studies
suggests that the shelf life of AECs may be as high as 20 years (Ref. 2).
This may be due to the fact that the recommendations are based on storage
temperatures up to 40ºC (104ºF) while most plant storage conditions would
be less than 30ºC (86ºF).
8.2
In Storage Maintenance Requirements (ISMRs):
Since the storage life of the power supply is generally limited to the life of
any AEC contained in the power supply, ISMRs should be established to
ensure the AECs do not reach their end of life.
The ISMR would energize the power supply. This energization would, in
turn, energize the capacitors. Per MIL-HDBK-1131 (Ref. 3), AECs should
be reformed within 1 hour. Based on this, one may suggest an ISMR that
energizes the power supply for one hour. However, it should be noted that,
the power supply design, as a rule, would never energize the capacitor to its
fully rated values. For this reason, a more conservative energization period
may be chosen.
Once energized for the specified period, the ISMR should verify the power
supply operability by ensuring the power supply meets its original
specifications. At a minimum, the performance should be verified for:




Output Voltage
Output Current
Output Voltage Ripple
Visual Inspection
A review of the individual power supply application and original
specifications should be evaluated to determine the critical power
performance criteria.
9.0
Recommendations
An effective program to minimize plant vulnerability to power supply
obsolescence must find the right balance in the following program areas:


Inventory Management: Ensure power supply inventory levels are
adequate to meet the plant demand.
Shelf life and In Storage Maintenance: Ensure the power supply
inventory is maintained in good working order.
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

Preventative Maintenance: Establish maintenance activities to
ensure power supply operability is maintained.
Identification of manufacturers/suppliers who retain specialized skill
for refurbishment of Power Supply.
By establishing a list of power supplies in the plant and prioritizing the list
based on the criticality of the power supplies’ applications, one can more
accurately determine the level of resources to expend in each of the
preceding areas.
A well-developed program should change as new data becomes available
(i.e. more power supplies become obsolete or reliability data suggests
maintenance/replacement intervals can be adjusted. Below are some items
to consider when addressing obsolescence of Power supplies
10.1
Prioritize actions (short term/long-term) based on vulnerability of
system failure due to obsolescence.
10.2
When looking for specific part replacements, use the OIRD database
for solutions already devised by other stations. Ensure station
engineers are aware of this tool.
10.3
Once a replacement power supply is established, identify the power
supply and replacement in OIRD.
10.0 References
References:
1. EPRI Technical Report TR-1003096, “Power Supply Maintenance and
Application Guide” Final Report, dated December 2001
2. EPRI Technical Report TR-112175, “Capacitor Application and
Maintenance Guide”, Final Report, dated August 1999
3. MIL-HDBK-1131, “DOD Handbook for Shelf Life and Reforming Procedure
for Aluminum Electrolytic Fix Capacitors”, Dated July 07, 1999
4. JUTG CGI’s on Power Supplies
5. EPRI NP-6406 for equivalent replacements
6. EPRI NP-5652, June 1988 - Guideline for the Utilization of Commercial
Grade Items in Nuclear Safety Related Applications
7. EPRI Technical Report, TR-102260 dated March 1994 – Supplemental
Guidance for the Application of EPRI Report NP-5652.
Page 18 of 19
ATTACHMENT 1
(Power Supply Replacement Flow Chart)
Page 19 of 19