Information Notice 1992-04, Potter and Brumfield Model Mdr Rotary Relay Failures
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UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
January 6, 1992
NRC INFORMATION NOTICE 92-04: POTTER & BRUMFIELD MODEL MDR ROTARY RELAY
FAILURES
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees of failures experienced with MDR series Potter
& Brumfield (P&B) rotary relays installed in safety-related systems at certain
nuclear power plants. It is expected that recipients will review the informa- tion for applicability to their facilities and consider actions, as appropri- ate, 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 January 14, 1986, September 17, 1987, and December 8, 1987, an emergency
diesel generator (EDG) failed an operability surveillance test at the LaSalle
County Station, Units 1 and 2. In each case, while the Commonwealth Edison
Company (CECO) attempted to synchronize the EDG to its bus, the EDG output
breaker would not close. CECO replaced all P&B MDR relays in the output
breaker closing circuits with General Electric HFA relays. The NRC staff has
received no reports of relay failures at LaSalle affecting EDGs since these
were replaced.
On October 10, 1988, the Arizona Public Service Company (APSC), the licensee
for the Palo Verde Nuclear Generating Station, submitted a report in accordance
with Title 10 of the Code of Federal Regulations, Part 21 (10 CFR Part 21).
This report documented 18 instances over a 2-year period in which P&B MDR
relays failed to change position.
APSC detected these failures during either routine surveillance testing or
actuation of the engineered safety features (ESF) actuation system or the
reactor trip switchgear. After replacing all P&B MDR series relays, APSC
experienced only two failures; improperly sized coils or contamination in the
insulating material of the switch caused these two failures.
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IN 92-04 January 6, 1992 On July 19, 1991, during a monthly surveillance test, the River Bend Station
experienced ESF actuation of containment isolation valves, control room filter
trains, the standby gas treatment system, and the fuel building filter trains
because of an MDR relay failure.
On July 23, 1991, during a monthly surveillance test at the River Bend Station, the failure of an MDR-5111-1 relay caused an ESF isolation of a reactor water
sample valve. The Gulf States Utilities Company (GSU), the licensee, committed
to replace all P&B MDR relays.
Discussion
P&B MDR relays are used in various safety-related applications in commer- cial nuclear power plants with reactors manufactured by the Babcock and Wilcox
Company; Combustion Engineering, Incorporated; the General Electric Company;
and the Westinghouse Electric Corporation. Industry records identify over
60 instances of P&B MDR rotary relays failing to operate properly since 1984.
An MDR relay failure may cause the loss of a train-of the ESF actuation system, the emergency core cooling system', or the reactor protection system. A common- mode failure may result in the loss of one or more of these systems.
GSU
performed a probabilistic risk assessment (PMA) of the reactor protection
system at River Bend, based on plant-specific, P&B MDR relay failure rates that
were greater than the generic failure rates by a factor of 5.1. This PRA
showed that the reactor protection system failure rate increased by a factor of
25 to 3.3E-4.
The principal failure mechanism of P&B MDR rotary relays'appears to be mechani- cal binding of the rotor'caused by deposits from coil varnish outgassing and
chlorine corrosion products. , A secondary failure mechanism appears to be the
intermittent continuity of electrical' contacts. A number of variables contribute
to these failure mechanisms and cause the relays to fail at random mostly
within 2 to 5 years of the in-service date. 'Failures may occur regardless of.
current or wattage, the use of ac or dc power, or whether normally energized or
de-energized. It is also important to note that a relay rotor can bind immedi- ately 'after a surveillance test.
Attachment 1 provides a detailed description of the failure mechanisms, con- tributing causes, and failure investigations. Attachment 1 also discusses
modifications made to P&B MDR series relays by the manufacturer to reduce
susceptibility to the failure mechanisms discussed above.
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January 6, 1992 This information notice requires
you have any questions about the
of the technical contacts listed
Reactor Regulation (NRR) project
no specific action or written response. If
information in this notice, please contact one
below or the appropriate Office of Nuclear
manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts:
K. R.
(301)
Naidu, NRR
504-2980
R. A.
(301)
Spence, AEOD
492-8609 Attachments:
1. Potter & Brumfield Model MDR Rotary Relays
2. Figure 1, Potter-Brumfield Model MDR Rotary Relay
3. Figure 2, MDR Non-Latching Relay
4. List of Recently Issued NRC Information Notices
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Attachment 1
IN 92-04 January 6, 1992 Potter & Brumfield Model MDR Rotary Relays
Description of the MDR Rotary Relay
Potter & Brumfield (P&B) manufactures two types of MDR rotary relays:
latching
and non-latching. Various series of these relays are provided for service at
28 and 125 volts (V) dc and 115 and 440 Vac, with from 4 to 24 pole double
throw (PDT) contacts. While each series has a different number of contact
stacks and has a different coil, power, and current capacity, each of the
series is similarly constructed and exhibits similar failure mechanisms.
Non-Latching Relays
The non-latching MDR relay has two coils connected in series inside the relay
which, when energized, rotate the relay rotor shaft, which operates the con- tacts through a shaft extension. The stator faces and stop ring limit the
rotor movement to a 30-degree arc. Two springs return the rotor to the stop
ring and the contacts to their normal positions when the coils are
de-energized. The non-latching MDR relays have two positions: 'energized" and
"de-energized."
(See figures 1 and 2).
Latching Relays
Each relay in the MDR latching series has two sets of series coils, which
provide a latching two-position operation. When one set of coils is energized, the rotor shaft rotates through a 30-degree arc, changing the state of the
contacts. The other set of coils must be energized to return the relay to its
original position.
Failure Investigations
The Commonwealth Edison Company (CECO) determined that the three events at the
LaSalle County Station resulted from the failure of P&B MDR-137-8 or MDR-138-8,
125 Vdc normally energized relay contacts to close. CECO performed diagnostic
testing after the earlier events but could not repeat the failure.
This lack
of repeatability is typical of MDR intermittent failures.
The Arizona Public Service Company (APSC) found that three of the P&B MDR relay
rotors at Palo Verde Nuclear Generating Station (PVNGS) would not move more
than 12 degrees of the complete 30-degree arc.
The failed relays, located
in cabinets without forced ventilation were in an ambient temperature of
95 to 104OF (the design limit is 1490F5 and had an external surface temperature
of 157 0F.
Attachment 1
IN 92-04 January 6, 1992 APSC detected no relay failures ii cabinets with forced ventilation which
provided an ambient temperature of 81'F or less.
Such relays had a temperature
of 112'F on their external surfaces. APSC determined that it had applied up to
39.8 Vdc to the 28 Vdc coils. APSC tested 7 of the 18 failed relays on an
18-month frequency and 10 on a 62-day frequency. APSC had the relay failures
analyzed and determined that varnish on the relay coils outgassed, condensed, and accumulated between the rotor shaft and the end-bell bearings, binding the
rotor and the bearings together.
The outgassing was due to excessive coil
temperatures that occurred when the coils were continuously energized at
voltages above their nominal ratings.
The heat also may have caused the
release of chlorine from (1) the PVC coating on the fiberglass tubing covering
the solder joint between the magnet wire and the Teflon coated lead wire, and
(2) the Neoprene rubber grommet through which the coil lead wires penetrate the
base of the relay. The chlorine corroded brass parts inside the relay.
P&B and
APSC concluded that long intervals between de-energizing of the relays may have
also contributed to the failures.
In May 1989, APSC installed replacement P&B relays at PVNGS that were manufac- tured with coils'coated with epoxy instead of varnish. APSC conducted tests and
found that 5 of the 42 relays tested would not rotate to their de-energized
position and that 5 other relays operated slowly. Two independent laboratories
observed that; (1) the relays' epoxy was not properly cured, (2) uncured epoxy
contaminated the rotor and (3) P&B did not de-aerate the epoxy prior to use, contrary to the manufacturer's recommendations. This caused the rotor and
stator surfaces to bond together, preventing the rotor from rotating freely.
P&B informed the NRC that APSC returned the 42 relays and that P&B rebuilt
them.-
On September 10, 1990, the General Electric Nuclear Energy Division (GENE)
issued Rapid Information Communication Services Information Letter 053 to
address P&B MDR relay failures reported at two GE boiling water reactors. P&B
believed that chlorine released from rubber grommets and polyvinyl chloride
sleeves caused corrosion and that varnish on the-coils outgassed while the
relays were continuously energized.' Both chlorine-corrosion products and
varnish accumulated in the bottom end-bell bearing and caused the rotor shaft
to bond to the bearing.
P&B suspected that the failed relays were exposed to
high ambient temperatures and could have been exposed to high coil voltages or
could have been rarely cycled.
On November 2, 1990, GENE-issued Potentially Reportable Condition 90-11 in
which it stated that both 24 Vdc and 120 Vdc'coils had lower coil powers than
the 125 Vdc relays and were therefore not vulnerable to this failure mode.
GENE concluded that no substantial safety hazard existed.
However, upon
investigating the failed MDR relays at River Bend as discussed below, the NRC
obtained results that may contradict these conclusions.
On July 19, 1991, a high resistance on one set of contacts on a P&B 24 Vdc, MDR-5111-1 rotary relay, which should have been closed, caused a voltage drop
to the downstream relays which opened their contacts and resulted in an ESF
Attachment 1
IN 92-04 January 6, 1992 actuation at the River Bend Station. The Gulf States Utilities Company (GSU),
the licensee, later performed bench testing of this failed relay and verified
that the relay actuated properly and all contacts changed state properly, and
exhibited proper continuity. The coil was meggered and found to be acceptable.
The contacts all appeared to be clean and shiny, with no evidence of pitting or
residue. GSU found no foreign material in the relay or on the rotor shaft and
found nothing that may have contributed to the high resistance across the
contacts.
On July 23, 1991, GSU investigated another MDR relay failure at River Bend and
found two MDR-5111-1 relay contacts open that should have been closed when the
coil was energized.
GSU also found that the contacts operated intermittently
with some contacts closing several minutes after the coil was energized or
sometimes not at all.
Both River Bend failed relays had been in service within tightly-regulated
design voltage and temperature conditions and were mounted inside stainless
steel isolation cans for divisional separation.
GSU measured the temperature
inside the isolation can at 1130F, while the ambient cabinet temperature was
920F. In each case, the failed relay had been recently cycled because of a
short loss of power to the coil that had occurred a few days before the relay
failure was discovered, and it appears that not all contacts engaged properly
when power was restored.
Failure Mechanisms
The primary failure mechanism of the P&B model MDR rotary relay appears to be
a mechanical binding of the rotor caused by organic outgassing and deposition
of contaminants and corrosion particles on the relay rotor shaft. The contamin- ants are deposited in the end bell bearings and sleeves and cause the rotor
shaft to bond or stick to the bearing, preventing the rotor shaft from fully
rotating when the relay coils are energized or de-energized. The principal
contaminant is outgassed material emitted from the brown enamel varnish used to
coat the relay coils. This contamination may not be apparent to the naked eye.
The corrosion results from chlorine released from the rubber grommets and the
polyvinyl chloride sleeves. Gulf States and P&B disassembled six operable and
two failed relays that had been in service since December 1984. The thickness
and color of the deposits on the rotor, sleeve, and end-bell bearings of the
relays varied widely among the eight relays, indicating varnish outgassing.
A secondary failure mechanism appears to be intermittent continuity of the elec- trical contacts.
High resistance and intermittent continuity may result from
chemical reactions on the fixed and movable silver contacts.
P&B tested a
MDR-5112-1, 125 Vdc relay that had been in service at River Bend and found
intermittent continuity on a set of clean, unused contacts.
A number of variables contribute to these failure mechanisms and reduce the
length of the operating life of the complex P&B MDR rotary relays. These
variables include coil wattage, applied ac or dc voltage, normally energized or
de-energized coil, manufacturing tolerances, ambient and coil temperatures, varnish thickness, mounting configurations and enclosures, cabinet ventilation,
e
Attachment 1
IN 92-04 January 6, 1992 relay breathing, testing frequency, operational cycling, the number of contact
decks, and the amperage and voltage of the contact load. These contributory
factors cause an apparent random failure history.
While each of the MDR
relays failed between 1 month to 13 years after it was placed in service, most
failed within 2 to 5 years.
Modifications to MDR Relays
P&B has made the following design changes to MDR series relays:
Changed the movable contacts from silver to silver-cadmium-oxide in
October 1985. However, P&B recommends against using MDR relays with
either silver or silver-cadmium-oxide in low current circuits.
Changed the coil coating from varnish to Dolphon CC-1090 epoxy resin in
February 1986. This reduced the coil outgassing rate.
However, P&B does
not de-aerate Dolphon CC-1090 prior to use, contrary to Dolphon's recommen- dations.
P&B informed the NRC that the epoxy manufacturer plans to cease
production of this currently used and tested epoxy. The NRC is unaware of
when P&B will change to a new epoxy.
Licensees may wish to determine if
PAB has examined the replacement epoxy for susceptibility to outgassing
after aging. Licensees may also wish to determine if P&B applies the
epoxy in accordance with the manufacturer's recommendations.
Replaced the brass switch studs in medium size MDR relays with stainless
steel studs in November 1986.
Began lubricating end-bell bearings in July 1988.
Changed chloride-containing materials to chloride-free materials in
June 1989.
Changed the rotor spacers from brass to stainless steel in May 1990.
Changed the brass spring retainer in small size MDR relays from brass to
stainless steel in May 1990.
Changed shims from brass to phosphor bronze in May 1990.
P&B had implemented all these modifications to its MDR rotary relay design by
May 1990.
When APSC reported having problems with MDR relays at Palo Verde in 1988, P&B
believed that only relays normally energized with excessive voltage and operat- ed infrequently were susceptible to the corrosion and outgassing failure mode.
PAB did not notify other licensees about these problems since this condition
appeared to occur only at plants with reactors manufactured by Combustion
Engineering, Incorporated. P&B informed the NRC that since 1988 it has only
supplied MDR relays as commercial grade components without accepting the
reporting requirements of 10 CFR Part 21.
Attachment 3
IN 92-04 January 6, 1992 MDR NON-LATCHING RELAY
SPACER
Figure 2
POTTER-DRUMWIELD MODEL 14DR ROTARY RELAY
ROTOR IN
DEENERGIZED
POSITION
ROTOR IN
ENERGIZED
POSITION
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COIL
HOUSING
ROTOR
ROTOR
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Attachment 4
January 6, 1992 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
Information
Date of
Notice No.
Subject
Issuance
Issued to
92-03
92-02
92-01
91-87
91-86
91-85
91-84
91-83
Remote Trip Function
Failures in General Electric
F-Frame Molded-Case Circuit
Breakers
Relap5/Mod3 Computer Code
Error Associated with the
Conservation of Energy
Equation
Cable Damage Caused by
Inadequate Cable Installa- tion Procedures and Controls
Hydrogen Embrittlement of
Raychem Cryofit Couplings
New Reporting Requirements
for Contamination Events at
Medical Facilities
Potential Failures of
Thermostatic Control Valves
for Diesel Generator Jacket
Cooling Water
Problems with Criticality
Alarm Components/Systems
Solenoid-Operated Valve
Failures Resulted in
Turbine Overspeed
01/06/92
01/03/92
01/03/92
12/27/91
12/27/91
12/26/91
12/26/91
12/20/91
All holders of OLs or CPs
for nuclear power reactors.
All holders of OLs or CPs
for nuclear power reactors.
All holders of OLs or CPs
for nuclear power reactors.
All holders of OLs or CPs
for nuclear power reactors.
All licensees authorized
to use byproduct materials
for human use.
All holders of OLs or CPs
for nuclear power reactors.
All Nuclear Regulatory
Commission (NRC) fuel
cycle licensees, interim
spent fuel storage licens- ees, and critical mass
licensees.
All holders of OLs or CPs
for nuclear power reactors.
OL = Operating License
CP = Construction Permit