Potter & Brumfield Model MDR Rotary Relay Failures
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
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 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.
9112300138
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IN 92-04
January 6, 1992
Page 2 of 3
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 (PRA) 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
mechanical 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 immediately 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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IN 92-04
January 6, 1992
Page 3 of 3
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts: K. R. Naidu, NRR
(301) 504-2980
R. A. Spence, AEOD
(301) 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
Page 1 of 4
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 contacts 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 104�F (the design limit is 149�F) and had an external
surface temperature of 157�F.
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Attachment 1
IN 92-04
January 6, 1992
Page 2 of 4
APSC detected no relay failures in 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
manufactured 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
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Attachment 1
IN 92-04
January 6, 1992
Page 3 of 4
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 113�F, while the ambient cabinet
temperature was 92�F. 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 contaminants 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
electrical 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,
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Attachment 1
IN 92-04
January 6, 1992
Page 4 of 4
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
recommendations. 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 P&B 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
operated infrequently were susceptible to the corrosion and outgassing
failure mode. P&B 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.
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