United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-68: Safety-Related Equipment Failures Caused by Faulted Indicating Lamps


September 27, 1994



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 the possibility that indicating lamp failures in
safety-related circuits could cause safety-related equipment to become
inoperable.  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 December 14, 1993, at the Wolf Creek Generating Station, a local indicating
lamp for a motor control center short-circuited in such a manner that it
caused the control power transformer secondary fuse to open, making the "B"
centrifugal charging pump room cooler inoperable.  The heat from the increased
current flow in the lamp was sufficient to melt a hole in the lens cover.

At Indian Point Unit 3, on February 9, 1992, an indicating lamp on the Train A
safeguards initiation rack failed in a manner that caused a short-circuit that
blew a 10-ampere fuse in the dc control power circuit.  This disabled the
automatic initiation feature of the Train A safeguards components, which
consisted of a safety injection pump, a containment spray pump, two
containment fan cooler units, an essential service water pump, a component
cooling water pump, and the Train A motor-operated valves.  The lamp circuit
did not have a current-limiting element.  The licensee attempted to correct
the problem by replacing the incandescent light bulbs with LEDs.  However,
during the replacement excess solder in an LED assembly lamp base caused
another short-circuit, blowing a fuse and extinguishing two racks of LEDs.

9409210064.IN 94-68
September 27, 1994
Page 2 of 3

Similar events had occurred previously at Indian Point Unit 3.  On January 23,
1992, an electrical fault in an indicating lamp on the local control panel for
the auxiliary boiler feed pump blew a 6.25-ampere control power fuse,
disabling the automatic initiation feature of the safeguards equipment on a
480-V vital bus.  On October 15, 1991, an electrical fault in an indicating
lamp on safeguards initiation Train B blew a 10-ampere control power fuse,
disabling the automatic initiation feature of the Train B safeguards


At Wolf Creek, similar local indicating lamps were used in other
safety-related motor control center starters in addition to the one that
failed.  These included those for the boron injection tank isolation valves.
If the indicating lamp associated with these valves had failed in the same
manner as the December 14th failure, these valves could have lost control
power, thereby delaying the initiation of high-head safety injection in an
accident.  This failure could have caused the simultaneous failure of other
safety systems as well.

The Wolf Creek licensee replaced all of the incandescent indicating lamps that
did not have voltage-reducing transformers with new light-emitting diode (LED)
bulbs.  The LED bulbs contain internal circuitry that prevents excessive
current from being drawn if the LED is short-circuited.

The Indian Point Unit 3 licensee modified all of the dc lamp circuits in the
safety-related control circuits that lacked current-limiting features, either
by installing fuses for the indicating lamp circuits or by replacing them with
assemblies having current-limiting resistors.

These events illustrate a type of design deficiency in which indicating lamps
are not electrically isolated from the associated control power circuitry.
One method of electrically isolating the indicating lamp circuit from the
control power circuit is to add a separate fuse to the lamp power supply
circuit.  The fuse for that circuit can be integrated with the fuse for the
control power circuit, such that a lamp or LED failure does not cause a loss
of control power for the associated safety-related circuits.  Another method
is to replace indicating lamps with lamp assemblies having current-limiting
features or resistors so that lamp failures will not cause an excessive surge
of current in the control power circuit.  In addition, the circuit can be
protected from socket faults if the current limiting resistor is part of the
circuit rather than the lamp itself.
.IN 94-68
September 27, 1994
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
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.

/s/'d by CIGrimes/for

Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation

Technical contact:  D. Nguyen, NRR
                    (301) 504-3202

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