Arc Fault Protection on Navy
Ships:
In the late 1970s the Navy
recognized that electrical fires were becoming a major problem in submarines.
About three fires per year were occurring in the main electrical distribution
switchboards across the submarine fleet. Those fires have had a major impact on
mission readiness and could potentially cause loss of life and ship. In
three-quarters of a second, the current from the smallest generator can burn a
fist-sized hole in the side of a switchboard. The damage initially increases
exponentially with time, limited only by available current from the generators.
Figure 1, a photograph from an older SSN 637 class submarine, shows extensive
damage from a typical electrical fire.
Figure 1. Damage from an arc-generated electrical fire.
Main
submarine electrical switchboards conduct thousands of amps over bare copper
bus bar 1–12 inches wide and 0.25–1 inch thick. On each ship, 15 to over 100 of
these switchboards exist. Large circuit breakers control the flow of current to
remote loads and smaller switchboards. An arc of several hundred amps can exist
and not cause a breaker to open since normal loads draw much more current. The
arc is not a short across the circuit, but a resistive load yielding heat;
therefore, the breakers do not open. Faulty connections due to corrosion,
faulty initial fastening, vibration, etc., cause 60–80% of arcs. Contamination
and foreign objects also cause arcs.
In 1979 the submarine force
became aware that JHU/APL had considerable experience in the creation and
control of large power electrical arcs for heating air that was used in aerothermal testing at the Avery Advanced Technology
Development Laboratory, now part of the Research and
Figure 2. An arc and submarine arc fault detector
components.
APL took the arc problem from
research and development to sensor development and into system development.
Level 3 production drawings were created and delivered to the Navy, and systems
were mass-produced after competitive bid. The first AFD systems were designed
for the older SSN 637 class submarines. APL has now designed systems for all
classes of submarines. More than 10,000 photosensors, 4,000 pressure sensors,
and 175 control units have been built and deployed on all classes of
submarines.
In AFD-protected submarine
switchboards, the photosensor portion of the sensor contains a narrow-band
ultraviolet filter and LEDs inside a hermetically sealed photodetector.
During built-in-test, the light from the LEDs is bounced from the back of the
filter into the photodiode for an end-to-end test of the sensor. Once a full
power arc is created, the air within the switchboard is rapidly heated and the
vents cannot relieve the pressure wave. A high-speed pressure switch closes if
the pressure inside the switchboard exceeds that outside the enclosure.
Solid-state switches inside the pressure transducer housing allow an end-to-end
test to be conducted when the central control unit performs the built-in-test.
Breakers that can cut off the flow of current to the
AFD-protected switchboards are identified upstream of the protected
switchboards. Because many switchboards have common feeds, removing power from
one entails removing power from several. Since the operation of almost
everything on a submarine depends on electricity, zones of protection are
defined to allow any switchboard sustaining an arc to be isolated so that a
minimum of other switchboards are affected.
A control unit receives
signals from the sensors and performs additional processing. When a valid arc
is recognized, the appropriate breakers are tripped. If the breakers are
tripped within less than 0.25 second, the damage will be limited to smoke
damage and major repairs will not be needed. More than 2000 arcs between a few
kilowatts and several megawatts were created to design and test the AFD system.
Figure 1 shows that
substantial pieces of copper bus are missing and that holes
were blown through the switchboard cover. Figure 3 shows that only light smoke
damage was sustained in a protected ship. The AFD system has now accumulated
over 400 ship years of operation without a single erroneous operation. It has
correctly detected and protected against six electrical fires since the first
installation in 1993.
Figure 3. Minimal damage from an
electrical arc in a protected circuit.
As part of the effort to
design an AFD system for the New Attack Submarine (NSSN), the
Navy asked APL to increase the performance of the system and to find ways of predicting imminent arcs. The photosensor
design was simplified, and its performance and life span were increased. The
production cost was reduced from $2500 to $600 per sensor, resulting in a
projected life cycle cost savings of $55 million in procurement costs.
As a part of the NSSN design,
a thermal ionization detector was developed to detect small particles released
into the air from overheated cables or from Glyptal-coated bus bar junctions.
Overheated insulation is detected at 200–300°C, well below the 1083°C needed to
melt copper and cause an arc. With this innovation, the AFD system becomes a
Continuous Thermal Monitoring (CTM) System, which can predict most arcs in time
to prevent them from happening.
Currently, the AFD/CTM effort
is toward redesigning the system for use on nuclear aircraft carriers. Because
of the Navy’s new design thrust, APL is making a major effort to reduce cost,
including using commercial off-the-shelf assemblies and parts rather than the
historical full MIL-SPEC design requirements. The new AFD/CTM system design
uses a star wiring that serves individual sensors on a network-based system
with remote intelligent sensor interface modules. Commercial enclosures (Fig.
4) and plastic sensor housings are used. The original Trident AFD hardware cost
$26,600 per protected switchboard. Through innovative design, the nuclear
aircraft carrier AFD/CTM design is projected to cost $4,500 per switchboard
despite having many new features. The current and future AFD systems
should be able to be used in a wide range of applications.
