51st AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference<BR>18th
12 - 15 April 2010, Orlando, Florida
AIAA 2010-2648
A Study of Over-Current Protection, Electrical Arcing, and
Fire Protection
Colin P. McCabe* and Daniel C. Cyphers†
Skyward, Ltd., Dayton, OH
The contribution of electrical wiring to ballistic threat-induced aircraft fires is a
recognized, but poorly quantified vulnerability, particularly as modern aircraft continue to
evolve. Electrical systems are increasingly replacing or supplementing mechanical systems.
Previous ballistic test programs have included live electrical wires, but primarily as a
secondary synergistic contributor to fire ignition. This paper details a laboratory-based
experiment, which preceded a ballistic test series, conducted on electrical wiring and OverCurrent Protection Devices (OCPD) representative of systems on modern aircraft. The
purpose of this experiment was to compare common OCPDs, and evaluate their capability to
protect against fire ignition in the event of the circuit incurring ballistic damage. A
secondary product of the experiment was an evaluation of the likelihood of a fire ignition in
a JP-8 fuel spray, given the energy present in an electric arc and the density or distribution
of that energy through the lifetime of the arc. This secondary evaluation was possible
because sample wires of several gauges (from 22 AWG to 4 AWG) were already a necessary
part of the experiment. It was suspected that a wide range of cross-sectional areas might
obscure each OCPD’s role in suppressing fire ignition by affecting arc characteristics. This
study contains an initial examination of the relationship, but is far from comprehensive. An
attempt is made to relate energy density, or flux, through a minimum area described by the
cross-section of a conductor supporting an electric arc. More elegant dependencies such as
electric repulsion, dielectric breakdown, and surface currents are left for future study.
Testing was conducted at the Aircraft Engine Nacelle (AEN) test facility, operated by the
U.S. Air Force, 46th Test Wing, 46th Test Group’s Aerospace Survivability and Safety
Operating Location (46TG/OL-AC). The AEN is part of the Aerospace Vehicle Survivability
Facility (AVSF) at Wright-Patterson AFB, OH.
Nomenclature
A
AC
AWG
DC
frps
ft
I
J
MHz
mm
ms
P
psi
RMS
s
V
VAC
*
†
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
Amperes
Alternating Current
American Wire Gauge
Direct Current
frames per second
feet
Current
Joules
Megahertz
millimeters
milliseconds
Power
pounds per square inch
Root Mean Squared
seconds
Volts
Volts Alternating Current
VDC
= Volts Direct Current
Engineer, Skyward, Ltd. 5100 Springfield Street, Suite 418, Dayton, OH, 45431, AIAA Member
Senior Engineer, Skyward, Ltd. 5100 Springfield Street, Suite 418, Dayton, OH, 45431, AIAA Associate Fellow
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Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
I.
Introduction
Electrical systems for modern aircraft are becoming ever more complex. An evolving objective of vulnerability
testing is to understand the contribution of the electrical system to platform fire vulnerability. Vulnerability analysts
have long been concerned with ballistic threat-induced electrical system damage acting as a fire ignition source.
This concern has not been fully addressed in previous live fire test programs, although it has been examined to some
extent. For example, powered wires have been involved in previous LFT&E programs. However, in these
programs, the electrical system damage was a potential secondary contributor added to allow for synergistic effects.
The ballistic threat was evaluated as the primary ignition source for fire/explosion vulnerability, and thus the testing
focused in other areas. Other limited research has been performed in the area of electrical arcing and fuel ignition,
but the data is not sufficient to make adequate inferences for newer platforms.1
Fire represents a significant vulnerability to all air vehicle systems due to the amount of flammable fluids carried
onboard. Combat aircraft are at risk due to the potential to encounter enemy threats that can ignite these flammable
fluids, which can take up a significant portion of the aircraft’s internal volume. These threats can either directly
ignite a fire due to their own energy or indirectly contribute to fire ignition when the threat penetrates another
potential ignition source. One secondary source is impact of an aircraft’s electrical wires, which can cause electrical
arcing. This arcing can occur in the vicinity of leaking fuel components also penetrated by the threat. A test
program recently conducted a developmental project to examine uncertainties in the contribution of electrical arcing
to fire ignition.2,3 Figure 1 depicts examples of arcing phenomenon observed in this testing.
a
b
c
Figure 1. Examples of Arcing Events: (a) Moderate Arc (b) Intense Arc viewed through
Filter (c) Low Density Arc
The purpose of this test series was to evaluate aircraft vulnerability to potential electrical arcing and fire ignition
hazards associated with its electrical system.2,3 Modern aircraft systems employ familiar 28 VDC and 115 VAC
circuits, as well as many newer higher voltage power systems that are used to operate the electric-powered hydraulic
actuators, which are replacing conventional hydraulic-powered actuators in pursuit of a more electric aircraft design.
The electrical system also contains a variety of protective devices, collectively called Over-Current Protection
Devices (OCPDs) that were examined for vulnerability reduction to fires.
Initial research indicated that while many prior programs have existed to establish the minimum ignition energy
for a variety of fuel-air and gas-air mixtures, the answers given were in terms of total energy, as opposed to energy
density, or energy flux. This seems to ignore critical aspects of the ignition equation, specifically; how much energy
in what space, and over what period of time? Therefore, while evaluating different types of circuit protection in
their roles in fire prevention, it was also attempted to quantify the role of energy density, (or energy flux) in ignition.
In this study, the effects of electric repulsion, dielectric breakdown, and surface currents have been ignored.
While the end goal of this test program was to examine electrical arcing and fire ignition from ballistic effects, a
significant laboratory phase using controlled, simulated damage test cases preceded ballistic testing. This was done
to select a more focused group of amperage/voltage/circuit combinations to evaluate in the ballistic phase of testing.
Each of these combinations required controlled damage wire samples ranging from 22 AWG to 4 AWG. The
current simulating normal operating conditions and overcurrent to be protected against differed by circuit design.
This necessitated the incorporation of 3 different types of OCPDs in the testing.
The three types of OCPDs tested were standard circuit breakers (SCB), solid state circuit protection devices
(SSPD), and contactor/breakers. The SCB used was the standard resistive circuit breaker commonly found in
residential electrical service boxes. They have a comparatively slow reaction time, as they rely on the heating of a
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bimetal strip to displace a switch. They are used for lower amperage currents, where circuit components have some
robustness to moderate or brief over-currents. SCBs must be reset manually once tripped. The SSPC is a solid state
relay switch, possibly controlled by monitoring software. They may be activated by control signals from
Programmable Logic Controllers (PLCs), PCs, Transistor-Transistor Logic (TTL) sources, or other microprocessor
and microcontroller controls. SSPCs may be reset automatically or manually, once tripped by interaction through a
software interface. A contactor /breaker is an electrically controlled switch (a relay) used for switching a power or
control circuit.4 A contactor is controlled by a circuit, which has a much lower power level than the power being
switched. There was some overlap within this test series regarding what type of OCPD was used with what circuit
combination, but invariably, SCBs and SSPCs were used in 28 VDC circuits, and contactor/breakers were used in
270VDC circuits. Contactor/breakers may also be reset automatically or manually through a software interface.
Throughout this test program, no OCPD was reset automatically in the course of a test. Once tripped, OCPDs
remained in an open (off) state until the conclusion of the test.
Three controlled damage conditions were evaluated in this test series. Each simulated a specific electrical fault
condition that could occur because of a ballistic impact. 1) Dynamic separation - the condition of severing a wire in
a circuit while it is operating under normal conditions. This simulates a wire being cut by a ballistic threat. 2) Short
circuit - the condition of some exposed portion of a circuit, operating under normal conditions, being shunted to the
common ground. This simulates a wire; either severed, crushed, or partially stripped; being electrically bridged
through the body of a ballistic threat, being forced directly by a ballistic threat, or by imparted momentum from that
threat; into electrical contact with the airframe or some other grounded component. 3) Partial diameter reduction the condition of material being removed from a wire by a ballistic threat or spall/debris without completely severing
it.
II.
Controlled Damage Testing
Test Facility
Testing was conducted by the U.S. Air Force, 46th Test Wing, 46th Test Group’s Aerospace Survivability and
Safety Operating Location (46 TG/OL-AC) at the Aerospace Vehicle Survivability Facility (AVSF) at WrightPatterson AFB, OH. The Aerospace Survivability and Safety Operating Location serves the 46th Test Wing’s 780th
Test Squadron based at Eglin AFB, FL. The AVSF is utilized to conduct research, development, and test and
evaluation of combat survivable aerospace vehicles by testing the system performance of today’s and tomorrow's
weapon systems and system components under realistic threat conditions. Tests were conducted at two different
facilities within the AVSF. Controlled damage testing was conducted at the Aircraft Engine Nacelle (AEN) test
facility shown in Fig. 2. A schematic showing the layout for controlled damage testing within the AEN test facility
is shown in Fig. 3.
A.
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Figure 2. Aircraft Engine Nacelle Facility
Figure 3. AEN Test Setup Schematic
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Controlled Damage Testing Approach
Controlled damage testing was conducted to determine the likelihood of initiating a fire for various
voltage/amperage/damage combinations under favorable fire conditions, and to provide a baseline for comparison
with ballistic damage tests. Although the results of controlled damage testing were used to select test cases for the
ballistic phase of testing, it also can stand alone as a valuable experimental test program. Testing was divided into
three categories: 1) short circuit, 2) separating arc gap, and 3) 90% conductor diameter reduction. The performance
of the OCPDs was also evaluated during the simulated damage events.
The short circuit tests were accomplished by causing a short to ground in a circuit, which up to that moment, was
energized and operating normally. This was accomplished by moving an exposed conductor from a wire segment
into contact with a grounded plate. The separating arc gap test condition, “pull-off”, was accomplished by causing a
break in an active circuit by separating the conductor from a grounded plate. The partial diameter loss tests were
accomplished by diverting current from a healthy circuit onto one in which 90% of a test segment’s diameter had
been removed. In each test condition, JP-8 fuel spray was directed at the region of interest from a distance of about
6 inches. An example of the formation of a sustained fire caused by one of these damage conditions is shown in
Fig.4.
B.
Maximum
arc intensity
Arc initiation
Fire initiation
Fire migration
towards sprayer
head
Fire sustainment
Figure 4. Typical Arc Ignition Sequence Showing Fire Sustainment
Test Fixture Description
A pre-existing test chamber, shown in Fig. 5, was used for this test program. The chamber was designed as a
large steel box reinforced to withstand overpressures from ullage explosions. This fixture was utilized as a “vacuum
test chamber” during early testing for pulling a vacuum (to simulate altitude) from normal atmospheric pressure
(~14.7 psi) to 4.3 psi. It remained as the primary test chamber during the remainder of testing. The test chamber’s
overall dimensions were 4 feet wide x 4 feet long x 4 feet high, with an internal volume of almost 35 ft3. Small,
circular ports on the sides and top of the test chamber were fitted with Lexan (i.e., polycarbonate) windows to allow
viewing of the event by video cameras and allow the installation of other equipment during testing.
C.
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Ports fitted
with Lexan
windows
Figure 5 Test Chamber Capable of Testing at Simulated Altitude
A smaller, “wire fixture” was used inside of the test chamber to hold the wire samples, instrumentation and a
fuel sprayer/fuel tank, as seen in Fig. 6. The wire fixture consisted of a pneumatic actuator, an aluminum grounding
plate, and a fuel sprayer. One section of the test wire was attached to the rod end of the actuator, which either: 1)
extended during the short-circuit testing to make contact with the grounding plate, or 2) retracted during the pull-off
testing to break contact with the grounding plate. The other end of the test wire was connected to terminals that
were attached through the Lexan covered ports, as seen in Fig. 7. The fuel sprayer was located approximately 6
inches away from and at a 45° obliquity to the test wire/aluminum grounding plate contact point. The fuel sprayer, a
Monarch F-80 AR, provided a pressurized (100 psi) spray of JP-8, with a fuel flow rate of 0.6 gallons per hour
(gal/hr), and in a 70° cone, directed at the location of the electrical fault. The fuel was at ambient temperature.
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Wire Test Segment
Pneumatic Cylinder
and Piston
Block for Shorting or
Completing Circuits
Fuel Sprayer
Figure 6. Example Test Fixture Placed in the Test Chamber
Vacuum Release
Valve
High Voltage Feedthrough Terminals
Fuel and Vacuum
Lines
Controls, Instrumentation,
and Video Wires
Figure 7. Terminal Attachments for the Test Wire Section
In all tests (controlled damage and ballistic), voltage was supplied from a battery bank (Fig.8) to a power
distribution panel. The power distribution panel (Fig. 9) was connected to a current limiting load bank, which was
used to limit the current from the battery bank to the desired current for each test setup. From the current limiting
load bank (Fig. 10), a connection was made from the power distribution panel and the Over Current Protection
Devices (OCPD). For unprotected tests, OCPDs were bypassed. From the OCPDs, a wire was connected to the test
chamber at feed-through terminals in the Lexan covered ports. The test section was then connected to the test
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chamber input side of the terminal. The input terminal was connected to the end of the test wire section attached to
the actuator. The other end of the wire (which was attached to the aluminum plate) was routed back to the terminal
and then directly out to a resistance load bank. The resistance and current limiting load banks were constructed of a
series of electric heaters (similar to those found in traditional home water heaters) that were connected to copper bus
bars, as pictured in Fig. 10. For each test, the bus bars were rearranged based on the resistance or current limit
needed for each voltage/amperage/wire configuration.
Battery Bank
Chargers
Figure 8. Battery Bank Used to Supply Voltage to the Test Fixture
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Power
Distribution
Panel
Figure 9. Power Distribution Panel
Configurable Bus Bars
Resistors
Figure 10. Current Limiting Load Bank (Resistance Load Bank Looked Very Similar)
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Controlled Damage Test Setup
In the controlled damage test setup, three different test configurations were used: 1) short circuit testing, 2)
dynamic separation testing, and 3) partial diameter loss testing. The test setup (Fig. 6) was almost identical for the
short circuit and the dynamic separation testing; the main difference being that the actuator was extended to close
the circuit in the short circuit testing, whereas it was retracted to open the circuit in the dynamic separation tests.
The aluminum plate was grounded in the short-circuit tests, whereas it was routed directly to the resistance load
bank in the dynamic separation tests. The point of contact for these tests is highlighted in Fig. 11. In the partial
diameter loss test setup, the wire test section was connected to both terminals on the Lexan cover port. The
aluminum plate and actuator were not part of the test setup for these partial diameter loss tests.
D.
Point of
Contact
Figure 11. Dynamic Separation and Short Circuit Contact Surfaces
The short circuit and dynamic separation portions of testing were accomplished by using an insulated pneumatic
cylinder, which held a test wire to extend to, or retract from, contact with a conducting plate. This plate was
grounded for short circuit tests, however, it was simply a completion of the designed circuit for dynamic separation
tests. The mechanism was designed to allow for remote manual operation of the cylinder while automating data
collection. Figure 12 and Fig. 13 depict the short circuit and dynamic separation mechanism and how it was utilized
for each controlled damage condition.
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Figure 12. Short Circuit
Figure 13. Dynamic Separation
The switch to actuate the pneumatic cylinder was manually controlled by the test engineer. During short circuit
pretesting, the switch was activated/deactivated within approximately 50-100 ms after the test engineer gave the
signal. Many times, upon initial contact between the wire and plate, comparatively weak arcing was observed.
However, upon deactivation (i.e. opening) of the circuit, it was discovered that more violent arcing events occurred.
This is due to the physical nature of the event. As the circuit opens, the last remaining microscopic points of contact
may be ablated, becoming a conducting and rapidly oxidizing gas. If the current density remains high enough,
rather than cooling through oxidation or condensation, the gas may remain in a conductive ionized state (i.e.
plasma). The ionized path between the aluminum shorting block and the wire ends allows the electric arc to be
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drawn across the increasing gap easily until, through convection and radiation, the ionized gas dissipates and cools,
breaking the conductive path of the arc. These larger arcing events allowed fires to occur without exception,
corrupting the results of tests intended to examine only a short circuit fault condition. It was obvious that a
combination of fault conditions, i.e. a dynamic separation from a short circuit, would always be a violently
exothermic event, and would always produce a fire as a result. Therefore, during subsequent testing, procedural
changes were made to treat pneumatic cylinder activation (i.e., short circuit configuration) and deactivation (i.e.,
dynamic separation configuration) actions as different controlled damage test conditions.
Instrumentation and Data Collection
1.
High Speed Video
High speed color video data was recorded with a Phantom V42 video camera, recording at 1,500 frames per
second. The camera resolution view was set at 256 x 256. The high speed video was triggered by a manual
operator’s signal given to a pneumatic actuator, initiating the fault condition to be tested. The high speed video
camera was mounted above the text fixture to observe the fault conditions through a Lexan window, directly above
the electrical fault.
2.
Standard Video
Standard color video was record with a Panasonic miniature camera with a fish-eye lens, recording at 30 frames
per second. The standard video camera was operated from a laptop also monitoring multi-meter readings using
LabView.
3.
Thermocouples
One type-K thermocouple was place in the fuel spray, near the location of the fault condition. It was monitored
at 1000Hz using Labview.
4.
Nicolet Data Collection System
Information was collected using a Nicolet Data Collection System, recording all channels at 1MHz. The
channels monitored were:
a. Ammeter: Measuring current at the load resistor
b. Ammeter: Measuring current leaving the power distribution panel, after the OCPDs
c. Voltmeter: Measuring voltage at the power distribution panel
d. K type thermocouple, same thermocouple in fuel spray as observed using Labview
e. Pressure Transducer
E.
Test Matrices
A total of 298 tests were conducted during this program: 253 controlled damage and 41 ballistic tests. Of the
controlled damage tests conducted, 111 were short circuit tests (48 without over current protection and 63 with the
OCPDs included), 92 were dynamic separation tests (59 without over current protection and 33 with the OCPDs
included), and 50 were partial diameter loss tests (48 without over current protection and 2 with the OCPDs
included). All of these tests were conducted with the fuel spray. The number of configuration repetitions was
determined during testing on a test-by-test basis. Tests were repeated to verify thresholds of voltage/amperage
combinations necessary to generate a fire. When a threshold was identified, tests bordering the threshold were
repeated to bolster confidence in the results.
Table 1 includes a simplified version of the test matrix, showing only the tests which are relevant to the focus of
this paper. The test combinations highlighted in red represent combinations which were tested. Combinations that
are crossed out were not tested, based on knowledge gained from the preceding tests. Originally, tests including 115
VAC circuits were also to be tested. Presenting a realistic 115 VAC short circuit test condition proved difficult
because of facility electrical service safeguards (i.e., breakers).5 The customer was in agreement that the 115 VAC
tests were of lower priority. On the aircraft, these circuits are protected with SCBs common to household electrical
service. Because of the nature of low frequency alternating current (<1 MHz), AC is less likely to sustain an arc.
The arc is self quenching every half cycle as the current reverses polarity. The power at maximum current within
each cycle is equivalent to the power available in a direct current circuit of the same configuration. The real power
available to do work in an AC circuit is the RMS value of the AC current (2/3 of maximum amplitude for a pure sine
wave), and is therefore always less than the available power for an equivalent DC circuit over periods greater than
one cycle. Because of this, evaluating the DC circuits effectively defines the boundaries of “worst case scenarios”
within the total selection of configurations available in the test matrix. A fault condition in an AC circuit will be
less of a threat for arcing and fires than the same fault in an equivalent DC circuit. Though the original intent was to
F.
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test AC circuit protection; for the stated variety of reasons, this testing was delayed and finally judged to be
unnecessary.
Table 1. Controlled Damage Test Matrix
General Test
Combination
#
1
2
3
4
5
6
Test Type
Controlled
Damage
In Total:
257 Tests were
Completed
Test Combination Description
270 VDC Baseline using a spray of JP-8 (48 tests completed)
28 VDC Baseline using a spray of JP-8 (65 tests completed)
115 VAC Baseline using a spray of JP-8 (not tested)
270 VDC w/ OCPDs using a spray of JP-8 (97 tests completed)
28 VDC w/ OCPDs using a spray of JP-8 (156 tests completed)
115 VAC w/ OCPDs using a spray of JP-8 (not tested)
Pretests: Effect of Altitude on Arcing and Fire Ignition
In theory, and in some physical circumstances, altitude affects electric arc maximum distance, intensity, and
potential for fire ignition/sustainment. The test article incorporated a vacuum chamber to simulate testing from sea
level (~14.7 psi) to an altitude of 30,000 ft (~4.3 psi). The fuel spray was provided by an oil burner nozzle that
produced a finely atomized fuel spray spread over a 70 degree cone, which moved at low velocity.
Results of altitude comparisons, as summarized in Table 2, indicated that current density (affected by both
current and wire gauge) had a larger effect on fire ignition/sustainment results than altitude. Within this set of test
conditions, the only differences caused by ignition at altitude versus ignition at sea level were observed in the
physical appearance of the flame front propagation during the early stages of ignition. Once a sustainable fire was
fully developed, the flame was generally more diffuse in conditions simulating 30,000 ft altitude. For the conditions
provided within the capabilities of this test article, there was general agreement in the results as to the likelihood of a
fire ignition. Examining fire results from unprotected circuits at the extremes of the test matrix variables: high and
low voltage, high and low amperage, and high and low wire gauge, short circuit fire results were almost always
identical, regardless of altitude. This revelation allowed testing to progress more rapidly by both shortening the test
matrix and streamlining the testing procedures, since testing simulating a higher altitude was discontinued. Example
flame images at sea level and simulated 30,000 ft. are shown in Fig. 14.
G.
Table 2. Comparison of Results at Sea Level vs. Simulated 30,000 ft Altitude
Ratio of Sustained Fire/Repeats, by Configuration, Altitude,
and Electrical Fault
Circuit Configuration
Voltage
Amperage
AWG
28
28
28
28
270
270
22.5
25
5
50
5
95
12
12
22
8
20
4
Altitude
Sea Level
30,000
pull-off
short pull-off short
1/3
0/3
0/3
0/3
0/3
0/3
0/3
3/3
0/3
3/3
0/3
1/3
2/3
3/3
3/3
3/3
3/3
3/5
3/3
2/4
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Flame Progression at Sea Level (14.7 psi)
Flame Progression at 30,000 ft (4.3 psi)
Figure 14. Flame Progression By Altitude
Results: Unprotected versus Protected Circuits
In controlled damage studies, the most likely form of damage to cause electrical arcing was a dynamic
separation. The least likely form of damage to cause electrical arcing was the conductor diameter reduction. No
overheating or catastrophic damage resulted from manually removing 90% of the diameter of the wires tested. It
was reasoned to be extremely unlikely that a ballistic event could result in a percentage removal greater than 90%,
without entirely severing the wire. Thus 90% removal was considered worst case. In general, for the controlled
damage conditions, as the total available power increased, so did the likelihood of initiating a sustained fire.
It was unlikely that testing 115 VAC would reveal new or helpful information regarding arc intensity.
Furthermore, an arc formed by alternating current self-extinguishes every half cycle, as the current passes through
zero while reversing direction. The effect of this limits AC power to shorter arcing distances than equivalent DC
power. At the voltages and atmospheric pressures present in this test program, the frequency of an alternating
current could not contribute beneficially to arc formation unless alternating at very high values (MHz), wherein
alternating electric and magnetic fields may participate to ionize molecules in air (or fuel and air) around the
electrical fault, then thus generating a conductive path for an arc..
The only damage condition which allowed OCPDs to react was the short circuit condition. Table 3 summarizes
the likelihood of starting fires within the controlled damage portion of the testing and compares protected and
unprotected circuits. Configurations listed as “untested” within this table were omitted based on knowledge gained
as testing progressed. It is notable that in the 28 VDC short circuit test cases, different types of OCPDs exhibited
very clear differences in their ability to react to an electrical fault.
It was apparent that some OCPDs do provide some added fire protection against fire ignition initiated from
electrical arcing, particularly involving low amperage/thinner wire 28 VDC circuits. The OCPDs demonstrated that
they may trip if a short circuit occurs, however, they did not always do so. Some devices were quite effective in
suppressing fires during controlled damage short circuits, while others were not particularly effective.
H.
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Table 3. Fire Ignition and OCPD Performance
Configuration
Pull-Off
Short Circuit
% Fires
(of #)
% Fires with
OCPDs
% OCPD
Trip
25% (8)
16.67% (6)
0% (6)
16.67% (6)
33.33% (3)
33.33% (6)
0% (3)
0% (9)
% Fires
with
OCPDs
Untested
16.67%
Untested
0%
0%
33.33%
0%
Untested
0% (3)
0% (6)
0% (6)
0% (6)
0% (6)
66.67% (6)
66.67% (6)
100% (6)
0%
0%
0%
0%
0%
0%
0%
0%
0% SCBs
0% SCBs
0% SCBs
100% SSPCs
0% SCBs
100% SSPCs
0% SCBs
100% SSPCs
22
0% (7)
Untested
100% (15)
0% SSPCs
66.67% SCBs
100% SSPCs
0% SCBs
4
8
12
12
16
20
100%
100%
100%
100%
100%
100%
Untested
Untested
Untested
Untested
Untested
Untested
100%
100%
100%
100%
100%
100%
55.56%
100%
100%
100%
33.33%
0%
100%
100%
100%
100%
100%
100%
Voltage
Amperage
Wire
Gauge
% Fires
(of #)
28
28
28
28
28
28
28
28
75
50
25
22.5
15
12.5
10
7.5
4
8
12
12
16
16
18
20
28
5
270
270
270
270
270
270
95
60
30
22.5
12.5
7.5
Arc Energy Density: Controlled Damage Arc Analysis
This section describes the analysis methodology used to estimate the amount of energy present in each of the
electrical arcs observed in controlled damage testing. Initially, in the test planning phases of this program,
determination of instantaneous energy available in the arc was the primary factor of concern. Evidence gained in
testing indicates that energy distribution was also an important factor. Estimates of arc energy density plotted
against arc duration, protection, and fire ignition show several things of potential interest. Calculations (using the
analysis method described below) for estimating arc energy density were performed using tools in a graphical
software package, DPlot, produced by Hydesoft Computing, LLC.
I.
Analysis Method
1. Only tests that produced an arc are be included in analysis and graphics. Arc times are roughly measured from
the high-speed video data and/or current and voltage data recorded by the Nicolet data acquisition system.
2. Fault/arc Nicolet data was trimmed from the beginning of the arc time to the end, or where fire ignition was
evident, whichever was shortest.
3. Power was calculated using P=IV, with millisecond resolution through entire arc duration.
4. Available energy was calculated by integrating the Power Curve with respect to time using the trapezoidal
method.
5. Energy density was estimated by dividing the available energy over the cross-sectional area of the conductor.
Raw data from an arcing event often shows a noisy transition occurring over a very short period of time, as seen
in Fig. 15. This circuit was operating normally as the electrical fault was introduced. As the break occurs, current
drops sharply to zero amps. Zooming in, as in Fig. 16 and Fig. 17, shows current and voltage fluctuating in the
circuit (blue and red, respectively). The dashed lines indicate the beginning and end of the electric arc. Figure 18
shows the processed data after voltage and current have been multiplied to produce the trace “Instantaneous Power”
(green). This is an expression of the power present in the arc at any given instant during the event. This curve is
integrated with respect to time, using the trapezoidal method (since the data is not a well behaved function). The
result of this integration yields the total arc power in watts (purple). To estimate the energy of the arc, integrated
power is multiplied by duration to obtain Joules. This value is then divided over the cross-sectional area of the
conductor to obtain an estimated energy density.
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Supply side
voltage rises
Noisy transition shows arcing
Amperage drops
Figure 15. Electrical Arc Raw Data
Figure 16. Arcing Event: Voltage and Amperage Zoomed
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Arc Duration
Figure 17. Arcing Event: Current and Voltage Trimmed
Figure 18. Integrated Arc Power
Once an estimate of arc energy density was obtained, energy densities were plotted as shown in Fig. 19. It is
evident that energy densities above 0.1 Joule per mm2 greatly increased the likelihood of a fire ignition in this JP-8
spray. The lower limit of energy density to initiate a fire was 0.00194 J/mm2. OCPDs made little difference in
protecting against fires once an electric arc formed for over arc durations of 19 to 78 ms, above 0.1 J/mm2. With arc
durations less than 19 ms, all protected circuits were successful preventing fire. A comparison of OCPD
performance against energy density is made in Fig. 20.
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Figure 19. Estimated Arc Energy Density vs. Duration for Tests where Arcing Occurs
Figure 20. Arc Energy Density vs. Protection for Tests where Arcing Occurs
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III.
Discussion and Conclusions
It was apparent at the conclusion of this program that some OCPDs do provide some added protection against
fire ignition initiated by electrical arcing. This was most evident during controlled damage tests, particularly
involving lower amperage in both 28 VDC and 270 VDC circuits. Contactor/breakers also showed some capability
to protect against fires in the highest amperage 270 VDC cases. However, the only circumstance in which any
OCPD may provide protection is in the case of a short circuit. The OCPDs demonstrated that they may trip if a
short circuit to the fixture occurs, however, they did not always do so. From the condensed OCPD performance
assessment in Table 4 it can be seen that SSPCs were quite effective in suppressing fires during controlled damage
short circuits, while contactor/breakers were not particularly effective. Recalling Table 3, conductor diameter and
circuit power had much larger roles to play in whether a fire was initiated, since the contactor/breakers tripped 100%
of the time. SCBs were slow to react and, therefore never tripped. SCBs tended to be used in circuits in which a
dead short was not likely to start a fire within the timeframe of the test. Had the test period been allowed to be
extended indefinitely, it is possible that SCB tripping would have been observed, and it is also possible that more
fires may have been initiated. Two fires were observed in SCB-protected short circuit tests that produced hot
surface ignitions before, or in the absence of arcing.
Table 4. OCPD vs. Controlled Damage Short Circuit Performance
OCPD Type
None
SCB
SSPC
Contactor/Breakers
Short
Circuit
w/ Fire
32
2
4
11
Short by OCPD
Type Total
48
18
30
15
OCPD
Tests
with
28
VDC
27
18
15
0
OCPD
Tests
with 270
VDC
21
0
15
15
28
VDC
Short
with
Fire
12
2
0
N/A
# of 28
VDC
OCPD
Trips
N/A
0
9
N/A
270
VDC
Short
with
Fire
20
N/A
4
11
# of
270
VDC
OCPD
Trips
N/A
N/A
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In controlled damage studies, the most likely form of damage to cause electrical arcing was a dynamic
separation. The physical reasons for this, though interesting and worthy of further investigation, were beyond the
scope of this paper. In controlled damage testing, the least likely form of damage to cause electrical arcing was
partial damage removal (90% was tested). No overheating or catastrophic damage resulted from manual removal of
90% of the diameter of the wires tested. It was reasoned to be extremely unlikely that a ballistic event could result
in a percentage removal greater than 90% without entirely severing the wire. In general, under controlled damage
conditions, as the total available power of the test configurations increased, so did the likelihood of initiating a
sustained fire.
The effects of altitude on arcing and arc induced fire ignition were of substantial concern at the beginning of this
program. Testing showed that in this particular setup, altitude did not have a noticeable effect on the likelihood of
ignition/sustainment of a fire. Testing did show observable differences in how an ignited fire propagated at altitude,
and what the final form of the sustained fire looked like.
Estimates of energy density allowed the construction of a graphical representation with which to view the
occurrences of a sustained fire and electric arc energy against any number of variables, including electrical fault
scenarios, circuit descriptions and values, protection methods, and protection behaviors. Only a few of these are
included in this paper. The analysis was helpful in determining an important energy density value for this setup.
The likelihood of fire ignition and sustainment noticeably increases with arc energy densities greater than 0.1 J/mm2.
The results of this analysis would be of future value as a basis for continued analysis of the physics of fire ignition,
or to calculate how much energy was actually required to ignite a JP-8 spray. This in turn could be useful in
vulnerability and fire prediction modeling.
Additionally, in future vulnerability testing related to electrical arcing, it is recommended that thermal and
photonic data collection be added to the setup. Heat flux gauges and photodiodes would help quantify heat/light
transfer to surroundings (e.g. fuel spray) and differentiate between energy present within the arc and the arc’s
contribution to fire ignition. This data would be helpful to other fire and vulnerability modeling programs.
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Acknowledgments
The authors would like to thank the many InDyne, Inc. technicians and test engineers, particularly Mr. Jeff Bird,
who designed the test fixtures and electronics for this program, as well as all the government and industry personnel
who were instrumental in the planning and execution of this endeavor. This includes the Joint Aircraft Survivability
Program Office, Mr. Pat O’Connell and Mr. Greg Czarnecki (46 TG/OL-AC), and Dr. Lenny Truett (Institute for
Defense Analyses).
References
1. Franklin A. Fisher. Some Notes on Sparks and Ignition of Fuels, NASA/TM-2000-210077 Lightning Technologies Inc.,
Pittsfield, Massachusetts, March 2000
2. McCabe, Colin P., and O’Connell, Patrick J., “Arcing Survivability,” Aircraft Survivability, (to be published).
3. McCabe, Colin P., and O’Connell, Patrick J.: Live Fire Test # 06, Electrical Arcing / Fuel Ignition Test, Final Report,
TERN XG SV-LF-06, CDRL A008 Volume 4.06, Doc. No. 2YMA00076, June 2009.
4. Croft, Terrell, and Summers, Wilford, American Electricians’ Handbook, Eleventh Edition, McGraw Hill, New York
(1987) ISBN 0-07013932-6, pages 6-124.
5. Ehman, Scott. AVSF Range 2 and 3 Operations, 46OG OI 91-1, September 2004.
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American Institute of Aeronautics and Astronautics