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Larry X Schneider
Pulsed Power Sciences Center
Sandia National Laboratories
Albuquerque, NM

The article entitled “Wired for Success: Ensuring Aircraft Wiring Integrity Requires a Proactive Systems Approach”, which was published in Vol. 8,
No. 3 of the AMPTIAC Quarterly (a predecessor to the AMMTIAC Quarterly), established the importance of maintaining wiring integrity, especially in aging aircraft. The article also emphasized the need for nondestructive approaches for evaluating the integrity of wiring systems and components. The current article answers the mail in terms of providing a nondestructive inspection (NDI) method to proactively ensure wiring integrity. – Editor

INTRODUCTION
Detecting and locating insulation defects in wiring systems is a nontrivial challenge. Ideally, the diagnostic should be nondestructive, capable of detecting a variety of insulation defects, such as cracking, chaffing, and abrasion, and be able to locate the defect accurately to reduce potential inspection and maintenance costs. The diagnostic should also have a low rate of false detection. Complicating this quest is the nature of complex wiring systems. For example, aircraft wiring systems are comprised of harnesses containing periodically bound single wire pairs* which create highly non-uniform impedance characteristics due to the varying distance between wire pairs.
Insulation defects may also be physically small, resulting in an immeasurable change in the impedance at the defect site. These characteristics tend to render impedance measuring or conventional time-domain-reflectometry (TDR)† concepts ineffective. Other approaches such as partial discharge or DC breakdown techniques may detect some types of insulation defects, but cannot locate them in complex impedance wiring systems (Figure 1).
PULSE ARRESTED SPARK DISCHARGE
Pulse Arrested Spark Discharge (PASD) has been demonstrated to be an effective insulation diagnostic for coaxial cables, twisted shielded pair, and more complex single wire bundles commonly

found in wiring systems in aircraft and control and monitoring devices. PASD employs a patented, low-energy, arrested-arc concept and time domain reflectometry techniques to produce a welldefined reflection from an insulation defect site. The PASD diagnostic can detect and locate a range of electrical insulation damage in cable/wire bundles with non-uniform impedance profiles.
PASD was first demonstrated on electrical wiring systems in 1996 under a Department of Energy (DOE) sponsored nuclear energy program. Success during the initial development of PASD for the
DOE led to a contract with the U.S. Navy to explore the use of this concept for wiring in control and power systems. This work, completed in August 2001, demonstrated the ability of this technique to identify and locate gross defects, as well as difficult to find small-volume dielectric defects in a laboratory environment [1].
This foundational work with PASD led to a three-year Federal
Aviation Administration (FAA) sponsored program, beginning in
October 2002, that focused on commercial aircraft wiring systems.
This FAA program culminated in the practical utilization of the
PASD technique. PASD has demonstrated great potential to locate aircraft wiring flaws‡, which are due to aging processes, manufacturing defects, maintenance, and installation damage, as well as a variety of other defects in more conventional wire/cable systems [2].

Figure 1. Inspection of Aircraft Wiring Systems.
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Reflected Transmitted
Pulse Pulse

Incident
Pulse

Insulation Defects
(Cracks, Abrasion, etc.)

Sensors

Insulator

Terminating impedance does not affect the
PASD
technique

Defect

Conductor
Insulation

PASD
Pulser

Insulation
Conductor

Ground Plane

Ground Plane

Figure 2. The PASD Breakdown Can Occur Between Two Adjacent
Conductors With Insulation Damage, or from a Conductor to a
Ground Plane, Such as Braided Shielding.

Figure 3. PASD Diagnostic Concept for Locating Defects in Electrical
Wiring Systems. (A Single Conductor is Shown Under Test.)

HOW PASD WORKS
The PASD technique uses a high-voltage (approximately 10 kilovolts), low-energy (a few millijoules), short pulse to induce an electrical spark discharge (breakdown) at the site of an insulation defect (Figure 2). The PASD pulse can be generated by a compact, battery driven pulser (Figure 3) and combined with diagnostics that includes software processing in a package the size of a small suitcase.
The pulse energy is similar in magnitude to the static discharge generated when walking across synthetic carpets and is insufficient to damage typical insulation or the conductors within wiring systems.
The discharge occurs from the conductor in the wire under test to an adjacent return path which typically is another wire or ground plane (e.g., aircraft skin). The development of an arc occurs within a few nanoseconds and produces a momentary short circuit that reflects energy back to the sensors at the injection point.
Conventional TDR techniques can then be used to accurately locate the defect site. This technique is a direct test of the dielectric strength of the insulation system. Since the PASD voltage is insufficient to breakdown any significant amount of bulk insulation, only defects which expose the center conductor are identified.
The technique begins with characterizing the impedance of the wire under evaluation, much like a conventional TDR technique, using the PASD pulse at a low voltage setting (hundreds of volts).
This establishes an impedance baseline which is critical to PASD’s ability to evaluate non-uniform impedance wiring systems. The injected pulse voltage is then raised in gradual steps to explore for defects. If a sufficient defect is present, the sensor waveform will deviate from the low voltage baseline characterization.
Figure 4 shows example waveforms derived from PASD tests on a coaxial cable with a 0.7 mm hole punched through the dielectric.
The inner conductor and outer shield remain fully conductive. The

two waveforms deviate at the location of the defect, 78 nanoseconds
(ns) down the line. In this example, the peak PASD pulse was
9.8 kV at a pulse width of 5 ns. The amplitude scale is the voltage measured at the sensor. The location of the anomaly between the two waveforms describes the physical location of the defect site.
Figure 5 shows PASD waveforms from a test of a damaged twisted shielded-pair cable. Although the insulation and electrical shielding is damaged, the inner conductors and shields remain electrically conductive. The impedance of this cable is highly nonuniform, as evidenced by the multiple reflections in the PASD baseline (solid line). The dashed line is the normalized PASD pulse
(4.5 kV) which creates a breakdown at the defect site. The two waveforms separate at approximately 45 ns, accurately indicating the location of the defect. A simple differencing algorithm can easily locate the point where the two waveforms deviate, as illustrated in Figure 4 and Figure 5.
PASD can be applied using several different techniques, including multiple pulse injection which improves PASD’s response in long lines. PASD has been effectively demonstrated in wire harnesses up to 200 feet in length
NONDESTRUCTIVE NATURE OF PASD
While PASD is an effective test method, it would not be a viable diagnostic if it were a destructive technique and degraded the properties of the wiring or connectors under evaluation. Therefore, a series of experiments were conducted to carefully quantify PASD’s impact on typical wiring insulation materials including Teflon®, polypropylene, celluloid, and Mylar® [3].§** Several types of experiments were conducted to evaluate the impact of a PASD breakdown on insulator surfaces. They included: (1) exposing the sample to a series of PASD pulse breakdowns to evaluate whether the breakdown

3

Amplitude (Volts)

2
1

Damage Signature

0
-1
Below Defect Detection Threshold

-2
-3

0

10

20

30

40

50

60

70

80

90

Time (10-9 Seconds)

Figure 4. PASD Waveforms from a Test of a Coaxial Cable with a Pin-Hole Defect in the Insulation that Extends to the Inner Conductor.
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2

Detection Threshold
4.5kV - 5ns

1.5
Amplitude (Volts)

1
0.5
0
-0.5
-1
Waveforms Differ After This Point
Solid Curve - Below Detection Threshold
-2 Dashed Curve - At Detection Threshold
10
20
30
40
50
60
70
80
0
Time (10-9 Seconds)

-1.5

90

Figure 5. PASD Waveforms from a Test of a Twisted Shielded Pair Cable.

ABILITY OF PASD TO LOCATE AIRCRAFT WIRING DEFECTS
The PASD diagnostic has demonstrated that it can detect and locate chaffing, cracking, pin-holes and other breached insulation defects under laboratory conditions and during testing at the
FAA’s Airworthiness Assurance Nondestructive Inspection (NDI)
Validation Center (AANC) in Albuquerque, New Mexico [4].
These types of defects are documented in the Aging Transport
Systems Rulemaking Advisory Committee (ATSRAC) publications:
Intrusive Inspection Final Report[5] and Aging Transport Systems
Task 1 and 2 Final Report[6], which were comprehensive joint government-manufacturer-operator (industry) studies sponsored by the FAA.
Several types of defects were tested at the AANC test bed facility.
A mockup of an aircraft fuselage panel is shown in Figure 6 with a variety of 10 foot wiring harnesses installed. Figure 7 shows the results of an insulation chafe on a 10 foot wiring harness in the

Figure 6. Mockup of an
Aircraft Fuselage Panel with a Variety of 10 foot
Wiring Harnesses and
Connector Types. Each
Harness has Multiple
Documented Insulation and Conductor Defects.

testbed. A 15 ft cable was used to isolate the 10 ft cable under evaluation. The PASD waveforms separate at 84.4 ns indicating the presence of an insulation defect. The velocity of propagation in the cable under test can be easily measured or estimated and is typically 3 ns/foot for loosely bound wires. The fault location can be

10000.00
84.4 ns

Amplitude (Arb)

5000.00
0.00
-5000.00
-10000.00
-15000.00
-20

30

80
Time (10-9 Seconds)

130

180

Figure 7. PASD Probe Waveforms for an Insulation Chafe Defect.

20.00
15.00

Amplitude (Current)

voltage decreased with subsequent pulsing; (2) performing a slowly rising DC breakdown test, applying a PASD pulse, and then repeating the DC breakdown test; and (3) determining the effect of high energy discharges on low energy voltage breakdown strength. PASD did not reduce the surface flashover strength of any of the materials tested. The PASD arc breakdown process occurs very rapidly, with the arc channel falling to 10% of its initial resistance value in approximately three nanoseconds. This is an important characteristic as it improves PASD’s ability to locate defects. When exposed to slowly rising DC after the application of the PASD pulse, the average breakdown voltages remained the same or actually increased. This is a common high voltage conditioning phenomenon.

10.00
5.00
0.00
-5.00
-10.00
Defect at 246 ns
-15.00
-20.00
-20

0

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320
Time (10-9 Seconds)

Figure 8. PASD Waveforms from a Test of a 100 Foot Wire Harness with an Insulation Defect at 80 Feet.

readily calculated using the propagation velocity and injected pulse width. PASD identified the defect at approximately 8.8 feet from the injection end of the harness. The actual location of the defect was at 8.0 feet.
Figure 8 shows PASD waveforms from tests on a 100 foot pair of wires using a multipulse approach. In this example, an insulation defect was present with a 1.0 mm air gap between two conductors that resulted when a small amount of insulation had been stripped away. The two waveforms deviate at the location of the defect, 246 nanoseconds down the line, which equates to 79.3 feet. The actual location of the defect was at 80 feet. PASD located the defect to within 8 inches in an 80 foot long section of wiring. This accuracy would save significant maintenance time, especially if wiring is hidden behind panels or bulkheads as typically found in aircraft or marine vessels, such as ships or submarines. The variations in the waveforms in Figure 8 are caused by the varying impedance profile down the harness. The nature of modest impedance variations in
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wiring systems is not important, since PASD uses a differencing technique to locate the defect.
PASD performed well against an array of challenges presented by the AANC wiring test bed. A summary of testing results at the
AANC is shown in Table 1. With a 1 mm air gap, PASD found 12 out of 15 defects present, including challenging cracked insulator defects. PASD was 100% effective in locating chafe and breach defects with air gaps up to 1 mm. These results have not been matched by other diagnostic techniques.
Table 1. Test Results at the FAA’s AANC Wiring Test Bed.
Gap Spacing††
Quantity
Defects Detected
Defects Undetected

Insulation Chafe Defect
7
1 mm Gap
3 mm Gap
5
Insulation Breach Defect
1 mm Gap
2
3 mm Gap
6
Cracked Insulation Defect
1 mm Gap
6
3 mm Gap
1
TOTAL
27

7
3

0
2

2
3

0
3

3
0
18

3
1
9

SUMMARY
PASD is effective at detecting and locating a variety of insulation defects in complex wiring geometries. It is highly immune to line impedance variations, an important property in aircraft or other complex wiring systems, and has been shown to be nondestructive to electrical insulation materials. Due to the simplicity of the PASD concept, the low energy PASD pulser and diagnostics can be readily implemented into a portable diagnostic system. PASD shows great promise as an effective diagnostic to find difficult to locate insulation defects such as breached insulation, chaffing, and physically small insulation cracks. Although the PASD technique will likely evolve as it enters into field applications (pulse shape, testing strategy, etc.) it is capable of making a near-term impact on the ability of inspection and maintenance organizations to detect and locate potentially hazardous insulation defects.

ACKNOWLEDGMENTS
The authors would like to acknowledge the Federal Aviation
Administration’s William J. Hughes Technology Center for supporting the development of PASD for application in commercial aircraft wiring systems.
NOTES & REFERENCES
There are typically many (up to 40) individual wires which form two wire circuits. † TDR is a method used to detect defects in an electrical transmission system (typically conductive wiring). A voltage is sent through the system and the reflected signal is superimposed on the original to identify any anomalies, which indicate the presence of defects.
‡ PASD can detect flaws of any size as long as the wire insulation is removed down to the inner conductor. The flaw, for example, could be nearly invisible to the unassisted eye.
§ The effect of PASD on Kapton® insulation was not investigated.
However, since the PASD pulse energy is very low, it is not expected to damage this type of wire insulation.
** Teflon®, Mylar®, and Kapton® are registered trademarks of E.I. du Pont de Nemours and Company
†† The gap spacing refers to the physical air gap from the exposed wire conductor to the nearest ground plane or adjacent grounded conductor.
*

[1] M.A. Dinallo and L.X Schneider, Pulsed Arrested Spark Discharge
(PASD) Diagnostic Technique for the Location of Defects in Aging Wiring
System, Sandia National Laboratories Report, SAND2001-3225, October
2001
[2] L.X Schneider and R.K. Howard, Pulsed Arrested Spark Discharge
(PASD) Wiring Diagnostic, 8th Joint FAA/DoD/NASA Conference on
Aging Aircraft, February 2, 2005
[3] S.F. Glover, M.B. Higgins, G.E. Pena, and L.X Schneider, Assessment of the Non-Destructive Nature of PASD on Wire Insulation Integrity, Sandia
National Laboratories Report, SAND2003-3430, September 2003
[4] M.E. Dinallo, Aircraft Wire System Laboratory Development: Phase I
Progress Report, Sandia National Laboratories Report, SAND2003-2923,
August 2003
[5] Intrusive Inspection Final Report, sponsored by the Aging Transport
Systems Rulemaking Advisory Committee as chartered by the FAA,
December 2000
[6] Aging Transport Systems Task 1 and 2 Final Report, sponsored by the
Aging Transport Systems Rulemaking Advisory Committee as chartered by the FAA, August 2000

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Larry Schneider is a Deputy Director at Sandia National Laboratories’ Pulsed Power Sciences Center. His group supports a broad portfolio of projects in high voltage pulsed power sciences, analytical and experimental electromagnetics, and particle accelerator systems development for Department of Energy National Security programs, other federal agencies, universities, and industry. He received his B.Sc. and M.Sc. in Electrical Engineering from Oregon State University in 1978 and is a Registered Professional Engineer.

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