Solid-state power controllers (SSPCs) are designed to replace mechanical circuit breakers, which work by tripping when the temperature of their internal sensors reaches a predetermined level (Fig. 1). Both forms of circuit protection are intended to prevent system wiring and components from overheating.
Like most circuit breakers, SSPCs operate in accordance with the time-current trip curve referred to in MIL-STD-1760 as the I2t curve, where I is the load current and t is time (Fig. 2). This curve plots out the allowable safe operating region for energy dissipated in a conductor before damage by I2R heating occurs. As the curve indicates, the larger the overload the shorter the conduction time allowed.
The SSPC senses the current in the load and either instantly trips in severe overloads or tracks the current behavior in minor overload conditions to determine if and when to trip. The controller also generates basic status signals for fault indication and for monitoring SSPC health. Unfortunately, some arcing conditions have the ability to damage or destroy wiring but do not trip standard thermal circuit breakers or today's SSPCs. This is because, despite their danger, such arcing conditions do not possess enough energy in the form recognized by thermal circuit breakers and SSPCs. The red dots in Fig. 2 depict how arcs can take place in otherwise safe operating regions well under the trip curve of the overload protection device.
What is an ARC Fault?
Arcs faults are discharges of current from one conductor to ground or to another conductor that result in some form of intolerable damage. All arcing events are not arc faults. In normal aircraft operation, the opening and closing of mechanical contacts, on a system carrying sufficient current and voltage results in some level of arcing. The variations in aircraft equipment and wire routing and bundling result in extremely complex and highly variable current signatures. These variations preclude the definition of arcing faults in physical terms such as luminescence, heat generation, rapid current rise or even regularly occurring flat portions at the zero cross over of an ac wave. All of these conditions may be found on a normally operating aircraft.
There are two major types of arc faults: series and parallel (Fig 3). Unsafe faults can occur as both types of arcs. A series arc can occur when the conductor in series with the load breaks, such as could occur at a loose terminal connection. The series configuration means the arc current cannot be greater than the load current the conductor serves. Typically, series arcs don't develop sufficient thermal energy to create a fire.
More dangerous is the parallel arc fault, which can occur as a short circuit or a ground fault. A conductor-to-conductor circuit arc decreases the dielectric strength of insulation separating the conductors, allowing a high-impedance, low-current arc fault to develop that carbonizes the conductor's insulation. This further decreases the dielectric of the insulation separating the conductors, resulting in increased current, exponentially increased thermal energy and the likelihood of a fire. The system impedance and the impedance of the arc fault itself limit the current flow in a short circuit, parallel arc fault.
A ground fault parallel arc fault can occur only when a ground path is present. A GFCI or an arc-fault circuit interruption (AFCI) can clear this type of arc fault. The RMS current value for parallel arc faults will be considerably less than that of a solid, bolted-type fault. Therefore, a typical circuit breaker might not clear this fault before a fire ignites. Fig. 4 shows two harnesses subjected to arc faults. The one on the left, which was protected with a conventional circuit breaker, was totally destroyed. The harness on the right was protected by a circuit breaker equipped with ARC Shield technology from Texas Instruments.
Arc Fault Circuit Protection in Aircraft
The issue of aircraft wiring safety has received widespread attention in recent years, highlighted by the unfortunate TWA 800 and Swissair 111 tragedies. As a result of these incidents and other concerns, the issue of wiring safety has been taken up by OEMs, regulatory agencies, the military and is being addressed as part of the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC).
According to industry sources, there is at least one “smoke-in-the-cockpit” incident per week in the United States on commercial aircraft. These can result in unscheduled landings and compromise aircraft safety. The military also has extensive documentation of arcing issues affecting its fleet of planes. Many of these smoke incidents as well as numerous unseen conditions in cargo holds and electronics bays are the result of wiring faults.
A number of recommendations have been made to improve aircraft wiring safety, including the development of arc fault circuit protection for wiring systems. Arc-fault circuit interrupters (AFCI) detect potentially hazardous arcing conditions and prevent catastrophic damage caused by electrical fires.
AFCI technology provides protection against dangerous arcing conditions that can result in fires. Arcing conditions are inherently chaotic and non-linear. They may be sporadic and intermittent, depending on the nature of the fault, the condition or age of the wiring and connections or other circuit parameters. Arcing conditions tend to worsen over time due to arc path carbonization.
The requirements for detecting and protecting against arc fault conditions are complex. The aviation industry requires enhanced protection that provides improved levels of safety, but cannot tolerate technology that results in nuisance trips that unnecessarily disable capable circuits.
Because space is limited space in aircraft, wiring often runs in bundles to minimize the space occupied by wire harnesses. Wire bundles on aircraft typically contain wires from many different circuits bundled together, sometime as few as three or four to more than 50 wires in a bundle. The sudden and extreme current changes due to arcing events on one line can couple a magnetic field that imposes an arc-like signal on adjacent wires in the same bundle. This is referred to as crosstalk.
Many loads are also powered by a common source. When an arc occurs on one branch of a source, it can drag down the voltage of that source in rapid succession, allowing an arc-like signal to be imposed on an adjacent circuit that shares that source. Ensuring these induced signals do not produce a nuisance trip is of prime importance.
A substantial amount of research has been conducted to understand and characterize arcing phenomena, and to qualify and quantify conditions specific to end applications. Key characteristics of reliable arc-fault detection include:
Ability to differentiate between normal load current and arc current
Insensitivity to cross-talk signals
Ability to sense a small arc current in the presence of large load currents
Insensitivity to RFI/EMI
Immunity to normal load start-up transients
High reliability — no false or nuisance trips.
Texas Instruments has developed intellectual property (IP) that addresses these characteristics, and provides an AFCI solution that can be tailored for aerospace, shipboard, land vehicle and other electrical circuit applications where the presence of an arc fault is intolerable. This IP is initially provided as an “arc shield module” with an embedded microcontroller and algorithms. This module (shown in Fig. 5) is then interfaced with the existing SSPC hardware of National Hybrid (NHI). The next generation of these products (currently in development) will absorb the algorithms into a custom ASIC with other SSPC functions.
Theory of Operation
On the surface, the implementation of arc-fault interruption is fairly simple. The hardware consists of one or more current probes processed by some simple circuitry and some software algorithms in a microcontroller or DSP. Monitor the current, check for abnormalities and sound the alarm. The problem occurs when you realize that to identify abnormalities you must first define normal conditions.
AFCI technology in aircraft applications must meet stringent electromagnetic interference (EMI) requirements. The arc detection circuit must not be damaged and must not be affected to the extent that it causes a nuisance trip during radiated and conducted EMI susceptibility testing, as well as lightning burst simulations. The AFCI circuit also must not emit EMI noise into the aircraft environment that will interfere with nearby sensitive electronic components.
Ideally, all circuits on an aircraft should be protected with AFCI technology to ensure complete system integrity. Wires for separate circuits are closely coupled and wire bundles are often routed adjacent to each other, resulting in a condition where a catastrophic fault of one wire could damage or impair multiple circuits in common or adjacent wire bundles. Thus, one unprotected circuit could compromise an entire wire bundle(s) of protected circuits.
Based on its proprietary technology, Texas Instruments developed a family of AFCI products that can mitigate hazardous arc-fault conditions and improve safety. NHI is an approved user of the IP that not only detects high-level arcing conditions, but also is sensitive enough to detect low-level series arcing without causing a nuisance trip.
Fig. 5 is the block diagram of a typical SSPC integrated with arc-fault detection technology. A sample of the output current is supplied to the arc-fault circuitry. The sample has an output sensitivity of 10 mv/A and is scaled to an appropriate level for the Arc Shield module in signal conditioning circuitry. An “Arc Trip” command from the AFCI circuit tells the SSPC that there has been an arc event. The “Reset” input allows the SSPC to reset the arc-fault detection circuit. The Reset input and Arc Trip outputs are both TTL-compatible signals.
NHI is integrating this arc-fault detection technology into its standard line of 28-V, 150-V and 270-Vdc, 10-A and 20-A SSPCs as well as in its 240-Vac, 25-A model. These enhanced units are the same size as the present models.
Advantages and Shortcomings of AFCI
The advantages of AFCI as previously mentioned are the ability to detect arc faults from normal arc events (i.e., switch and contact closures) and the ability to interrupt or stop an abnormal arcing event before further damage occurs. The addition of AFCI to SSPCs also exhibits an attractive cost-to-benefit ratio.
A shortcoming of present arc-fault technology is that it is not foolproof in preventing some small level of nuisance trips due to unique loads. This limitation still exists, but improved algorithms are reducing these occurrences.
The incorporation of Arc Shield Technology into Solid State Power Controllers for aircraft and other applications provide enhanced protection for wiring harnesses and associated equipment, and will greatly reduce the possibility of fires. The addition of AFCI in any SSPC or circuit breaker will produce a safer electrical environment, especially in aging aircraft. Next-generation AFCI will add arc-fault location as a feature, which will save hours of searching in harnesses for worn spots subject to further arc events.
Thomas Potter and Michael Lavado, P.E. Texas Instrument White Paper Presentation, “Arc Fault Interruption Requirements for Aircraft Applications.”
Thomas Potter and Michael Lavado, P.E. Texas Instrument Presentation. “Aging Aircraft 2003; Methods of Characterizing Arc Fault Signatures in Aerospace Applications,” July 2003.
Hendry Telephone Products Presentation. “Arc Detection, Arc Fault Circuit Interruption.” Presented to the Protection Engineering Group, April 12, 2000.
Kevin A. Mussmacher, P.E., and William L. Froeb. “SSPCs Handle Heavy Loads with Foldback Current Limiting,” Power Electronics Technology, January 2003.
For more information on this article, CIRCLE 335 on Reader Service Card