DN Staff

October 1, 2001

31 Min Read
The target: Electronics 'sweet spot'

Demand for more electrical power in cars is being paced by inclusion of more and more electronic functions-tasks including communications, navigation, entertainment, telematics, and energy saving electrically activated systems.

One example of energy saving systems is electric steering, or drive-by-wire. This system would eliminate a power-and mileage-robbing hydraulic pump that continuously runs on current cars. Among other features, emissions can be reduced and mileage boosted using infinitely adjustable, electrically activated engine valves and variable geometry air intake systems. One particularly bright component under development to take full advantage of the higher power level is the integrated starter/alternator. Mounted to the engine bell housing, it would permit such functions as stop/start, to eliminate idling fuel consumption, and regenerative braking to recover energy. A similar device is used on hybrid cars to boost engine power as well.


This representative 42V architecture features only one battery and a separate alternator and starter. Other proposals use an integrated starter/alternator to allow such functions as stop/start, to eliminate idling fuel consumption, and regenerative braking. System voltage (42V) when the alternator is running is slightly higher than the 36V battery voltage. A similar dual-battery systems places a 12V battery to the right of the 42/14 dc/dc converter to isolate starting and ignition off-loads from the main battery.

Lucky number. An advanced automotive working group based at MIT, now the 42V Consortium (formally the Consortium on Advanced Automotive Electrical/Electronic Components and Systems), agreed on the 42V value in the mid-nineties. Organization members include most companies who build and supply parts and the automakers as well. The consortium does research, and has assumed a leadership role in standards development (http://auto.mit.edu/consortium). The latter includes concerns such as battery terminal size and shape selection to avoid confusion and accidents with 14V-system components and interfaces, as during a jump start.

While more power is going to be needed, higher voltage seems the way to go. Just remember from circuit theory that voltage is electrical 'pressure' that forces current through wires. The greater the voltage, the more power (voltage x current) that can be carried by existing diameter wires-thus avoiding an increase in weight of wiring.

But why 42V to push the added power around? Why not 28 or 56V instead of the 14V electrical systems standard on today's vehicles? The answer lies in the cost of silicon chips balanced against the potential for wire arcing. On the silicon side, in the mid-nineties, David Perreault, now assistant professor at MIT, developed the switched-mode rectifier (SMR) alternator. He essentially removed the triple-diode pack in a conventional alternator for producing dc, and substituted power metal-oxide silicon field effect transistors (MOSFETs) instead.


As speed increases, MOSFET-equipped switched-mode-rectifier (SMR) alternators produce higher power levels than alternators using diodes. Better power factor (i.e., less phase shift) also produces more power from the same current in the machine.

Simply put, the SMR acts as an electronic version of a transformer, chopping voltage so the windings "see" a more optimal voltage than the battery-level voltage. As its speed of rotation increases, the power produced in the SMR alternator still increases, rather than leveling off as in the standard alternator (see figure).

But power MOSFETs cost more, and cost is proportional to the area of silicon contained in the chips. Cost comes down as the supply voltage increases, however, but the curve levels out around 42V (see figure). Under diminishing returns, any further cost reduction sees a much greater increase in voltage, which in turn raises concerns about arcing from exposed or broken wires causing fires.

42V is not a magic number, however. Any auto electrical system does not operate at one voltage. In fact, 42V is only the nominal running voltage for the system architecture. Different off-nominal conditions dictate different voltages where the system must work or survive. MIT's Tom Keim, consortium director, notes that on a cold day, wire resistance goes down and "nominal" voltage may surge to 50V continuously. As currently envisioned by the consortium, 21V would be the lower limit for full safety systems functionality during start, with 30V the limit while running. Maximum dynamic voltage would be limited by alternator design and suppression methods to 58V during a load-dump pulse of up to 400 msec. A load-dump occurs when the alternator is cranking out maximum power and its load becomes disconnected (i.e. a battery cable comes loose).


Silicon area of the power portion of a semiconductor switch, and thus cost, is inversely proportional to the norminal supply voltage -- until roughly 42V, when diminishing returns kicks in. This down trend must be balanced against the increasing potential for arcing as voltage goes up.

Transition. While arcing may be the major concern in upping automotive electronics' voltage, other safety and safety- related issues have arisen. One concerns the transition to 42V systems, according to Joe Fadool, director of Siemens Auto-motive's Electrical and Electronics Distribution Systems Group (Troy, MI). Hybrid 14V/42V systems will come on line first, giving carmakers experience in limited applications. While one or a few systems can be accommodated with just up-converting voltage, as a greater fraction of electronics transitions to 42V dual batteries, will likely be installed, (a conventional 12V and a 36V system running voltage when the alternator generates power is slightly higher than the battery voltage level.) Such a dual architecture introduces added complexity and more failure modes as well.

Durability and performance of 42V components is an issue because of pushing higher currents through parts like switches and relays, says Shahram Zarei, technical specialist in research and vehicle technology at Ford. Redesign will be needed for components such as fuses and circuit protection devices. And, Zarei adds, a business case has to be made for 42V introduction. It has to have a high benefit/cost ratio for customers to accept and needs the economy of scale to push costs down.

Senior Sales Engineer David Goff of automotive electrical component supplier Omron Electronics (Schaumburg, IL) says the company is tackling power generation and distribution issues stemming from the higher voltage. Omron began prototyping relays in 1999 and switches in 2000 and looks to supply manufacturers with 42V electronic control units (ECUs) as well.


Many 42V electrical components, such as relays developed by Omron, will require coil rewinding for the higher voltage and dual series contacts (dividing voltage in half) with sufficient gaps to avoid higher voltage arcing that can fuse a contact. The 42V relay (center), which replaces either of the two present-size relays, can handle cojmparable power at 10A as the larger 14V mini relay (left) does at 30A.

In relays, Goff notes coils need to be rewound to function at 42V. "Contact gaps have to be sufficient to handle arcing," he adds, "and dual, series contacts will reduce this to an acceptable level." Hybrid relays that combine electronics to dissipate transient arcing energy then break the mechanical relay contact may also be developed.

"For high-current switches, contact redesign to address arcing is similar to relay issues," says Goff. With switches, relays, and connectors in 42V systems, Goff says higher dielectric materials will be needed for insulating conductors from each other within the same dimensions.

ECUs use regulators to convert system voltage down to the logic-level value of 5V. To minimize the conversion losses over the greater step down from 42V, diode-based switching regulators can be used rather than linear voltage regulators that have higher heat losses. For dual voltage interim systems, the ECUs would stay on the 14V-bus system.

Ford's Zarei adds, "Many devices cannot operate at 42V, such as lighting incandescent lamps for example. Perhaps voltage could be modulated for these. While the coils in small electric motors may need to be rewound by a factor of three, the space may not be available to then house them. There is also the issue of motor brush life."

One concern that turns out not to be an issue is electromagnetic interference (EMI), notes MIT's Keim. "This doesn't occur in this voltage range, and while some effects scale with current, these are still low," he says. "42V will not cure EMI, but it won't make it worse."

Keim and Zarei both conclude that 42V systems will first appear on luxury cars where any price increase is a smaller fraction of total cost. "42V will be used if and only if it will work," Keim says. He stresses the potential fuel economy benefits of having electric-driven systems that only draw power when needed (power steering, braking) as well as new ones more precisely controlled for economy and emissions reduction (integrated starter/alternators and electrically-activated intake and exhaust valves). Good design engineering will probably see practical realization of many of these benefits.


Finding (arc) fault

Taking automotive electrical systems to 42V introduces greater risk of arcing faults. Arcing is not only an issue between exposed (i.e. chaffed) conductors and ground (parallel arc fault) or between the ends of a broken or cracked wire (series fault), but also for design of mating contacts in switches and relays.

Engelbert Hetzmannseder, group chief of mechatronics for EATON's Innovation Center (Milwaukee, WI) says while an arc fault can look spectacular, "It is not a showstopper, but a challenge. Arc physics has been around for a long time and there are solutions for switching and detecting" (see DN 9/4/2000, p. 76). Arc-fault protection devices based on microelectronics and detection algorithms for ac circuits are available for building wiring and under test for aircraft systems. "For automotive, the principle is the same," notes Hetzmannseder, "but the algorithms are different for dc" in determining normal and abnormal transients. With dc, the voltage doesn't change polarity. If an arc fault is detected, electronics will then trigger a relay, breaker, or switch.

He adds that EATON has prototype devices but is looking for consensus from the OEMs on switching, packaging, and basic architecture (i.e. mechanical or solid state). Questions on standards for given wire lengths, current loads, and voltage ranges will also affect architecture decisions, Hetzmannseder says. He adds the OEMs may be treading cautiously because they haven't had to do such current and voltage sensing before.
Other issues are traditional switch and relay concerns, such as interrupt-arc contact lifetimes. Here for example, going to lower-load, solid-state switching FETs (field-effect transistors), which can also current sense and do diagnostics, may be worth their added cost, offers Hetzmannseder. He concludes, the bottom line for any 42V developments will be whether any increase in system costs are offset by greater fuel efficiency over the life of the car.


42VDC Arc Faults: Physics and Test Methods
Dr. Engelbert Hetzmannseder, Dr. Joe Zuercher
EATON Innovation Center

Abstract

The new 42VDC automotive power system raises a number of challenges that were not applicable to standard 14VDC systems, not the least of which is arcing fault mitigation.

Arc physics teaches us that, for supply voltages larger than about 20VDC, an arc can be sustained theoretically indefinitely, while at lower voltages, i.e. 14VDC, the current can be interrupted with a very short arc.

An arcing fault, which can occur at any point in the vehicle's life, might not be detectable with present short circuit or overload protection devices. In the case of an arc fault in parallel with the load for systems with supply voltages larger than 20VDC, fuses or breakers may trip late or not at all due to the intermittent nature of the arc with significantly lower peak current. If the arc is in series with the load, a fuse will never open since the fault current is always lower than the load current.

Metal-to-metal short circuits occur fairly seldom. Unintended arcing faults are the norm at supply voltages larger than 20VDC. In a new 42VDC automotive power system, an unintended arcing fault may cause a reliability issue or even a fire hazard.

Several test methods have been developed to simulate various potential series and parallel arcing faults' causes at 42VDC and to investigate their effects.

  1. Introduction

    The automotive industry is migrating to a 42VDC power distribution system as a result of an ever increasing number of electrical components and system's requirement to meet the user's needs for reduced exhaust emission, improved fuel economy, safety, comfort, and convenience. A 42 VDC system provides more power and higher efficiency. This higher voltage system is required to commercialize innovations in starter-alternators, electric steering, electric water pump, electric air conditioning compressors, and electric braking as well as electric drives for hybrid-electric and fuel cell vehicles.
    Many products and systems are being developed utilizing innovative solutions consistent with higher voltages. The power distribution and protection system is one of the first systems requiring change.
    Fuses, circuit breaker, or electronic-based devices protect conventional electrical AC and DC circuits from current faults. They are designed to interrupt when metal-to-metal short-circuits or current overloads stress the wiring harnesses' insulation. It has become apparent that arc fault protection is necessary in addition to overload and short circuit protection.

    Interruption of arc faults in AC circuits has already been investigated for residential [

    1

    ] and aerospace [

    2

    ] applications. Circuit breakers, with arc fault current interruption capability, are commercially available [

    3

    ] for residential applications. Arc fault interruption devices for aerospace circuits are under development with prototypes being tested for flight qualification [

    4

    ].


    Nuisance tripping of arc fault interruption devices is an issue, e.g. the interruption device in response to switching a load "on", or in response to an intermittent current or voltage situation. Nuisance tripping is particularly undesirable in aerospace applications.
    This paper will discuss the physics, the differences between 14VDC and 42VDC arcs and test procedures for simulating various DC arc faults. Arc voltage and test current traces for 100W - 500W circuits at 14VDC and 42VDC will be presented. Movies were recorded to visualize the effects of 14VDC and 42VDC arcing faults (presentation at conference, only).

  2. Arc Physics

    1. Creating Arcs, Arc Mechanism, and Arc Characteristics

      An electrical arc is generated during circuit interruption, if the circuit's characteristics make the creation of an arc possible. From physics, an arc fault is the same phenomenon as the interruption of a circuit with a switch. However, an arc fault is an unintended occurrence in which the contact area is not designed for making and breaking an electrical current.
      When interrupting a circuit with a pair of metallic electrodes, the contact force decreases as the contacts start to separate resulting in the heating of the contact spots until they melt. After further contact separation, the molten metal evaporates.
      The high electrical field in the very short contacts' separation gap and the evaporating metal yield ideal conditions for creating an arc.
      In case of an arc fault, i.e. when a "live" wire slightly touches the circuit ground, the initial contact force between the separating conducting surfaces is low. The contact spots melt and rapidly evaporate. An arc quickly forms.
      In most cases, an arc plasma is essentially super heated, conductive air at temperatures at which the molecules (mainly N2) become dissociated (N2(R) N+N, at about 6000 degrees C) and ionized (N(R) N+ & e-, at about 14000 degrees C).
      The current times voltage gradient (10VDC/cm - 50VDC/cm) of the arc column heats the arc column to maintain the temperature necessary to keep the arc conductive. High current arc column's temperature may be as high as 30000 degrees C.
      The existence of an arc requires a minimum current (IArc,min, &1A) and a minimum arc voltage (VArc,min 3 12VDC). These two arc characteristics depend on the electrode materials and the surrounding atmosphere (Table 1).


      Contact material


      UArc,min [V]


      IArc,min [A]


      Cu


      13


      0,43


      Fe


      13-15


      0,35...0.55


      C


      20


      0,01...0,03


      Ag


      12


      0,4


      Au


      15


      0,3


      Ni


      14


      0,4...0,5

      Table 1

      : Minimum arc current and minimum arc voltage of free burning arcs in air [

      5

      ]


      The anode and cathode voltage drops at the electrodes determine the minimum arc voltage. Hence, there is an arc voltage even at a very small contact gap. For a given electrode material, current value, and local ambient atmosphere (usually air), the arc voltage increases with increasing arc length.
      It is very important to note that an arc can occur at any supply voltage. At supply voltages lower than the minimum arc voltage, e.g. 12VDC or even 5VDC, the arc is fed by the energy stored in the inductance of the circuit. Once the arc dissipates the inductive energy, the arc extinguishes. Since practically every circuit has some inductance, an arc occurs at every current interruption at any supply voltage if the current is larger than the minimum arc current. In many cases the arc is not noticeable since its duration is very short, i.e. tARC &10 -9 seconds. The arc still maintains it's minimum arc voltage ( 3 12V, Table 1) due to dI/dt.
      At supply voltages larger than the minimum arc voltage, an arc can theoretically be sustained indefinitely. The difference between VSUPPLY and VARC maintains the current, which heats the arc. The principal of interrupting DC circuits is to create an arc voltage greater than the supply voltage. Two well known arc voltage generation schemes are arc splitting (2 arcs have at least twice the minimum arc voltage) or arc lengthening (the arc column adds 10VDC/cm - 50VDC/cm to the minimum arc voltage). In case of an arc fault, the arc can be lengthened due to electrodes' separation or electrodes' melting.

    2. Interruption of DC Circuits

      A simple resistive-inductive DC circuit with a series arc is shown in Fig. 1. If a "live" wire touches the system ground the load is the only circuit impedance. The separation of two metal contact members creates an arc as described in chapter 2.1.

      The current heats the arc as long as there is current, i.e. as long as the arc voltage is smaller than the supply voltage (42VDC). This is described by the V-I characteristic [

      6

      ] of the free burning arc in air as shown in Fig. 2. There is one characteristic curve for a specific arc length. These curves can be measured in resistive circuits for stable arcs with local thermal equilibrium, i.e. while the contacts open slowly.


      While the metal contacts touch (1 in Fig. 2), there is no arc voltage drop (according to the resistive load line). As soon as the contacts open just a little, the arc voltage across the small gap jumps to the minimum arc voltage along the resistive characteristic line (2). At the same time, the current is reduced (the arc limits the fault current). The arc voltage is opposed to the driving source voltage (R) (VSOURCE - VARC) reducing the net system voltage driving the current. Increasing the contact gap, the active point moves to (3) and then to (4) with ever increasing arc voltage and correspondingly decreasing current. Eventually the voltage across the gap jumps to the supply voltage (5) extinguishing the arc since there is no voltage left to drive the current.


      The points between (2) and (4) are all stable [ 7 ] arcs and can theoretically be maintained indefinitely if the arc length is kept sufficiently short. In practice, the very hot arc (TARC>6000 degrees C) destroys everything in its immediate surrounding thus increasing its length due to melting of the metal electrodes. In a 14VDC circuit, the resistive line [(1) - (6) in Fig. 2] does not cross any of the V-I curves. However, the inductive energy stored in the circuit will always be converted into arc energy during contacts' "break". Once the arc consumes this energy, the arc extinguishes interrupting the circuit.
      In 14VDC systems, dangerous arc faults do not occur since arcs cannot be sustained.
      The difference between a 14VDC and 42VDC interruption phenomenon was also illustrated in a series of tests conducted at Ford [ 8 ].

    3. Parallel and Series Arc Fault

    While there is a difference between a series and a parallel arc fault, the arc characteristics described above are applicable to both a parallel and a series arc.
    A series arc fault (Fig. 3a with high side switch) occurs on loose lugs, terminals, or broken wire. Disconnection of "live" connectors also falls under this category. Since these faults are in series with the load, the fault currents are less than nominal circuit currents. Fuses or breakers do not detect these faults!
    A parallel arc fault (Fig. 3b with low side switch) occurs when damaged wires (shaved, chafed, or aging insulation or broken wires) touch the system ground or an auxiliary low voltage bus. These faults are parallel to the load and hence higher than nominal currents are expected. However, due to the current limiting nature of arcs, the resulting fault currents can be significantly lower than bolted fault currents. Consequently, fuses or breakers clear the fault late or not at all! In a circuit with a low side switch, as shown in Fig. 3b, a parallel arc fault can occur even when the circuit is switched "off", e.g. a parked car.
    Both a series and a parallel arc fault can cause a significant reliability and fire hazard.

  3. Test Set-ups and Traces

    For 3A and 10A arc fault tests, the circuit impedance was a 5m long wire with cross sections of 0,732mm2 and 1,71mm2, respectively. The load currents were 3A and 10A resistive. The circuits were protected with fuses.
    Existing test standards for arc faults and wire harnesses were adapted and new test procedures established to simulate typical automotive arcing.

    1. Parallel Arc fault - Dangling Wire Test

The dangling wire test simulates a broken, dangling wire, such as may be found in the motor compartment. In the laboratory, an uninsulated automotive wire is brushed over a grounded metallic plate at about .50 m/s. The contact force, between the wire and the grounded metal plate, is achieved by the weight of the 0,3m long wire, only. Arc voltage and current traces are shown in Fig. 4.

Three possible circuit states can be identified in Fig.4:

  • Temporary short - this could result from temporarily welded contacts or a sliding contact. Source voltage and short circuit impedance determine the current.

  • Arcing - the wire lifts off the plate resulting in a contact gap. Due to the arc voltage, the current is significantly lower than the short circuit current.

  • Temporary open - the wire lifts off the plate resulting in a contact gap that is sufficiently large to interrupt the circuit.

Significant localized heating occurs during arcing when both current AND arc voltage are present. RMS responding monitoring devices "underestimate" local heating effects due to the lower duty cycle and high concentration of fault power.

The three circuit states above are also valid for the arc fault traces described in the following tests.

  1. Parallel Arc fault - Salt Water Drip Test

    The salt-water drip test simulates humidity and winter salt contamination conditions.
    The aerospace standard for wet arc-propagation resistance, MIL-STD-2223, part 3006, has been adapted to automotive circuit conditions.
    Small droplets of a saturated water/salt solution were applied every 5 seconds to two the exposed wire sections of insulated, adjacent, wires. The wires were arranged as shown in Fig. 5. At the beginning of the test, there was no or very little arcing between the wires. After a few minutes intermittent arcing of one second or longer occurred. Melting of the conductors usually terminated the test. The fuse usually did not protect the fault circuit.
    Fig. 5

    : Wire arrangement according to MIL-STD-2223, part 3006


    The arc fault lasted longer than shown in Fig. 6, since the capture time was only 1 second. Both current and voltage traces are very noisy and show short spikes of arc currents with temporary system voltage drops to the arc voltage level.

    • Parallel Arc fault - Saw Test The saw test simulates abrasion and shaving of wires' insulation, as described in aerospace standard, MIL-STD-2223, part 3007 (Fig.7). This standard defines the sawing frequency, blade dimensions and blade material (Al). For 42VDC automotive tests, the blade was constructed from automotive sheet metal and electrically grounded.
      Multiple, long arcing times are for each swipe of the saw blade (Fig. 8). In the 2nd event of Fig. 8, the average power was 2,1kW lasting 60ms. This represents over 120 joules of energy delivered to the localized fault area!

    • Parallel Arc faults - Guillotine Test (UL) The guillotine test simulates a cut wire, e.g. during an accident. The test fixture is similar to that specified in UL1699 for producing parallel faults and for qualifying arc fault circuit interrupters. As presently adapted for automotive use, the blade is grounded. Multiple current spikes result depending on the blade's motion and the blade's metal evaporation, Fig. 9. The RMS current values are much less than the peak currents recorded.

    • Parallel Arc Fault times To demonstrate the magnitude of the arc fault issue, the arc fault times of all the performed tests backed with fuses have been compared to the fault times of metallic short circuit faults backed with fuses (Fig. 11).
      Metallic short circuits pose little or no fire hazard since there is no hot arc involved (assuming proper fault clearance by the fuse). Depending on the type of test, the majority of arc fault times are significantly longer than the times for metallic short circuits in spite of the presence of fuses. The fuses do not shorten the fault time due to the reduced current and duty cycle.

    • Series Arcing Fault - Loose Connector / Lug Test

The loose connector and lug test reproduces loose connections utilizing automotive connectors and lugs. The loose contacts were opened and closed manually or by vibration. Disengagement of electrically "live" connectors also falls under this category of testing. Series arcs in a broken wire can be simulated and may occur when a broken wire is held in place by insulation or by a wire bundle.

The current and voltage traces, from the slow separation of connectors, are shown in Fig. 12. When a small gap (1mm) is maintained between conducting surfaces, a series arc can be maintained for many seconds significantly heating the metal. Series arcs usually are not as violent as parallel arcs since the fault current is always lower than the nominal current. While the heating effect is equivalent to a 100 watt light bulb glowing for 0,15 sec, a 15 joule event, it still represents a possible fire hazard due to the energy's concentration.

  1. Discussion

14VDC vs. 42VDC Arc Fault:

Only when the supply voltage is higher than the minimum arc voltage, can a stable arc be sustained. At supply voltages below 20VDC, e.g., 14VDC, usually only very short duration arcs occur in inductive circuits. These arcs extinguish once the arc has dissipated the inductive energy. At 42VDC, an arc fault may be sustained even though the current is significantly reduced below the bolted value since the voltage driving the circuit impedance is reduced by 30%-50%.

Parallel Arc Fault vs. Short Circuit:

During most fault conditions, the contact force is low, e.g. a "live" wire slightly touching the circuit ground. The contact spot melts and evaporates quickly forming an arc. This type of fault is defined as a parallel arc fault although in many cases this type of fault is wrongly identified as a short circuit. A short circuit, per definition, is a metal-to-metal fault. It can occur when the molten contact spot welds or when the contact force is high enough to carry the fault current preventing arcing. Since an arc limits the fault current, the arc fault duration is always lower than the short circuit duration. Additionally, depending on the arc cause and the motion of the arcing conductors, the arcing duty cycle can be significantly less than 100%. While the RMS value of the fault current is reduced, the arcing still represents a local heating hazard. Fuses or breakers trip late or not at all!

While fuses, circuit breakers, or conventional electronic fault protection devices cannot detect arc faults, they are still necessary for overload and short circuit protection!

A parallel arc fault is an issue for a supply voltage between 20VDC and about 120VDC (higher in AC systems). Below 20VDC, the arc is not stable (minimum arc voltage). Above 120VDC, the current limiting effect of the arc (VSOURCE - VARC = voltage that drives the current) is small permitting the short circuit protection device to operate. For AC circuits this 120V limit is not applicable due to the current zero crossings allowing intermittent arcs.

Series Arc Fault:

In a series arc fault (the arc fault is in series with the load), the fault current is less than the nominal current preventing detection by a fuse or a breaker!

A series arc fault is an issue at any voltage higher than 20VDC becoming more severe the higher the voltage. Since less molten material is ejected by a series arc (reduced power), the total energy of a series arc fault event can still reach substantial levels before the arc is extinguished.

Both a series and a parallel arc fault can spread to adjacent circuits resulting in a significant reliability and fire hazard.

Testing:

Existing test standards for arc faults and wire harnesses and new test procedures have been adapted to reproduce close-to reality faults in cars.

Test set-ups for series arc faults:

  • Connector test "Live" connector disconnect

  • Loose lug / connector test Loose terminations

  • Broken wire test Fatigue from bending or vibration

Test set-ups for parallel arc faults:

  • Saltwater Drip Test Humidity, Salt in winter

  • Saw Test Wire abrasion

  • Dangling Wire Test Broken wire

  • Guillotine Test Cut wire

The voltage and current traces for series and parallel arc fault tests show that the fault times were significantly longer when compared to short-circuit fault times, even though fuses were used in both cases.

The traces exhibit characteristics that differ substantially from residential and aerospace AC circuits' arc faults.

  1. Conclusion

  • An arc is essentially super heated, electrically conductive, air at extremely high temperatures (6000 degrees C to 30000 degrees C).

  • For a given current value and local ambient atmosphere (usually air), the arc voltage is a function of the characteristic minimum arc voltage (Table 1) and the arc length.

  • The minimum arc voltage depends on the contact material and can quickly change due to carbonization effects.

  • While at 14VDC a stable arc does not exist, a stable arc can be maintained at 42VDC.

  • Parallel arc fault is an issue for supply voltages from 20VDC to 120VDC (not true for AC).

  • Investigation of arcing faults in test set-ups is necessary to reproduce realistic arc faults.

  • Parallel arc fault times are usually significantly longer in duration than bolted short circuit times.

  • A fuse trips late or not at all when a parallel arc fault occurs, while not at all for a series arc fault.

  • Parallel arc fault, in most cases, is shorter in duration (&1 second) at a higher current compared to a series arc fault, which can last for many seconds at a lower current.

  • Series arc fault is an issue at any voltage higher than 20VDC.

  • Arc fault can cause potential reliability issues or even a catastrophic event like a fire.

  • Arc fault can spread from one circuit to other adjacent circuits (wire bundles).

Many arc fault detection methods have been patented for AC applications; e.g. EATON Corp. has more than 50 patents issued and pending. Only a few patents have been published for DC arc fault detection methods, e.g. Hendry, EATON Corp. Eaton Corporation is the world leaders in arc fault detection and protection.

  1. Acknowledgement The authors would like to thank Dick Baumann, Jason Bobrowitz, and Chuck Tennies for conducting the tests and Pete Moldovan and Dave Turner for proof reading the paper (all EATON Innovation Center).

  2. References


[1]


R.J. Clarey, T.M.Doring, J.C.Engel, "Arc fault Circuit Interrupters: New technology for Increased Safety", a detailed discussion of the development of a residential arc fault circuit interruption device.


[3]


S. Schmalz, J. Zuercher, "Arc Fault Detection Utilizing Algorithm to Compare Integrated Cycle-to-cycle Load Current" PAT 5933305 (8.3.99).


[2]


J. Engel, E. Hetzmannseder, J. McCormick, P. Theisen, M. Walz, "Aircraft Electrical System Safety Considerations", SAE 2000 Advances in Aviation Safety Conference and Exposition, April 11-13, Daytona Beach, Florida.


[4]


"Aircraft Arc Fault Circuit Interrupter," PAT PENDING 09/504421.


[5]


A. Keil: Werkstoffe fur elektrische Kontakte. Springer-Verlag,
Berlin/Gottingen/Heidelberg, 1960.


[6]


R. Holm, "Electric Contacts - Theory and Application", 4th edition.,
Springer-Verlag, Berlin/Heidelberg/New York, 1967.


[7]


W. Rieder, "Die Stabilitat geshunteter Gleichstromlichtbogen. ELIN-Zeitschrift Bd. 7, No. 3, 1955.


[8]


S. Zarei, S. Alles "Effect of 42V on Automotive Relays and Switches", SAE 200-01-3054, 2000.


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