issue: October 2006 APPLIANCE Magazine
No Tolerance for Arc Faults
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by Dr. Allen C. Eberhardt, mechanical design and test engineer, Product Safety Research
Closely evaluating end-of-life and pulse-current failure scenarios can make small appliances more robust and protect against potential fires.
Figure 1. Split-phase motor (no fuse) peak-current 60-A stair-stepping waveform from end-of-life testing.
When looking for enhanced fire and circuit protection for small appliances—beyond the safety choices of thermal cutoffs (TCOs), ground fault circuit interrupters (GFCIs), arc fault circuit interrupters (AFCIs), and other safety devices—evaluate the product end-of-life and pulse-current failure scenarios, and apply some “fuseology” for fire safety. The appropriate selection of a fuse can add robust fire protection against abuse, misuse, line-cord damage, contaminants, surges, and final-failure events.
Fire Cause and Origin Investigations
In the aftermath of a fire, forensic findings are often disputed among insurers, investigators, experts, and manufacturers. Localized melting with the appearance of beads of molten copper on wires and connections suggest that the circuit was energized at the time of the fire. Fire cause and origin investigators realize that arcing often occurs as a consequence of fire. Damage from secondary fires and structural collapse can degrade physical evidence to a degree that causation cannot be determined by application of the scientific method, as required of investigators under NFPA 921.
Unfortunately, melting of conductors and secondary arcing in any appliance near the area of fire origin may be cited as a potential cause, rather than a result of the fire. As a fire progresses, heat chars PVC insulation, making it conductive, and thereby creating opportunities for arcs that can occur repeatedly before the circuit branch is opened by a breaker. There is a large current available to support such intermittent arcs, as a 15-A breaker can allow a draw in excess of 150 A before producing an “instantaneous” trip.
FLimited Role for AFCIs
High-current devices such as air-conditioners, heaters and portable appliances are primary targets for AFCIs and LCDIs. Operating currents below a few amps have not been targeted for detection by AFCIs, although these appliances continue to be identified by investigators as a potential cause of many fires. As a result, appliances that draw only up to a few amps in normal operation must fail rather dramatically to activate the 150-A instantaneous threshold of the branch-circuit breaker. Such high currents require a parallel-path rather than an “impedance-limited,” series-fault path to cause the breaker to open. Often, if the series-fault generates enough heat to compromise insulation, the breaker-tripping parallel-fault occurs. This scenario gives fire initiation an undesirable second-chance opportunity.
A similar situation may arise with low-current series type faults in low-power appliances. The low current produces a lower level signature and a lower signal-to-noise ratio for arc detection by an AFCI-type device. AFCI design is complicated by the diversity of conditions that create arc-type ignition events, from high current parallel-faults, to intermediate current series-faults, to low current uncharacteristic signatures. At low-current signal levels, the arc signature may be lost in the noise. Increasing the AFCI sensitivity for low-level signals may produce the unwanted side effect of nuisance tripping.
Figure 2. Stopping before the peaks (fused motor). Evaluating the result of fuse selection by interrupting early during the stepping increase.
Identifying Ignition Features for AFCI
AFCIs require recognition of a predetermined current signal “feature-set” for arcs, which may be sensitive to timing intervals, amplitudes and frequency content. The feature set characteristics then become a specification for an application specific AFCI. The appliance arc-failure characteristics must be identified and quantified. The known or experimentally evaluated failure modes and end-of-life characteristics are used to assign features that identify dangerous arcs. The features must be differentiated from normal line-current fluctuations and noise. Noise is not limited to just the appliance being protected. The detection must be sufficiently robust to avoid nuisance tripping from parallel-connected branch-circuit devices such as dimmers, switches, fluorescent ballasts, vacuum cleaners, blenders, and air-conditioners.
Having robust features is essential, but not adequate; time is also a factor. The detection algorithms must provide rapid response to prevent a fire. The response must limit the pulse energy (current-squared multiplied by the time duration). In addition to response at the pulse energy limits, the device must ignore anticipated pulses from the following: surge currents, start-up currents, inrush currents, and transients. Loose terminal connections and long duty cycles are problematic if intermittent low-current arcs defy detection, intermittent I2R (current-squared times resistance) heating may create a progressive failure, or repeated events such as arcing through char can have long duty cycles.
There are still many questions regarding AFCI protection in applications for which the limitations may be unknown and difficult to quantify. Experience over time will clarify appropriate use. A number of questions remain with a lingering uncertainty:
• Can recognition be established for faults across a broad range of appliances?
• Can carbonized path and arc-tracking problems be handled with safety?
• Is the “reset” feature of an AFCI following a “trip” an asset or a liability?
Identifying Ignition Features for Fuse Applications
Unlike circuit breakers, which are designed as a last resort to interrupt at the branch-circuit location, or AFCIs, which respond to features at normal operating currents, appliance specific fuses are selected to respond quickly as current rises above a much more restricted and specifically selected time-current energy threshold. The appropriate fuse must be selected to handle a number of tough requirements:
• quick response to limit the total energy
• response to current levels that rise in milliseconds
• arcs that can produce 5,000°F (2,760°C) across a loose connection or carbonized material
• arc temperatures that may produce projectile molten metal
• arc-tracking or carbonization arcing that is subtle and persistent
Unlike complex decisions like choosing arc detection features, current and time characteristics of appliance failure and end-of-life current can be quantified. The time-current envelopes are typically measurable, often repeatable and thus, predictable. Series arcing is often current-limited by the impedance of the appliance itself.
In fused applications, duty cycles for repeating arcs need not complicate the decisions. Fuses act only once; thus, the requirement is to interrupt the first cycle that exceeds the envelope of time-current limits. A fuse selection is tailored to the specific limits of current, timing and repetitions dependent on the appliance.
An example of the current characteristics during failure of a fractional-horsepower fan motor is shown in Figure 1. In this example, arcing of the windings produces a stair-stepping increase in current, ultimately opening the circuit with an arc of greater than 60 A. The same motor had previously been tested with an appropriately selected fuse in the circuit, as shown in Figure 2. Note that the result in this second figure is dramatically different; the fuse responds to the stepping increase in current at a 5-A peak. The result is that no visible arc is produced.
For many appliances there are a number of advantageous characteristics that can be detected:
• short arc durations of 8 ms (half-cycle 60 Hz) are not unusual
• currents and impedances can change rapidly during a failure
• current can escalate through a short sequence of mini-arcs
• rapid stair-stepping of current is common in motor windings
• events quickly progress to exceed predetermined current limits
• rapid interruption can intervene to arrest the failure while in progress
Along with the advantages, however, are some limits to applicability of arc-current interruption by fuses. Most significant is that fuses can only respond to failures that produce over-currents. Some common advantages and limitations are as follows:
• ideal for low-current applications (4 A and below)
• offer less certain I2R protection forslow-heating in higher-current applications (greater than 6 A)
• fuse selection typically protects only a limited number of specific failure modes
• wide range of fuse values are available to match time-current characteristics
• time-current response curves are well-documented by suppliers
• predictable, dependable and inexpensive
• available in a range of voltages, speeds and form factors
• long history of wide use and acceptance as fire-safety and equipment-protection devices
• can be incorporated upstream into existing plugs or connectors
Application-specific fuses can provide rapid response to arcing over-current conditions, but are not AFCIs:
• AFCIs do not require over-currents to detect an arc.
• AFCIs distinguish arc features within normal operating currents.
• AFCIs may respond to arcing currents that do not spike above normal loads.
• AFCIs may not respond well to low-current arcs at or below a few amps.
An AFCI can be programmed to respond to over-currents of short duration that may be characteristic of arcing in an appliance. However, response to over-currents of limited duration is a key capability of fuses. Moreover, fuses are a mature technology, can be selected to meet specific time-current characteristics, are reliable, and are typically failsafe.
Figure 3. Moving the fuse “upstream,” as in this fan application, adds protection to cords and connections, as well as controls and motor windings.
Fuses can be very effective in detecting and preventing over-current arc-fault fires in low-current applications. As a test, a carefully selected fuse can be compared in head-to-head competition with an AFCI specification or actual device. There are a series of steps in evaluating an appliance for fused protection. The key to identifying the appropriate response characteristics for a fuse (or for an AFCI) is understanding the failure modes and corresponding current waveforms. Waveform details can be obtained through an experimental evaluation in which arcing failure conditions are intentionally introduced to simulate conditions such as age, heat, moisture, vibration, shorting, or any known end-of-life scenario. An experimental evaluation coupled with a field investigation often reveals recognizable and repeatable current features and failure modes, over-currents, durations, identified modes and statistics from field-returned failures, and test data on units removed from service.
Whether testing new product or units removed from service, the following steps should be taken:
• Monitor the time-current range of normal operating signatures.
• Identify the time-current characteristics for each failure mode.
• Isolate the failure features that exceed the normal operating current:
–A rapid current rise or stair-stepping to failure.
–High-amplitude current transients.
• Select a fuse to interrupt the circuit based on I2t at the lowest time-current threshold and above normal operational transients.
• Adapt if required to meet interrupt time-response limits:
–Materials adaptation—select material ignition energy thresholds above fuse thresholds.
–Materials isolation—isolate, minimize, encase the fuel package.
• Perform new laboratory and product field evaluations:
–Initiate on a single product line.
–Analyze all open-circuit-fuse field returns:
- Confirm and then replace the open fuse.
- Energize and monitor the current.
- By-pass the fuse.
- Follow the current response to failure.
- Analyze the data.
When considering fuse placement, protection may be limited to a component, subassemly, or encompass the entire product, using the placement shown in Figure 3.
Zero Tolerance Circuit Interruption
As a final note, the once-only response of a non-replaceable fuse at its I2t pulse limit offers no second-chance, gives no option for reset other than replacement and is one-strike-and-out “zero tolerance” response.
Unlike the fuse, the AFCI can be reset to respond repeatedly following a problem. However, the “reset option” is not always the best choice. For example, consider the characteristics of a moist or dirty environment that may produce parallel arcing through char. The scenario is as follows:
• carbonized-path current draw between phase and neutral
• short-duration arc-current bursts that partially clear the path
• temporary reduction of current following each burst
• modest pulse energy that varies with each burst
• long duty cycles between current bursts
• pulse-current that will not open a circuit breaker
Under similar sequences, an AFCI may be challenged and may respond by opening the circuit. If seen as a nuisance trip by the user, the AFCI will be reset, allowing the char to continue through one or more rebuilding cycles and subsequent arcing.
Many appliances that operate at currents of no more than a few amps can benefit from enhanced fire protection by selection of an appropriate fuse. Unlike the I2t pulse limit a fuse, a 15-A circuit breaker will not respond to high-current, low duty-cycle events, including arcs.
Appliances that operate at higher currents, particularly in the range of up to 6 A, can also be evaluated for benefits resulting from fused fire protection. At higher normal operating currents above 6 A, reduction of pulse-current and ignition energy can be dramatic with the addition of a fuse. This is because there is a large, unprotected region for short-duration events with circuit breakers—typically ten-times the breaker rating, 150 A for a 15-A breaker.
Although some modes of I2t heating failures may be undetected, arc current amplitude, duration and resulting pulse energy can be reduced dramatically by appropriate fuse selection.
About The Author
Dr. Allen C. Eberhardt is a mechanical design and test engineer at Product Safety Research, an engineering consulting firm based in Raleigh, North Carolina, U.S. If you wish to contact Eberhardt, e-mail email@example.com