Determination of Arc Resistance Performance of Materials: Standards, Methods and Applications

Arc resistance testing measures how long a solid insulating material can withstand a high-voltage, low-current arc discharge across its surface before a conductive path forms. This article explains the principle and significance of arc resistance determination, the governing standards such as ASTM D495, IEC 61621 and GB/T 1411, specimen requirements, test procedures, and typical applications in material selection and quality control for electrical equipment.

Technical Overview

Arc resistance is the ability of a dry, solid electrical insulating material to withstand the action of a high-voltage, low-current arc discharge moving slowly over its surface without forming a conductive path. During the test, a controlled electric arc is struck between two electrodes placed on the specimen surface, and the elapsed time until material failure is recorded in seconds. The longer the material resists the arc, the higher its arc resistance, which indicates better reliability when the material is exposed to arcing conditions in service, such as during switching operations, contact separation, or insulation defects.

When an arc propagates along an insulating surface, the concentrated thermal energy can degrade the material through several distinct mechanisms. The dominant failure mode determines both the measured arc resistance value and the practical significance of the result for a given application. Common failure mechanisms include:

• Carbonization and tracking: formation of a conductive carbonized track that carries current between the electrodes
• Surface flashover or breakdown of the air gap along the degraded surface
• Ignition and burning of the material, with sustained flaming or glowing
• Melting, flowing, or erosion of the surface that distorts the electrode gap
• Cracking or decomposition that undermines the insulating function of the part

Several internationally recognized standards describe the determination of arc resistance for solid insulating materials. The principal methods are technically equivalent and share the same electrode system, voltage level, and severity programme:

• ASTM D495: Standard Test Method for High-Voltage, Low-Current, Dry Arc Resistance of Solid Electrical Insulation
• IEC 61621: Dry, solid insulating materials — Resistance test to high-voltage, low-current arc discharges
• GB/T 1411: Chinese national standard for arc resistance testing of dry solid insulating materials, aligned with IEC 61621
• UL 746A: Polymeric materials — Short term property evaluations, which adopts arc resistance as part of its material assessment scheme

Performance Classification

For material comparison and datasheet reporting, arc resistance results are frequently grouped into performance level categories based on the measured time to failure. A widely used classification scheme assigns indices according to the following ranges of arc resistance time:

• Level 0: 420 seconds or longer (no failure at the end of the standard programme)
• Level 1: 360 to 419 seconds
• Level 2: 300 to 359 seconds
• Level 3: 240 to 299 seconds
• Level 4: 180 to 239 seconds
• Level 5: 120 to 179 seconds
• Level 6: 60 to 119 seconds
• Level 7: shorter than 60 seconds

Testing Samples

Virtually any dry, solid insulating material that can be produced as a flat, rigid or semi-rigid specimen can be evaluated for arc resistance. The test is most commonly applied to polymeric materials used in electrical and electronic equipment, as well as to ceramics, laminates, and molded compounds.

Thermosetting Materials

Thermosets are frequent candidates for arc resistance testing because they are widely used in switching devices, arc chutes, and insulating structural parts. Typical materials include epoxy molding compounds and cast resins, phenolic molding materials, unsaturated polyester compounds, melamine and urea formaldehyde moldings, and silicone resins. Inorganic-filled formulations, such as those containing silica or alumina trihydrate, generally achieve higher arc resistance than unfilled organic systems.

Thermoplastic Materials

Engineering thermoplastics used in connectors, housings, and interrupter components are also routinely tested. Common examples include polyamide, polycarbonate, polybutylene terephthalate, polyethylene terephthalate, polyphenylene sulfide, polyetherimide, and fluoropolymers. Unfilled thermoplastics often show moderate arc resistance, while glass-fiber or mineral-filled grades tend to perform better under arc exposure.

Ceramics, Elastomers and Laminates

Alumina and other technical ceramics typically exhibit very high arc resistance and frequently complete the full 420-second programme without failure. Silicone rubbers and other elastomers used in insulating components can be evaluated provided they are supported to maintain the correct electrode geometry. Laminated sheets, copper-clad laminates for printed circuit boards, and impregnated or coated insulating systems are additional sample categories commonly submitted for testing.

Specimen Preparation and Conditioning

To obtain valid and reproducible results, specimens must satisfy specific geometric and surface requirements before testing:

• Flat panels with a surface area comfortably larger than the electrode arrangement, commonly around 50 mm by 100 mm or greater
• A thickness of approximately 3 mm or more, so that failure occurs by surface tracking rather than breakdown through the material
• Smooth, clean surfaces free from mold release agents, dust, grease, fingerprints, and other contaminants that could alter tracking behavior
• Conditioning in a standard laboratory atmosphere, typically 23 °C plus or minus 2 °C and 50 percent plus or minus 5 percent relative humidity, for at least 24 hours prior to testing
• Multiple specimens, typically five, tested for each material to characterize the scatter inherent in arc-induced surface degradation

Testing Items

The core deliverable of an arc resistance determination is the measured time, in seconds, for the specimen to fail under the standardized arc exposure. A complete test report generally contains the following elements:

• Individual arc resistance times for each specimen, together with the mean or median value as specified by the applied standard
• Description of the observed failure mode, such as tracking, carbonization, ignition, or melting
• The test standard used, including any deviation from the reference procedure
• Specimen identification, nominal thickness, and conditioning history
• A statement that testing was performed in the dry condition, since moisture and surface contamination are excluded by the method
• Assignment of a performance level category when required for material comparison or datasheet purposes
• A statement of "no failure at 420 seconds" for specimens that withstand the complete programme

Testing Methods

Test Apparatus

The determination is performed with a dedicated arc resistance tester comprising a high-voltage source, a controlled arc gap, and precision timing instrumentation. The essential configuration includes:

• A high-voltage transformer supplying an open-circuit secondary voltage of approximately 12.5 kV at power frequency
• A current-limiting and control circuit that regulates the arc current in discrete steps from 10 mA up to 40 mA
• Two tungsten rod electrodes with precisely machined tips, positioned with a fixed gap of 6.35 mm between the tips at a defined angle to the specimen surface
• An automatic timing and sequencing system that executes the standardized programme of increasing severity
• A ventilated test enclosure with safety interlocks to contain UV radiation, ozone, and any combustion products

Test Procedure

The conditioned specimen is placed on a grounded support, and the tungsten electrodes are lowered onto the surface with the specified contact geometry. The arc is initiated, and the instrument automatically executes a programmed sequence lasting up to 420 seconds in total. Severity increases each minute: the interrupted arc duty cycle becomes progressively heavier, the discharge becomes continuous, and the arc current is raised stepwise from 10 mA to 40 mA. The test continues until the material fails or the full 420-second programme is completed.

Endpoint and Failure Criteria

The endpoint is the moment at which the material ceases to perform its insulating function under the arc. Recognized endpoint criteria include:

• Formation of a conductive path across the electrode gap, indicated by the arc extinguishing as current flows through the carbonized surface track
• Sustained ignition or flaming of the specimen
• Significant erosion, melting, or flow that fundamentally changes the electrode-to-specimen geometry
• Surface tracking that bridges the electrodes with a visible conductive deposit

Expression of Results

The arc resistance of each specimen is the time in seconds from the start of the arc to the endpoint. Results from replicate specimens are averaged or reported as a median in accordance with the applied standard, and the failure mode is documented. When a specimen survives the entire programme, the result is reported as 420 seconds or greater. It should be noted that the method is a comparative, dry-surface test: the measured value characterizes resistance to clean, dry arcing and does not by itself predict behavior under wet or contaminated conditions, where tracking tests such as the comparative tracking index are more appropriate.

Complementary Tracking and Ignition Tests

Because arc exposure is only one of several electrical degradation scenarios, arc resistance is often evaluated alongside complementary standardized methods to build a complete surface performance profile:

• Comparative tracking index and proof tracking index tests under wet, contaminated conditions (IEC 60112 / ASTM D3638)
• Inclined plane tracking and erosion resistance at high voltage (IEC 60587 / ASTM D2303)
• High-voltage arc tracking rate and ignition assessments within short-term material evaluation programmes (UL 746A)
• Dielectric strength and insulation resistance measurements to characterize bulk dielectric performance

Applications

Arc resistance data directly support the safe design, material selection, and qualification of components in which arcing can occur. Typical fields of application include:

• Low- and medium-voltage switchgear, contactors, relays, and circuit breakers, where insulating walls and arc chutes are exposed to switching arcs
• Terminal blocks, connectors, brush holders, and current-carrying plastic parts located near make-and-break contacts
• Insulators, bushings, busbar supports, and stand-off components in power distribution equipment
• Enclosures, switch housings, and structural insulating parts for electrical appliances and power tools
• Copper-clad laminates and insulating substrates for electronic assemblies operating near high-voltage sources
• Research and development of polymeric formulations, including studies of filler type and loading on arc endurance
• Incoming inspection and production quality control of molding compounds, laminated sheets, and ceramic insulators
• Regulatory compliance support, with testing performed by accredited third-party laboratories under recognized international standards

From a materials engineering perspective, arc resistance results guide formulation decisions. Replacing organic fillers with inorganic fillers such as silica or alumina trihydrate, increasing inorganic loading, and selecting resin chemistries with low carbon yield under thermal stress are established strategies for raising arc resistance. Design measures, including adequate creepage and clearance distances, arc barriers, and arc runner geometries that move the arc away from polymeric surfaces, further mitigate arcing damage at the equipment level.

FAQ

What is arc resistance and in what units is it expressed?

Arc resistance is the time, expressed in seconds, that a dry, solid insulating material withstands a standardized high-voltage, low-current arc discharge on its surface before a conductive path forms or another defined failure occurs. Higher values indicate better resistance to arc-induced surface degradation.

Which standards are used for arc resistance determination?

The most widely applied standards are ASTM D495, IEC 61621, and GB/T 1411, which define technically equivalent dry arc test procedures. UL 746A incorporates arc resistance results into its short-term evaluation scheme for polymeric materials. The selection of standard usually follows the market or specification to which the product is sold.

How should arc resistance results be interpreted?

Results are comparative rather than absolute service-life predictions. Materials reaching 420 seconds or more are considered highly arc resistant, while values below 60 seconds indicate limited suitability near arcing sources. Interpretation should always consider the observed failure mode, the equipment design, and complementary tracking and ignition data.

How does arc resistance differ from the comparative tracking index?

Arc resistance evaluates surface endurance under a dry, energized arc supplied by the test electrodes. The comparative tracking index evaluates resistance to tracking under wet, contaminated conditions with a much lower applied voltage. The two tests address different degradation mechanisms, and both are commonly required for a complete assessment of insulating performance.

What factors influence arc resistance test results?

Material formulation has the strongest influence, particularly the type and loading of fillers and the carbon yield of the polymer matrix. Specimen thickness, surface cleanliness, conditioning history, ambient humidity, and electrode condition also affect reproducibility, which is why standardized specimen preparation and laboratory atmosphere control are essential.

How can the arc resistance of a material be improved?

Effective approaches include incorporating inorganic fillers such as silica, alumina, or alumina trihydrate, increasing filler loading, using resin systems that erode rather than carbonize under arc heat, and applying surface treatments or inorganic coatings. At the design stage, arc chutes, barriers, and adequate spacing keep arcs away from vulnerable polymeric surfaces and extend component life.