Arc Resistance vs Track Resistance — Key Differences for Electrical Insulation

Arc resistance and tracking resistance measure completely different failure modes — arc resistance measures how long a material survives a high-voltage arc on its surface before carbonizing, while tracking resistance (CTI) measures how well a material resists forming a conductive carbon path under contaminated voltage stress. Specifying the wrong one leads to premature field failure.

TL;DR — Key Takeaways

• Arc resistance (ASTM D495): How long (in seconds) before a HV arc carbonizes the surface — relevant in circuit breakers, arc chutes, and disconnects where arcing events are expected
• Tracking resistance (CTI, IEC 60112): The minimum voltage (in volts) at which a contaminated surface forms a conductive track — relevant in switchgear, motor terminals, and anywhere electrolyte contamination can deposit
• G5 and G9 (glass-melamine) lead in arc resistance (300–400 sec); GPO-3 leads in arc resistance for structural grades (~200–250 sec)
• G10 and FR4 have moderate arc resistance (60–120 sec) but good CTI (~175–200V for standard FR4)
• High-CTI FR4 variants and GPO-3 achieve CTI 600 (the highest Comparative Tracking Index category)

What Is Arc Resistance?

Arc resistance is measured by ASTM D495 (High-Voltage, Low-Current Dry Arc Resistance of Solid Electrical Insulating Materials). The test applies a series of high-voltage arcs of increasing duration directly to the surface of the material. The result is reported in seconds — the time until the surface carbonizes and becomes conducting.

ASTM D495 Test Procedure

1. Two tungsten electrodes are placed on the specimen surface, spaced 1/4 inch apart
2. A series of arcing exposures is applied: short arcs (1/8 sec on, 7/8 sec off) for the first stage; progressively longer arcs for subsequent stages
3. The test ends when the material begins to track (surface conducting) or ignites
4. Result = total elapsed time in seconds at which failure occurs

What Arc Resistance Predicts

High arc resistance indicates the material:

• Does not rapidly carbonize under arcing conditions
• Will not quickly create a short-circuit surface path during an arc event
• Is appropriate for components where arcing occurs (circuit breaker interiors, arc chutes, knife-switch contacts)

High arc resistance does NOT predict good tracking resistance — they are independent properties. Phenolic grades can have high arc resistance but poor tracking resistance; melamine grades combine both.

What Is Tracking Resistance (CTI)?

Tracking resistance is measured by the Comparative Tracking Index (CTI) test per IEC 60112 or UL 746A (identical procedure). The CTI is the highest voltage (in volts) at which a material withstands 50 drops of contaminated electrolyte (ammonium chloride solution) without forming a conductive track.

CTI Test Procedure

1. 0.1% ammonium chloride solution is dropped at 30-second intervals between two platinum electrodes on the specimen
2. The test voltage is held constant; the number of drops until tracking failure is recorded
3. The CTI is the voltage at which ≥50 drops are survived without failure

CTI Categories (IEC 60664)

Higher CTI = smaller required creepage distance = more compact equipment design.

Arc Resistance and CTI by Thermoset Grade

Why Epoxy Grades (G10 and FR4) Have Lower Arc Resistance Than Phenolic/Melamine

Epoxy resin carbonizes relatively quickly under arcing — the carbon formed is conductive, and once a carbonized path bridges the electrode gap, the arc test ends. Phenolic and melamine resins form hard, insulating char under arcing, which is why G5/G9 (melamine) and phenolic grades consistently outperform epoxy grades in arc resistance.

This is also why:

• G10 and FR4 is excellent for PCBs and structural insulation but not ideal for arc chutes
• G5/G9 is specified specifically for circuit breaker interiors and arc chutes where sustained arcing is expected

Why Melamine Outperforms Everything in Arc Resistance

Melamine (1,3,5-triazine-2,4,6-triamine) has a nitrogen-rich ring structure. When exposed to a sustained arc:

1. The melamine ring breaks down, releasing nitrogen gas
2. The nitrogen gas helps quench the arc
3. The remaining char is dense, hard, and non-conductive

This mechanism produces arc resistance values of 300–400+ seconds — 3–4× higher than glass-epoxy grades.

Which Property Do You Actually Need?

Creepage Distance and CTI in Equipment Design

IEC 60664-1 specifies the minimum creepage distance between live parts as a function of working voltage, pollution degree, and material group (based on CTI). For a given working voltage:

• Group I material (CTI ≥ 600): Shortest allowable creepage distance
• Group IIIb (CTI < 175): Largest required creepage distance (can be 3–4× longer than Group I)

Selecting a Group I material (GPO-3, high-CTI FR4) instead of Group IIIb (standard FR4) can reduce required creepage distances by 30–50%, enabling more compact equipment layout. This is often the deciding factor between GPO-3 and standard FR4 in high-density switchgear panels.

More related guides

Materials

• G10 Material Hub
• FR4 Material Hub
• Phenolic-Glass Material Hub

Specifications

• ASTM D709 Explained
• MIL-I-24768 Explained

Applications

• Bus Bar Insulation Materials
• Motor Insulation Materials