Comparative Tracking Index (CTI, IEC 60112) and Flame Retardants: Test Method, Values by Polymer and 6 Additive Families That Move It

The comparative tracking index (CTI) is the highest voltage, up to 600 V, at which five specimens of a plastic withstand 50 drops of ammonium chloride solution between two platinum electrodes without forming a conducting track or a persistent flame, measured under IEC 60112:2025, and 6 additive families move that number up or down. A neat polyamide 6 reaches 600 V and the same polyamide with 15 % of an aluminium diethylphosphinate flame retardant drops to 550 V, so which additives cost tracking resistance, which protect it, and how is a compound formulated to keep CTI 600 V?

Phosphorus flame retardants can lower the CTI slightly in unfilled polyamide, while published glass-filled grades still reach 600 V. Brominated flame retardants with antimony trioxide lower it as their bromine share rises, mineral flame retardants such as aluminium hydroxide (ATH) can lift it above 600 V, zinc borate is used against arc tracking, copper halide heat stabilizers lower it through their halide, and mould release agents change the surface the test reads.

The number moves with the formulation: six families of plastic additives push it up or down, four of them flame retardants or synergists. This page covers tracking and PTI, the IEC 60112 drop test and its 2025 edition, material groups and PLC bands, the CTI of 16 plastics, the 6 families, 5 formulation rules, the parts that need CTI 600 V and the limits of the test.

Table 1 summarises the comparative tracking index in 11 properties.

Edition current on 24 October 2026. Apparatus values marked in Table 2 are from the 2003 text where the 2025 text could not be read.

What Is the Comparative Tracking Index (CTI)?#

The comparative tracking index is the numerical value of the maximum voltage at which five test specimens withstand 50 drops of electrolyte without tracking failure and without a persistent flame, the definition IEC 60112:2025 gives in clause 3.5, and it is the standard number for comparing how well solid insulating plastics resist surface tracking. In plastics manufacturing, CTI is a compound property: it sits on the data sheet of an electrical-grade polyamide, PBT or polycarbonate next to the UL 94 rating and the glow-wire values, and incoming material is checked against the CTI that the part specification names. IEC 60112:2025 states that composition and surface condition both directly influence the result, so one resin can give two CTI values from two moulding conditions. If CTI is a voltage, what physically fails at that voltage?

What is electrical tracking on a plastic surface?#

Tracking is the progressive formation of conducting paths on or inside a solid insulating material under the combined effect of electric stress and electrolytic contamination, the definition IEC 60112:2025 uses, and on most plastics those paths are carbon left behind by the polymer itself. Riba, Moreno-Eguilaz and Bogarra (Universitat Politècnica de Catalunya, 2023) describe the 4 steps of the tracking process, listed below.

1. Wet the surface: moisture and dissolved salts form a conductive film between two conductors.
2. Heat the film: leakage current dries part of it, and a dry band of high resistance forms.
3. Arc across the dry band: small discharges break polymer chains at the surface.
4. Bridge the gap: carbonised residue accumulates until a continuous conductive track joins the conductors.

The same review correlates CTI with a material's tendency to carbonise: PTFE forms no carbon track and erodes instead, and mica flakes or ATH stop decomposition products from forming a continuous carbonaceous path. An additive that leaves more char on the surface can therefore lower the CTI, and an additive that interrupts the char path can raise it.

CTI and PTI: the two tracking indices IEC 60112 defines#

CTI is the highest voltage a material withstands in the drop test, found by stepping the voltage up, while PTI, the proof tracking index, is a pass or fail result at one voltage the specification names, so CTI characterises a material and PTI accepts a part. IEC 60112:2025 describes PTI as "used as an acceptance criterion as well as a means for the quality control of materials and fabricated parts" and CTI as "mainly used for the basic characterization and comparison" of materials. A compounder therefore publishes CTI on the data sheet, while a product standard or customer specification names a proof voltage, such as 175 V or 250 V, that the moulded part has to survive for 50 drops. The 2003 text recommends 5 specimens for a PTI, requires the proof voltage to be an integer multiple of 25 V, and reports "Pass PTI 175" or "Fail PTI 175 M".

What does "CTI 600" or "CTI 175 M" on a data sheet mean?#

Read a CTI line as the 50-drop voltage first and the suffixes second: "CTI 600" means five specimens withstood 50 drops of Solution A at 600 V, the highest voltage the method applies, and "CTI 175 M" means 175 V with the more aggressive Solution B. A number in brackets, as in "CTI 400(350) M", is the 100-drop result, printed when it is more than 25 V below the 50-drop value, and a figure after a hyphen is the erosion depth in millimetres, as in "CTI 275-1.2", which IEC prints with a decimal comma. "CTI > 600" on a supplier sheet means passed at 600 V, not measured above it.

How Is the CTI Test Performed? The IEC 60112 Drop Test Step by Step#

A CTI test is run by placing two chisel-edged platinum electrodes 4 mm apart on a flat plastic specimen, applying an AC voltage between 100 V and 600 V, and letting a drop of ammonium chloride solution fall between them every 30 seconds under IEC 60112:2025, until the surface tracks, flames or survives the drop count. The CTI test is the electrical member of the fire-safety group of methods for testing plastic additives. IEC 60112:2025, edition 6.0, published in June 2025, is a basic safety publication that covers plaques as well as pieces cut from parts of equipment.

Table 2 lists the 10 test parameters of IEC 60112, each with its status for the current edition.

Values marked as 2003 text are from IEC 60112:2003, published in full as the identical Indian standard IS 2824:2007. The 2025 edition's clauses 4 to 11 were not available to us; values corroborated by a current test laboratory are stated as current practice.

Specimen, conditioning and surface state#

Three specimen conditions decide whether two CTI values can be compared: a thickness of at least 3 mm, a clean, smooth surface, and the same moulding history, because the 2025 edition of IEC 60112 states that the surface condition influences the result as directly as the composition does. The 2003 text (IS 2824:2007) recommends a flat area of at least 20 mm × 20 mm, allows thinner pieces to be stacked to 3 mm because a thin specimen loses more heat to the glass support, and conditions specimens for at least 24 h at 23 °C and 50 % relative humidity.

The same text notes that fabrication conditions and flow direction can change the CTI, asks for plaques made by the part's own process, and reports the lower of the two directions for anisotropic material. Two CTI values should only be compared when they were measured under the same edition, with the same solution and on specimens of the same thickness and moulding history.

Electrodes, test solution and drop sequence#

The apparatus has three controlled parts: a pair of platinum electrodes that cannot react with the contaminant, a standard ammonium chloride solution, and a dropping device that releases one drop between the electrodes every 30 seconds. The 2003 text specifies platinum of at least 99 % purity with a 30° chisel edge, a 4.0 mm gap and 1.00 N per electrode, because platinum is the most inert electrode metal available; results with copper, brass, stainless steel, gold or silver electrodes "do not qualify as either comparative or proof tracking indices".

Solution A, about 0.1 % ammonium chloride at 3.95 Ω·m (listed as 395 Ω·cm in UL Solutions' current service), is the default contaminant. Solution B adds a wetting agent and earns the suffix "M"; the 2020 edition added a Solution C aligned with IEC 60587 and moved Solution B to an annex; and the 2025 edition specifies the solution by conductivity instead of resistivity. This page gives no conductivity figure, because the 2025 clause 7.3 was not available to us.

One drop falls every 30 s from 35 mm, so 50 drops take about 25 minutes, and UL Solutions gives each drop as 20 mm³ (0.02 ml).

Failure criteria: tracking current, persistent flame and erosion#

A specimen fails the CTI test in two ways and can be marked down in a third: a tracking current trips the over-current device, a flame burns for more than 2 seconds, or, when the specification asks for it, the surface erodes beyond a stated depth. The 3 criteria are listed below.

• Tracking failure: a current of 0.5 A held for 2 seconds trips the over-current device, the value UL Solutions lists for current practice.
• Persistent flame: a flame burning for more than 2 seconds fails the specimen under definition 3.6 of IEC 60112:2025.
• Erosion: a depth measured with a 1.0 mm hemispherical probe after 50 drops, reported after the CTI value when the product standard requires it.

An arc through the air above the surface trips the device but is not tracking, and the 2003 text repeats that test as invalid. IEC 60112:2025 tolerates a short flame but prefers materials that make none, so a flame retardant can matter to CTI twice: through the char it leaves and through the flame it prevents.

How the CTI value is determined#

CTI is not calculated: it is found by testing at stepped voltages until the highest voltage is reached at which five consecutive specimens survive 50 drops, and the result is reported with a statement on how the material behaves at 100 drops. The 2003 text sets out the determination in 5 steps, listed below.

1. Start at 350 V when the material is unknown.
2. Run 100-drop tests first at integer multiples of 25 V, changing the voltage by no more than 50 V per step above 400 V.
3. Record the highest voltage at which five specimens pass 100 drops.
4. Test upward at 50 drops until the highest passing voltage for five specimens is found.
5. Report the 50-drop voltage as the CTI, with the 100-drop voltage in brackets when it is more than 25 V lower.

Note 1 of the 2025 definition allows no failure at any lower voltage, and since 2020 a screening test catches materials that withstand high voltages yet fail at lower ones. The 350 V start and the 50 V cap are 2003 practice, because the 2025 procedure clauses were not available to us.

IEC 60112 editions: what changed in 2020 and 2025#

IEC 60112:2025, the sixth edition, published in June 2025, is the current CTI standard, and its only significant technical change from the 2020 edition is that the test solution is now specified by conductivity instead of resistivity. IEC technical committee TC 112 prepared it as a technical revision that replaces the 2020 fifth edition. Table 3 lists the three editions of IEC 60112 and the 4 standards a CTI value is read against.

Editions are those current on 24 October 2026. Where no edition is shown, the standard's own text was not available to us.

The 2020 edition, published on 28 October 2020 and withdrawn on 23 June 2025, added Solution C and the screening test, so a CTI quoted "to IEC 60112" without a year can come from either procedure, and European adoptions such as BS EN IEC 60112:2025 and OVE EN IEC 60112:2022 carry their own years. A CTI value is only placeable with its edition and its solution letter.

Is ASTM D3638 the same as IEC 60112?#

No, not for current editions: ASTM D3638-21e1 declares itself technically equivalent only to an older version of IEC Publication 112 and warns that the IEC 60112 fourth edition "yields numerical CTI values that are very likely to differ", so a CTI measured to one standard does not satisfy a specification written to the other. One sourced difference is the contaminant, specified at 3.85 ± 0.05 Ω·m in ASTM D3638 against 3.95 ± 0.05 Ω·m in IEC 60112, according to Riba et al. (2023).

What Does a CTI Value Mean? Material Groups and PLC Categories#

A CTI value is read in two banding systems: the IEC material groups I, II, IIIa and IIIb that insulation-coordination standards use to set creepage distances, and the six UL 746A performance level categories, PLC 0 to PLC 5, printed on UL recognition data. Both band the same number, but group IIIa (175 V to 399 V) spans PLC 2 and PLC 3, so a group cannot be converted into a PLC without the raw CTI value. Table 4 maps each CTI range to its group and PLC.

Material groups as listed by UL Solutions and, for medical equipment, in IEC 60601-1:2005 (cited by Riba et al., 2023); PLC bands as listed in UL's Prospector database and in Riba et al. (2023). The texts of IEC 60664-1 and UL 746A were not available to us.

IEC material groups I, II, IIIa and IIIb#

A CTI class is the material group a plastic falls into by its CTI: group I at 600 V or more, group II from 400 V to below 600 V, group IIIa from 175 V to below 400 V, and group IIIb from 100 V to below 175 V. UL Solutions lists these bands, and Riba et al. (2023) cite the same banding from IEC 60601-1:2005 for medical electrical equipment. Equipment standards size creepage distances by group, and Riba and co-workers note that a group IIIa material needs a longer creepage distance than a group I material at any pollution level. A flame-retardant polyamide 6 at 550 V therefore sits in group II, one group below its 600 V base resin.

UL 746A performance level categories (PLC 0 to 5)#

A performance level category is UL 746A's banding of a CTI value into six steps, where PLC 0 means a CTI of 600 V or more and PLC 5 means less than 100 V, so a lower PLC is a better tracking result. The middle bands run from 400 V to 599 V (PLC 1), 250 V to 399 V (PLC 2), 175 V to 249 V (PLC 3) and 100 V to 174 V (PLC 4). A band can keep a test with known scatter from implying a precision it lacks: Riba et al. (2023) note that CTI tests "typically have a high standard deviation" because the apparatus and the environment affect them.

Does a higher CTI allow a shorter creepage distance?#

Yes, a higher material group permits a shorter creepage distance in insulation-coordination standards, but IEC 60112:2025 itself warns that a CTI result is not directly suitable for evaluating a safe creepage distance, which is set together with overvoltage levels and the pollution degree under IEC 60664-1. Creepage is the shortest distance between two conductive parts measured along the insulation surface, as Riba et al. (2023) define it, while clearance is the shortest distance through air. This page gives no distance figures or calculator, because the IEC 60664-1 tables are outside our verified sources.

CTI Values of Plastics: Which Polymers Track and Which Do Not#

The polymer sets the CTI ceiling before any additive moves it: aliphatic polymers such as polyethylene, polypropylene and polyamide reach the 600 V top of the test, while aromatic, char-forming polymers such as polycarbonate, PPS and PEEK track between 175 V and 400 V. Table 5 lists indicative CTI values for 16 plastics and polymer groups, from the highest to the lowest.

Indicative values compiled by Riba, Moreno-Eguilaz and Bogarra (Universitat Politècnica de Catalunya, Polymers 2023) from supplier and test-house sources. Grade, filler, flame retardant, colour and moulding change them; use the compound's own data sheet.

These values are indicative: Riba, Moreno-Eguilaz and Bogarra compiled them from supplier and test-house sources, not from one test series, and "> 600" means the material passed at 600 V. A compound's own CTI, measured with its flame retardant, filler and colour, is the value that goes on a part specification. Which flame-retardant system each of these polymers takes to reach V-0 is set out under flame-retardant plastics.

Aliphatic polymers that reach 600 V: PE, PP, PA, PMMA and fluoropolymers#

Polyethylene, polypropylene, polyamide, PMMA and the fluoropolymers reach the 600 V top of the CTI test because their chains leave little conductive char when a discharge breaks them, and PTFE leaves none at all: it erodes instead of tracking. Riba et al. (2023) add PFA, FEP and silicone rubber to the "> 600" band and list PVC and ABS at 600 V. In these polymers the CTI headroom is spent by additives, not by the resin, which is why a flame-retardant polyamide 6 measured 550 V where the neat resin measured 600 V in the 2026 study by Peng, Hua and co-workers.

Aromatic and char-forming polymers below 400 V: PC, PPS, PEEK, PI and phenolics#

Polycarbonate, PPS, PEEK, polyimide and phenolic resin sit between 125 V and 400 V because their aromatic rings turn into conductive char under a discharge, the same char that helps them resist ignition. The Riba compilation gives polycarbonate 175 V to below 400 V, PPS 175 V to below 250 V, PEEK 175 V, polyimide 150 V and phenolic resin 125 V. This is the trade-off the page is built around: char helps a plastic pass a fire test and hurts it in a tracking test, and Acquasanta and co-workers at the University of Bologna (2011) found that CTI appeared to follow the char-forming tendency of the glass-filled polymers they tested, a finding read here from the published abstract.

Polyesters and glass-filled engineering compounds: PBT, PET and glass-filled nylon#

Among glass-filled polyesters, PBT tracks less than PET and polycarbonate at the same 30 % glass content, according to Acquasanta and co-workers (University of Bologna, 2011), and Clariant reports glass-filled PBT and glass-filled nylon compounds with phosphinate flame retardants at CTI 600 V. PBT carries an indicative 500 V in the Riba compilation, so a 600 V glass-filled PBT grade is a compound result, not a polymer property.

Glass fibre itself is not the variable these sources isolate, and our knowledge base holds no measurement of its own effect on CTI, so this page states no direction for it. The sizing, stabilizer and flame-retardant package that a CTI 600 V grade of glass-filled nylon carries is set out on its own page.

Electrical PBT grades depend on the same additive choices, and their full package is under additives for PBT.

How Do Additives Change the CTI? The 6 Families#

The 6 additive families that move a plastic's CTI are phosphorus flame retardants, brominated flame retardants with antimony trioxide, mineral flame retardants, zinc borate and other synergists, copper halide heat stabilizers, and mould release agents with other surface-active additives, and the phosphorus systems come first because they carry the only published before-and-after CTI measurement. Table 6 gives each family's direction, a sourced example and the evidence type, from the strongest evidence to the weakest.

Directions are those reported in the cited sources for the stated polymer and loading. No source in our library isolates the CTI effect of glass fibre, carbon black or pigments, so those are not listed.

Compare phosphinate, mineral and polymeric flame-retardant grades by CAS, not by trade name, with the plastic additive supplier finder.

1. Phosphorus flame retardants: phosphinates and melamine polyphosphate#

Phosphorus flame retardants can lower the CTI slightly in an unfilled polyamide: Peng, Hua and co-workers at the Beijing University of Chemical Technology (2026) measured 600 V for neat PA6 and 550 V once 15 % aluminium diethylphosphinate was added. The study in Polymers tested 20 × 20 × 3.2 mm specimens, 5 per condition, and adding a phosphorus, nitrogen and zinc synergist (PABG@Zn) left the CTI at 550 V. The 50 V fall moves the compound from material group I to group II. Melamine polyphosphate (MPP), the nitrogen-phosphorus synergist used with phosphinates, has no CTI value of its own in our sources.

Clariant reports CTI 600 V for glass-filled PA6 and PA66 with 15 to 20 wt% of its Exolit OP 1312 phosphinate blend, and CTI up to 600 V for glass-filled PBT with Exolit OP 1240, in its 2019 thermoplastics brochure; these are supplier values for complete compounds. Grade-by-grade loadings for PA and PBT are on phosphinate flame retardants (DEPAL).

Both results hold, because the study measured one loading in unfilled PA6 while the supplier figures describe glass-filled compounds of unpublished formulation. Phosphinates are one of the six classes of halogen-free flame retardants, and the only class with a published CTI pair in our sources.

2. Brominated flame retardants and antimony trioxide#

Brominated flame retardants can lower the CTI as their share rises: Chiu and Cheng (2006) found in the Journal of Applied Polymer Science that the surface CTI of a copper-clad epoxy laminate increased as the brominated epoxy content decreased. The aromatic and polymeric types of brominated flame retardants used in connectors are compared on the class page. The laminate is a thermoset, so its numbers do not transfer to a polyamide or PBT compound, and our sources hold no CTI value for a brominated thermoplastic compound.

Why do bromine and tracking interact? The sources support two parts of an answer, the direction Chiu and Cheng measured and the carbonisation correlation of Riba et al. (2023), and none describes a halogen-acid mechanism, so this page proposes none.

ICL markets poly(pentabromobenzyl acrylate), sold as FR-1025 with about 71 % bromine, for glass-filled PA66 and PBT connectors and lists "high CTI" among its benefits, with thermal ageing resistance, recyclability and no blooming. FR-1025 belongs to the polymeric flame retardants, whose size keeps them from blooming onto the surface the test reads. Antimony trioxide (CAS 1309-64-4) is the usual synergist in these brominated systems.

3. Mineral flame retardants: ATH and MDH#

Mineral flame retardants can raise the CTI, because a filled surface gives the discharge less polymer to carbonise: Chiu and Cheng (2006) report that an epoxy laminate with aluminium hydroxide (ATH) can exceed 600 V, and Riba and co-workers (2023) describe ATH as stopping decomposition products from forming a continuous carbonaceous path. Loadings and particle grades for mineral flame retardants (ATH and MDH) are on the class page. Riba et al. (2023) also report that micro- and nano-sized ATH improves the tracking and erosion resistance of silicone rubber.

ATH releases water from about 200 °C, so polyamide and PBT compounds processed at 240 to 320 °C use magnesium hydroxide (MDH, stable to about 320 °C) or boehmite instead, and our sources hold no CTI value for either in those polymers. The 200 °C onset that limits it is documented on aluminum trihydrate (ATH).

4. Zinc borate and other flame retardant synergists#

Zinc borate is the synergist used against arc tracking: Firebrake ZB (2ZnO·3B2O3·3.5H2O, CAS 138265-88-0) releases water above 290 °C and leaves a glassy borate layer that can cover the char instead of letting it form a continuous carbon path. Our knowledge base records its anti-arc-tracking function from a secondary source, next to its roles as char promoter, smoke and afterglow suppressant and partial antimony trioxide replacement. Clariant's Exolit OP 1312 and OP 1314 phosphinate blends contain more than 3 % zinc borate, but no study in our library gives a CTI with and without it, so this page states its function, not a number. Grades are on zinc borate, one of the flame retardant synergists alongside antimony trioxide.

5. Copper halide heat stabilizers in nylon (polyamide)#

Copper halide heat stabilizers can lower the CTI of nylon through their halide: the iodide or bromide that accompanies 0.001 to 0.03 wt% copper is dosed at 0.1 to 5 wt%, and halide ions both corrode metal contacts and reduce tracking resistance. Copper(I) iodide is paired with potassium iodide or potassium bromide and rated by suppliers for long-term heat up to 180 °C. The full package of additives for nylon (polyamide) is on the polymer guide.

The fix the market sells is to take the halide out: Brüggemann rates its halogen-free copper complexes BRUGGOLEN H3376 and H3377 at a CTI of up to 600 V for electrical parts, and offers halogen- and metal-free stabilizers, TP-H2062 and TP-H2217, that keep UL 94 V-0, all as supplier ratings. How copper halide and halogen-free heat stabilizers for nylon compare on long-term heat is set out separately.

6. Mould release agents, lubricants and other surface-active additives#

Mould release agents and other additives that reach the surface can change a CTI result without changing the compound's chemistry: IEC 60112 names mould release among the contaminants that may influence the result, and the 2025 edition states that the surface condition influences the result as directly as the composition. The 2003 text lists "dust, dirt, fingerprints, grease, oil, mould release or other contaminants" in clause 6.2 and requires any cleaning procedure to be reported, so no direction is stated here for any single release agent.

A moulder therefore reports the cleaning, tests the as-moulded surface when the part sees contamination in service, and re-tests after a release-agent change. External release agents, lubricants such as metal stearates and waxes, and migrating antistatic agents all reach the surface, and internal and external mold release agents differ in how much of the additive gets there.

How to Formulate a Flame-Retardant Compound for CTI 600 V: 5 Rules#

Five rules keep a flame-retardant engineering compound at CTI 600 V, and the first is to start from a resin and a flame-retardant system that are published at 600 V rather than to rescue a low-CTI formulation afterwards. The 13 chemical types of flame retardants for plastics are compared on the family hub. The 5 rules are listed below, each with its source.

1. Start from a 600 V resin and a published system: neat PA6 measures 600 V (Peng, Hua et al. 2026), and Clariant reports glass-filled PA6, PA66 and PBT phosphinate compounds at up to 600 V.
2. Keep the halogen content as low as the fire target allows: CTI rose as brominated epoxy content fell (Chiu and Cheng 2006), and non-blooming polymeric bromine sources are the brominated route marketed for high CTI.
3. Break the char path with mineral content where processing allows: ATH lifted a laminate above 600 V, but it releases water from about 200 °C and is excluded at 240 to 320 °C.
4. Replace copper halide stabilizers with halogen-free copper complexes or metal-free stabilizers in electrical grades.
5. Re-test the moulded part after every formulation change, with the cleaning procedure and flow direction recorded.

Char that helps a part reach the glow-wire ignition temperature of 775 °C, which IEC 60335-1 clause 30.2.3 asks of insulating parts in unattended appliances, can hurt its CTI, so both numbers are specified together and neither is inferred from the other. A compound that changes flame retardant should be re-tested for CTI, because a value does not carry over from one system to another. Polyamide-specific systems are compared under flame retardants for nylon.

To shortlist systems by polymer first, use the flame retardant selector by polymer and UL 94 rating.

Which Electrical, Electronic and E-Mobility Parts Need CTI 600 V?#

CTI 600 V is specified for connectors, circuit breakers, switches, plugs and appliance parts moulded from glass-filled polyamide and PBT, where it sits in one specification set with UL 94 V-0 at 0.4 to 1.6 mm, a glow-wire ignition temperature of 775 °C and long-term heat ageing at 150 to 230 °C. The aluminium diethylphosphinate systems that reach this set are also used in household appliances and rolling-stock parts. The full set of rules and systems for flame retardants for electrical and electronic equipment is on its own node.

High-voltage connectors for electric vehicles add requirements: Clariant's e-mobility connector case for PBT lists UL 94 V-0, CTI 600 V, orange colour fastness and no copper contact corrosion. Riba et al. (2023) note that DC buses of 800 V and 1,000 V are now common in electric mobility, above the 600 V AC the CTI test applies, and that a higher CTI lets creepage shrink in compact designs. Thermal conductivity, orange colour and laser marking join CTI in additives for EV battery and e-mobility plastics.

What the CTI Test Cannot Tell You#

A CTI value cannot answer 5 questions that designers regularly put to it, and each one needs a different standard or test. The 5 questions are listed below with the standard or test that answers each one.

• A safe creepage distance: IEC 60112:2025 says its results are not directly suitable for this, and the distance is set under IEC 60664-1 with overvoltage levels and the pollution degree.
• Behaviour above 600 V or under DC: the method is designed for up to 600 V AC because higher voltages and DC reduce test severity, and Riba et al. (2023) report lower tracking resistance under DC, with no international DC tracking standard yet.
• Outdoor tracking and erosion: the inclined plane test of IEC 60587 uses larger specimens and higher voltages, and can rank materials in a different order.
• Behaviour against the real contact metal: only platinum-electrode results count as CTI, so a copper or brass contact needs its own test.
• Fire performance: a flame of up to 2 seconds is allowed in the CTI test, so flammability is judged by UL 94 and the glow-wire test.

CTI results also carry a high standard deviation (Riba et al. 2023), so a CTI value ranks materials under one contamination and is not a service rating for a part.

Which Other Electrical and Fire Tests Are Read Next to CTI?#

A CTI value is read next to 5 other tests, because a part that resists tracking still has to resist ignition, flame spread, static charge, outdoor erosion and long-term heat. The complete map of testing plastic additives and additive-dependent properties shows which of the 42 methods answers which formulation question.

Glow wire test (GWIT, GWFI)#

The glow-wire test is the ignition partner of CTI on electrical data sheets: IEC 60335-1 clause 30.2.3 asks for a glow-wire flammability index (GWFI) of at least 850 °C and a glow-wire ignition temperature (GWIT) of at least 775 °C. The wire is applied for 30 s, and Clariant reports a GWFI of 960 °C in glass-filled PA66 and a GWIT of 775 °C for its Exolit OP 1312 blend, the system it also reports at CTI 600 V. How the wire and the two indices work is set out on the glow wire test (GWFI, GWIT) page.

UL 94 and the limiting oxygen index#

UL 94 and the limiting oxygen index rank how a compound burns, which CTI does not: a V-0 rating means each afterflame stops within 10 seconds at the stated thickness, and the LOI is the lowest oxygen concentration that keeps the material burning. All 12 classes and their loadings are on UL 94 flammability ratings.

The LOI is measured in vol % of oxygen under ASTM D2863-23e1 and ISO 4589-2, against about 21 vol % in air. LOI values by polymer are tabulated on limiting oxygen index (LOI).

Surface resistivity and dry arc resistance#

Surface resistivity measures how well a dry, clean surface insulates, in ohms to ASTM D257, while CTI measures the voltage the same surface withstands once it is wet and contaminated. Antistatic surfaces sit at 10⁹ to 10¹³ ohm and insulative ones above 10¹⁴ ohm. ASTM D495 dry arc resistance is the older dry counterpart, and Riba et al. (2023) cite the wet CTI test as the superior measure. Antistatic and insulative ranges are on surface resistivity.

The inclined plane test (IEC 60587) for outdoor insulation#

The inclined plane test of IEC 60587 is the outdoor counterpart of CTI: it runs a contaminant down a tilted specimen at higher voltages for longer, and IEC 60112 itself warns that it can rank materials in a different order. Its contaminant adds 0.02 % of a non-ionic wetting agent to 0.1 % ammonium chloride, and Riba et al. (2023) report that during arcing the centre of the carbon track exceeds 1,200 °C.

RTI and the UL Yellow Card#

A recognised compound's CTI reaches the buyer as a PLC on its UL Yellow Card, printed with its UL 94 rating by thickness and colour and its relative thermal index (RTI). How a compound gets its card is covered under UL recognition and Yellow Cards.

The thermal index on the same card, set under UL 746B, comes from long-term heat aging and RTI.

Where can you get a CTI test, and what does it cost?#

A CTI test is run by electrical-testing laboratories such as UL Solutions, which offers it to IEC 60112, and the cost depends on the number of voltage steps the material needs, because each step uses 5 fresh specimens or sites. A PTI at one named voltage needs only a single step. This page quotes no test or tester price, because no verified figure exists in our sources.

What is the CTI of FR4 circuit-board laminate?#

Standard FR4 glass-epoxy laminate sits in group IIIa, with an indicative CTI of 175 V to below 250 V in the compilation by Riba et al. (2023), and higher-CTI laminates exist because the brominated epoxy and filler content can be changed, as Chiu and Cheng (2006) showed. PCB layout and creepage design are outside this reference, which covers the plastics compound.

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