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Arc Duty Is Not Flammability: What UL 94, CTI, Arc Resistance, and FST Data Each Actually Claim
"Fire-rated" is the most dangerous phrase in barrier procurement, because it lets four different data sets impersonate each other. A flammability class describes how a specimen behaves in a flame test. A tracking index describes how a surface resists carbonized leakage paths. Arc resistance describes survival under a high-voltage, low-current dry arc. And FST data describes what a material emits when it burns in an enclosed room.
The H-O arc flash and fire protection application page keeps these four apart by designation, UL 94, IEC 60112, ASTM D495, and ASTM E662 / E162, and this article walks what each one actually claims, what none of them claim, and the failure patterns that follow when they are confused.
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UL 94: a flame-test class, per grade, and nothing more
UL 94 classes describe specimen behavior in a standardized flame test, and they belong to specific grades as tested by their manufacturers: FR4 carries V-0 where plain G10 is unclassed; Durostone UPM grades carry V-0 file classes; the ManniGlas grades carry V-0 file classes; the FST neoprene reports V-0. Two confusions follow the class around.
First, the class is assumed instead of read: the program says "fire-rated barrier material," a generic laminate is ordered, and the audit finds a TDS with no class at all; the fix is to name the grade on the drawing, not the polymer family, and keep the TDS with the order file. Second, and subtler: the class is mistaken for radiant-heat longevity.
Organic materials, including good flame-retardant grades, age fast under continuous radiant load even when they never ignite, browning, embrittling, and shrinking back from their fasteners. The flammability class describes behavior in a flame test, not longevity under radiation; surfaces that face continuously hot zones climb the ladder to inorganic ManniGlas glass-fiber paper or mica sheet.
CTI and arc resistance: the arc-side data
Switchgear air is not clean. Dust, moisture films, and conductive contamination let surface leakage currents carve carbonized tracks across an insulating surface, and a tracked barrier is a failed barrier: the next overvoltage event follows the track. Two TDS lines speak to this duty.
The comparative tracking index per IEC 60112 ranks surfaces by track resistance and is reported on the Durostone composite TDS and on glass-epoxy laminate data; high-voltage low-current dry arc resistance per ASTM D495 is reported on the RF-120 heat-shield foam TDS. Mica does not carbonize at all, which is half of why it owns the chute.
The parent page's rule for polluted or condensing compartments: check the CTI line on the TDS before the dielectric line, because designs fail by tracking long before they fail by puncture.
What Each Line on the TDS Actually Claims
| Data set | Method | What it claims | What it does not claim |
|---|---|---|---|
| Flammability class | UL 94 (per grade TDS) | Behavior in a flame test | Longevity under continuous radiant load; smoke behavior |
| Tracking index (CTI) | IEC 60112 | Surface resistance to carbonized tracks | Bulk dielectric strength; arc-plasma survival |
| Dry arc resistance | ASTM D495 | Survival under HV low-current dry arc | An assembly arc rating; a flammability class |
| FST data | ASTM E662 (smoke), ASTM E162 (radiant flammability) | Emission behavior for enclosed, occupied spaces | Structural or tracking performance |
| Assembly arc rating | IEEE C37.20.7 | A tested complete switchgear design | Anything about a material in isolation |
Methods by designation as cited on the parent application page; classes and values are per each grade's current TDS, and accessibility-type designations (including Type 2B) belong to tested designs.
FST: the enclosed-room question
Inside an enclosed electrical room, a substation basement, or walk-in gear (the work environment NFPA 70E governs), the question expands from "will it burn" to "what does it emit": flame, smoke, and toxicity together. Every material in the room can carry a flammability class and the room can still fail the safety engineer's question, because smoke obscuration and toxic emission decide survivability in an enclosed space.
The fix is to add the FST lines to the material callout: ASTM E662 specific optical density and ASTM E162 radiant-panel data, as reported on the TDS. The FST neoprene reports E662 data with UL 94 V-0; the V-0 silicone sponge family's super-soft grade reports E162 / E662; and the inorganic families sidestep the question by not burning. "Fire-rated" in a catalog title is not data.
Arc duty, fire duty, and the drawing that says which
The parent page's specification principle ties the four data sets together: an arc barrier manages plasma, pressure, and tracking, so its callout asks for CTI, arc resistance, and dimensional stability under thermal shock; a fire barrier manages ignition, flame spread, and heat transmission, so its callout asks for the UL 94 class, non-combustibility, and conductivity data.
Mica and Durostone are arc-side materials; ManniGlas and the heat-shield foam are fire-side; glass-epoxy laminates serve both within their classes. Write the duty on the drawing, state whether the exposure is continuous or fault-event-only, and the rest of the selection follows.
Assembly-level arc performance belongs to designs tested under IEEE C37.20.7 and the IEC 62271 family; the materials support the tested design rather than carrying a rating alone.
How H-O Products helps
H-O Products is a precision converter, not a test laboratory.
We convert mica, laminates, glass-fiber paper, heat-shield foam, and FST elastomers to your drawing, die-cut, waterjet-cut, machined, and laminated, and we carry the maker's documentation with the order: the TDS whose UL 94, CTI, ASTM D495, and FST lines your specification depends on, with material traceability and lot-code records under our ISO 9001:2015 certified quality management system.
When a drawing says "fire-rated" without naming a data set, that is the first question our engineering review asks.
Frequently asked questions
Is a UL 94 V-0 material safe next to a continuously hot surface?
Not necessarily. The class describes behavior in a flame test, not longevity under radiation: organic materials, including good flame-retardant grades, brown, embrittle, and shrink back from fasteners under continuous radiant load even when they never ignite. For surfaces facing continuously hot zones, climb the ladder to the inorganic families, glass-fiber paper or mica, and state on the drawing whether the exposure is continuous or fault-event-only.
What is CTI and why does it matter for switchgear barriers?
The comparative tracking index, per IEC 60112, ranks how well an insulating surface resists forming a carbonized conductive track under contamination and voltage. A tracked barrier is a failed barrier: leakage carbonizes a path millimeter by millimeter until flashover follows it. For barriers in dusty, humid, or condensing compartments, check the CTI line on the TDS before the dielectric line, prefer high-CTI laminates or inorganic sheet, and detail the part so creepage paths across it stay long.
Does a material with ASTM D495 arc data make my switchgear arc-resistant?
No. ASTM D495 is a material-level method: survival under a high-voltage, low-current dry arc, reported on TDSs such as the heat-shield silicone foam's. Arc-resistant switchgear is qualified as a complete assembly under IEEE C37.20.7, and the accessibility-type designations the industry quotes belong to tested designs. Material data informs the design; the rating belongs to the assembly's test reports.
Which barrier materials carry UL 94 V-0 on their TDS?
Per the parent page: Durostone UPM 203 and UPM S16 carry V-0 file classes; FR4 (NP511) is the V-0 member of the glass-epoxy family where plain G10 is unclassed; the ManniGlas grades carry V-0 file classes; the FST neoprene reports V-0; and the heat-shield foam reports its own flammability data on its TDS. The class belongs to the specific grade as tested by its manufacturer, so name the grade on the drawing and read the class line on that grade's TDS before ordering.
Have the data requirements sorted before the audit does it for you
Send the program spec lines and the barrier locations, and engineering will map which TDS data each part actually needs: UL 94 class, CTI, arc resistance, or FST smoke data.
Samples typically ship in 3 to 5 business days for common die-cut configurations on materials we keep on hand; standard production runs about two weeks after drawing approval; expedited service is available. MOQ varies by material and part.
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- UL 94 (flammability classes, per grade TDS); IEC 60112 (comparative tracking index); ASTM D495 (high-voltage low-current dry arc resistance); IEC 60243 (dielectric methods).
- ASTM E662 (specific optical density of smoke) and ASTM E162 (surface flammability under radiant heat), the FST data set; NFPA 70E (the workplace context).
- IEEE C37.20.7 (testing complete switchgear assemblies for internal arcing faults) and IEC 62271 (HV switchgear and controlgear).
- Manufacturer technical data sheets for the mica, laminate, composite, glass-paper, heat-shield foam, and FST neoprene grades named on the parent application page.
This article explains the distinct claims of flammability, tracking, arc-resistance, and FST data sets using the methods and materials cited on the parent application page. Classes and values belong to specific grades on their current technical data sheets; assembly arc ratings belong to complete designs tested under IEEE C37.20.7; and no temperature limit or rating is invented here.
All parts are custom manufactured to order; minimum production run quantities vary by material and part. H-O is a converter, not a raw-material producer or test laboratory, and does not certify finished switchgear.