Stacking Fire Requirements: UL94, Glow Wire and CTI in One Part

Quick answer: UL94, glow wire and CTI fail materials for different reasons, so a compound can clear one and miss another with no warning. Only the first is really a flame test — glow wire is about a sustained hot element, and CTI is not a fire test at all. Specify all three from the start and design to whichever is hardest, because for most connector and appliance work that is not the burn rating.

A customer sends a specification: PA66-GF30, V-0 at 0.8 mm, GWIT 775 °C, CTI 600 V. Four numbers, one material. Each is achievable on its own; together they eliminate most of the formulation space, and the combination that survives is not obvious from any single datasheet. This is the everyday reality of electrical component compounding, and it is where the most expensive iteration loops happen.

What Each Test Actually Measures

Note that only the first two are really about burning. GWIT is about resisting ignition from a sustained hot source — the failed solder joint or overheating contact inside real equipment — and CTI is not a fire test at all. It measures electrical surface degradation under contamination and moisture. Treating all four as “the fire requirements” is how the conflicts get missed.

Where They Fight Each Other

Q: Why does improving my UL94 rating hurt the other tests? Because the mechanisms that pass a brief flame test are not always the ones that survive sustained heat or resist surface tracking.

Three tensions come up repeatedly:

1. Brief flame versus sustained heat. UL94 applies a small flame for ten seconds; a glow wire holds 750–960 °C against the specimen continuously. Formulations that rely on rapid self-extinguishing after flame removal can still ignite under sustained contact. This is the most common surprise: V-0 achieved, glow wire failed, and the fix is a char-forming system rather than a faster-extinguishing one.
2. Tracking resistance versus additive chemistry. CTI depends on how the surface behaves when contaminated and electrically stressed, and it is sensitive to what is in that surface. Ionic and conductive species tend to hurt it — which is one reason halogen-free phosphinate systems became the default for high-CTI connector applications, and why carbon black pigmentation deserves a check on electrical parts, as noted in our article on colour matching.
3. Everything versus thin walls. Reducing the wall makes UL94 harder, as covered in our guide to thin-wall flame retardancy — and glow-wire performance is also thickness-dependent. A stack that is comfortable at 1.6 mm can become genuinely difficult at 0.8 mm across several requirements at once.

A useful point of agreement in the middle of all this: anti-drip control helps in both directions. Flaming drips fail V-0, and dripping that ignites the tissue below also fails the glow-wire flammability index. Fibrillating PTFE at fractional loadings is one of the few additives that serves several requirements simultaneously — the wider synergist family is mapped in our guide to flame retardant synergists.

The Method: Hardest Requirement First

Do not start with UL94 simply because it is the familiar one. Work in this order:

• Rank the requirements by difficulty for your specific material and wall. For glass-filled polyamide at thin walls, glow wire and CTI are frequently harder than V-0 — the opposite of what most project plans assume.
• Formulate for the hardest one. Its constraint usually decides the chemistry family outright, and the remaining requirements get tuned within that family rather than fought across families.
• Verify the whole stack on the same specimens. Every time. A change made to fix one number can move another, and testing sequentially across months hides that.
• Re-test the stack after any change — colour, carrier, glass content, PCR fraction, resin grade. Each of these can move at least one of the four.

For the common case — glass-filled PA66 or PBT connectors needing V-0 at thin walls plus glow wire plus high CTI — the industry has largely converged on halogen-free phosphinate chemistry with a nitrogen synergist, which is why aluminium diethylphosphinate paired with melamine polyphosphate is the default rather than one option among many. It is not that this system wins every individual test; it is that it clears the whole stack at once, which is the only thing that matters.

Certification Scope: What Your Approval Actually Covers

One administrative point that costs money when misunderstood. A material’s listing is specific along several axes at once:

• Thickness — the rating applies at the certified thickness and above, never below.
• Colour — colours are covered explicitly; adding one usually needs testing rather than an assumption of coverage.
• Grade — the specific compound, not the chemistry family.

So “our material is V-0” is never a complete statement. The complete statement names the grade, the colour, and the thickness — and for an electrical part it names the glow-wire and CTI results alongside. When you request data from a supplier, ask for the stack at your thickness and colour rather than a headline rating; how to read the resulting documents is covered in our guide on sample evaluation.

If you are holding a specification with four numbers on it and no obvious material that meets all of them, send us the full stack — resin, glass content, wall thickness, colour and every test clause. We ladder the package against the hardest requirement first and burn-test the whole stack before quoting, which is slower than sending a datasheet and considerably faster than three failed qualification rounds. One administrative point deserves its own read — what a UL listing actually covers, and why an additive cannot carry one: UL Yellow Card explained.

FAQ

What is the difference between GWFI and GWIT?

GWFI measures how the material behaves when a glowing wire is applied — how long it burns and whether it ignites tissue below. GWIT identifies the temperature at which ignition occurs at all. Specifications usually name one or the other with a temperature; check which, because they are not interchangeable.

Can one material pass UL94 V-0, glow wire and high CTI at the same time?

Yes — halogen-free phosphinate systems in glass-filled polyamide and PBT do this routinely, which is why they dominate connector applications. It requires designing for the whole stack from the start rather than optimising the burn test and hoping the rest follows.

Why does my part pass UL94 but fail the glow-wire test?

Because the tests model different fire scenarios. UL94 uses a brief flame; the glow wire applies sustained high temperature. Formulations that depend on rapid self-extinguishing rather than char formation are the usual failures. The fix is typically a condensed-phase char system, not more of the existing additive.

Does a UL Yellow Card cover every colour and thickness of a grade?

No. The listing states the thicknesses and colours evaluated, and the ratings apply only within that scope. Adding a colour or thinning a wall generally means additional testing — budget for it at design stage rather than discovering it during certification.

These three tests rarely arrive one at a time. Connectors, switchgear and appliance parts routinely carry a burn grade, a tracking class and a glow wire value in the same specification — the component-by-component view of that is on our flame retardant for electronics page.

Final Thoughts

The lesson from running these four tests together is to start from the hardest requirement, not from UL94. In practice that is usually tracking or glow wire, and it often forces halogen-free before any environmental policy does. Our phosphinate and MPP systems exist for exactly that case – send your CTI and GWIT targets with the wall thickness and we will tell you what is achievable together.

—— Rectivas Materials 团队 老陈