Fire Safety · Standards Guide
UL94 Flammability Ratings: A Practical Guide
In new-energy vehicles, electronics and electrical insulation, a material's reaction to fire can be the difference between a contained fault and a catastrophe. UL94 is the most widely used standard for rating the flammability of plastics and coatings — and the rating you specify directly affects safety, compliance and certification.
This guide explains what each UL94 rating means, how the tests work, and how to choose a flame-retardant coating for fiberglass insulation sleeves.
The rating ladder (TL;DR)
From lowest to highest fire performance: HB → V-2 → V-1 → V-0 → 5VB → 5VA. For most flexible insulation and electrical parts, the target is UL94 V-0.
| Rating | Test | What it means |
|---|---|---|
| HB | Horizontal | Slow horizontal burn — the lowest UL94 level |
| V-2 | Vertical | Self-extinguishes, but flaming drips are allowed |
| V-1 | Vertical | Self-extinguishes, no flaming drips |
| V-0 | Vertical | Fastest self-extinguish (≤ 10 s), no flaming drips |
| 5VB | 500 W flame | Severe exposure — a hole may form in a plaque |
| 5VA | 500 W flame | Most stringent — no hole permitted |
What is UL94?
UL94 is the "Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances." It measures how a material reacts to a small flame using two main test orientations — horizontal (the HB rating) and vertical (the V and 5V ratings). The vertical tests are far more demanding because the flame travels upward through the material.
V-0, V-1 and V-2 — how they differ
In the vertical burn test, a flame is applied to a specimen twice for 10 seconds each. Testers measure how long the specimen keeps burning (afterflame) and whether any flaming drips fall and ignite a cotton indicator below. The three vertical ratings share the same method but differ in how strict the limits are:
| Criterion | V-2 | V-1 | V-0 |
|---|---|---|---|
| Afterflame per specimen | ≤ 30 s | ≤ 30 s | ≤ 10 s |
| Total afterflame (5 bars) | ≤ 250 s | ≤ 250 s | ≤ 50 s |
| Flaming drips that ignite cotton | Allowed | Not allowed | Not allowed |
The 5VA and 5VB classifications use a larger 500 W flame for the most demanding enclosures and are stricter still than V-0.
Why halogen-free matters
A high UL94 rating tells you how the material behaves in a flame, but not what it releases. Traditional flame retardants often rely on halogens (bromine or chlorine), which can emit toxic and corrosive gases when they burn. Halogen-free systems reach the same V-0 performance without those by-products, making them safer for people and equipment and easier to keep RoHS and REACH compliant.
Choosing a flame-retardant coating for fiberglass sleeves
For fiberglass insulation sleeves, the flame performance comes from the coating system applied to the sleeve. Tentuo's halogen-free flame-retardant acrylic coating (TT-AC-FR) offers an adjustable rating up to UL94 V-0, is water-based, and is RoHS and REACH certified — protecting fiberglass insulation sleeves used in new-energy battery packs and high-temperature wiring.
Frequently asked questions
What does UL94 V-0 mean?
UL94 V-0 is the most stringent of the common vertical-burn ratings. To qualify, a specimen must stop burning within 10 seconds after each flame application and must not release flaming drips that ignite cotton placed beneath it. It is the rating typically required where fire safety is critical.
Is V-0 better than V-1 or V-2?
Yes. All three are vertical-burn ratings, but V-0 is the strictest: it allows only a 10-second afterflame per specimen and no flaming drips. V-1 allows up to 30 seconds and no flaming drips, while V-2 allows 30 seconds and permits flaming drips that ignite cotton.
What is the difference between UL94 HB and V-0?
HB is a horizontal-burn test measuring how slowly a material burns — it is the lowest UL94 classification. V-0 is a much stricter vertical-burn rating that requires rapid self-extinguishing and no flaming drips.
Are halogen-free flame retardants as effective as halogenated ones?
Yes. Modern halogen-free formulations can reach UL94 V-0 while avoiding the toxic, corrosive gases that brominated or chlorinated systems release during combustion — which is why halogen-free is preferred for RoHS/REACH-compliant and safety-sensitive applications.
Does a coating change a fiberglass sleeve's flammability rating?
It can. A flame-retardant coating such as Tentuo's TT-AC-FR is applied to fiberglass insulation sleeves to raise the finished sleeve's fire performance, with an adjustable flame-retardant rating up to UL94 V-0.
Need a UL94 V-0 flame-retardant coating?
See the full specifications for our halogen-free flame-retardant acrylic coating, or contact our engineers for samples.