Welding Blanket FM 4950 Class A vs B vs C — How to Choose per Hot Work Permit NFPA 51B

In Thai factories, construction sites, and railway infrastructure projects, the phrase "welding blanket Class A/B/C" is in common use — but the ANSI/FM Approvals 4950 standard does not actually use Class A/B/C letter designations in the way ASTM does. It defines three product types by application form and severity of use. This article clears up the confusion and provides a selection guide based on NFPA 51B and OSHA 1910.252.

What FM Approvals 4950 Actually Defines

The ANSI/FM 4950 standard, most recently published by FM Global, classifies products into three types based on method of use:

1. Welding Curtain — Hung vertically as a drape, separating the welder from the production area. Catches sparks and slag projected laterally. (Industry shorthand: "Class A")
2. Welding Blanket — Laid horizontally or draped around structures to protect against light-to-medium welding work. Catches slag falling from above. (Industry shorthand: "Class B")
3. Welding Pad — Receives direct molten metal and heavy slag. Used in plasma cutting, oxyfuel burning, and slag tapping operations. (Industry shorthand: "Class C")

In FM Global's official documentation, search for "4950" not "Class A/B/C" — but Thai and Chinese suppliers use the Class A/B/C terminology because that is what customers ask for. The meaning is equivalent.

Comparison Table — Three Product Types

Flowchart: Selecting by Hot Work Scenario

flowchart TD
  A[Identify Hot Work task] --> B{Orientation of
spark protection?}
  B -->|Vertical
area partition| C[Welding Curtain
Class A · 550°C]
  B -->|Horizontal
lay/drape| D{Severity?}
  D -->|Light MIG/MAG
spark protection| E[Welding Blanket
Light · Fiberglass]
  D -->|TIG/Arc
heavy| F[Welding Blanket
Class B · Silica 1000°C]
  D -->|Plasma/oxyfuel
molten splatter| G[Welding Pad
Class C · Ceramic 1260°C]
  C --> H[NFPA 51B
11 m radius]
  E --> H
  F --> H
  G --> H
  H --> I[Fire Watch
+ 30 min after work]

NFPA 51B Section 4.3 — Coverage Requirements

NFPA 51B (2024 edition) section 4.3 specifies the following requirements for the hot work area:

• 4.3.3.1 Remove all combustible materials within a 35-foot (11-metre) radius of the work point.
• 4.3.3.2 Where removal is not possible, cover with flame-retardant material meeting a recognised standard such as FM 4950, EN 1869, or equivalent.
• 4.3.4 Seal any gaps, cracks, holes, or openings in walls or floors within the radius to prevent falling slag from penetrating.
• 5.3 A Fire Watch must be on duty throughout the work and for 30 minutes after completion (approximately 50% of hot-work fire incidents occur within the first hour after welding stops).

OSHA 1910.252 (b)(4)(i) uses the same basis as NFPA 51B but is enforceable under US federal law. In Thailand it is not directly mandatory, but EPC contractors for Saudi Aramco, ExxonMobil, PTTGC, and similar operators enforce it through contract clauses.

Substrate Materials — Fiberglass / Silica / Ceramic Fibre

Materials on the Thai market that meet FM 4950:

• E-glass Fiberglass — Most affordable. Suitable for general MIG/MAG work. Continuous temp 250–550°C. Woven in twill or satin patterns, 0.43–1.5 mm thick.
• Texturized / Bulked Fiberglass — Extra coverage layer, suitable as a heavier alternative for Arc work. Temperature ~700°C.
• High-Silica (SiO₂ > 96%) — For heavy TIG/Arc work and slag handling. Continuous temp 1,000°C, short-term 1,200°C. Usually vermiculite-coated.
• Ceramic Fibre (Al₂O₃–SiO₂) — For furnace work, plasma cutting. Temp 1,260–1,425°C. Most expensive — use only where necessary.
• Pre-oxidized Aramid / Carbon — Handles sudden heat surges but lower continuous temp than silica — used in aerospace applications, not common in general industrial work.

Surface coating is equally important as the base fabric:

• Silicone — Flexibility + water resistance, good chemical resistance. Suitable for vertical Curtains.
• Vermiculite — Expands under heat, better slag blocking. Suitable for Welding Pads.
• Acrylic/Plain — Lowest cost, lightest weight. For light spark protection only.

6 Procurement Guidelines

1. Specify FM 4950 in the TOR — Do not simply write "welding blanket". State the type (Curtain/Blanket/Pad) + continuous temperature rating.
2. Request FM Approvals certification — Verify the manufacturer name at fmapprovals.com/listings. If the manufacturer does not appear in the listings, they are citing the standard but are not actually FM-approved.
3. Fabric weight (g/m²) is related to service life — 600 g/m² fiberglass lasts 2–3 times longer than 350 g/m² in service.
4. Sizes — Standard sizes available in Thailand: 1×1, 1×2, 2×2, 2×3, 3×3 metres. Custom cutting is available but adds 30–50% to price.
5. QC hold point — Randomly test per ASTM D4029 (flame propagation) or EN 1869 before releasing final payment.
6. Storage — Keep in a dry environment at 5–40°C; do not stack heavy objects on top. Inspect before every use (holes, brittleness, loose weave).

Summary

FM Approvals 4950 classifies products into three types (Curtain / Blanket / Pad) by form and work severity — not by "Class A/B/C" letter designations, even though that is the common trade usage. Silicone-coated Fiberglass is sufficient for general MIG/MAG and spark protection. Vermiculite-coated Silica is required for heavy Arc/TIG work. Ceramic Fibre is necessary for plasma cutting and oxyfuel work involving molten splatter.

Sahawatthanakit supplies FM 4950-rated welding protection products in all three types, with custom-cut sizes and Mill Certificates to EN 1869 — contact us to request test samples before placing a large order.

Frequently Asked Questions

What exactly are FM 4950 Class A/B/C? FM 4950 officially defines three product types by application: Welding Curtain (vertical hang), Welding Blanket (horizontal lay), and Welding Pad (direct molten splatter). Industry uses A/B/C as shorthand — the meaning matches, but FM does not use these letter designations in its official documentation.

How many jobs before replacement? Light-to-medium Welding Blanket: 100–300 cycles. Curtain not directly struck by slag: 1–3 years. Welding Pad receiving direct molten splatter: 50–150 cycles. Inspect before every use — replace immediately if holes appear or fabric is brittle.

What protection radius does NFPA 51B require? 35 feet (11 metres) around the work point. If clearance is not possible, cover with flame-retardant material meeting FM 4950 or EN 1869. Fire Watch must remain on duty throughout + 30 minutes after completion.

How do Fiberglass / Silica / Ceramic Fibre differ? Not just temperature. Fiberglass (550°C): MIG/MAG work. Silica (1,000°C): heavy TIG/Arc. Ceramic Fibre (1,260°C): furnace and plasma cutting — 3–5 times more expensive, use only where necessary.

Vermiculite vs Silicone coating? Vermiculite expands under heat — blocks molten slag at 1,000°C+, best for Welding Pads. Silicone is flexible and water-resistant — general Curtain work at 550°C.

What documents are needed for a Hot Work Permit in Thailand? Type and temperature rating of the blanket, coverage area + 11 m protection radius, Fire Watch name and watch duration, location of nearest fire extinguisher, signature of permit issuer / safety officer.

Order & Inquiries

Sahawatthanakit (1988) Co., Ltd. — Welding blanket and fire protection product supplier across Thailand, VAT invoices available.

• Tel: +66-2-096-2118
• Email: info@sahawatthanakit1988.com
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