MIL-I-24768 Explained — Military Thermoset Laminate Specifications

MIL-I-24768 is the U.S. Department of Defense specification covering industrial laminated thermosetting electrical insulation materials — it defines performance requirements for the same NEMA-grade laminates used in commercial switchgear, but adds military-specific qualification, testing, and documentation requirements.

TL;DR — Key Takeaways

• MIL-I-24768 covers all major thermoset laminate families (paper-phenolic, fabric-phenolic, glass-epoxy, glass-melamine, glass-silicone)
• Each "type" within the spec maps to a commercial NEMA grade (GEE = G10, GEF = FR4, FBM = CE phenolic, etc.)
• The specification is divided into numbered slash-sheets (MIL-I-24768/1 through /27+) — each slash-sheet covers a specific grade
• Procurement to MIL-I-24768 requires certified test reports (CTR), Qualified Products List (QPL) sourcing in some cases, and traceability documentation
• Many commercial thermoset laminates meet MIL-I-24768 requirements but are not formally QPL-listed — verify with your supplier

Structure of MIL-I-24768

MIL-I-24768 is an umbrella specification. The base document defines:

• Scope and classification system
• General requirements (workmanship, dimensions, tolerances)
• Quality assurance provisions (first article, periodic, acceptance testing)
• Packaging and marking requirements

The actual material-specific requirements appear in slash-sheets (appendix documents identified as MIL-I-24768/1, /2, /3, etc.). Each slash-sheet specifies a particular resin-reinforcement system, property requirements, and test methods.

Designation Code Structure

MIL-I-24768 uses a three-letter type code that encodes the laminate system:

Letter 1 — Reinforcement:

• P = Paper
• F = Fabric (woven organic: cotton, linen)
• G = Glass (woven glass fabric)
• M = Glass mat (random-fiber, non-woven)

Letter 2 — Resin:

• B = Phenolic (base resin)
• E = Epoxy
• M = Melamine
• S = Silicone
• P = Polyester

Letter 3 — Grade modifier:

• E = Standard electrical grade
• G = Low-loss electrical (equivalent to NEMA "low-loss" modifier)
• F = Flame retardant
• B = High temperature / high strength
• M = Melamine variant

MIL-I-24768 Type Designations — Complete Mapping

Note: Slash-sheet numbers may vary by revision level. Always reference the current issue of MIL-I-24768 for procurement.

Key Slash-Sheets in Detail

MIL-I-24768/14 — GEE (G10) and GEF (FR4)

This is the most frequently invoked slash-sheet in electrical and military programs. It covers both GEE (glass-epoxy without flame retardant = G10) and GEF (glass-epoxy with flame retardant = FR4).

Critical distinction: GEE and GEF are different materials within the same slash-sheet — they share the same structural and most electrical requirements but differ in:

• Flammability: GEE is HB (horizontal burn); GEF is V-0 (self-extinguishing)
• GEF uses brominated epoxy resin (tetrabromo bisphenol A, TBBPA) to achieve V-0
• Both achieve essentially the same dielectric strength (~500 V/mil at 1/16″)

When a drawing calls out "MIL-I-24768/14" without specifying GEE or GEF, request clarification — the two grades are interchangeable mechanically but not for fire-code compliance.

MIL-I-24768/15 — GEB (G11)

G11/GEB is G10 with a higher-temperature epoxy system (multifunctional or tetra-functional epoxy vs. the difunctional base in standard G10). The Tg of GEB is ~170°C versus ~130°C for GEE.

Specify GEB when:

• Continuous service temperature exceeds 130°C
• The laminate is used in proximity to power transformers or motors that run hot
• Dimensional stability at elevated temperature is critical

MIL-I-24768/12 — GME (G5)

G5/GME uses a glass-woven fabric with melamine resin — melamine produces exceptionally high arc resistance and is self-extinguishing without halogenated additives. Used almost exclusively in arc chute applications (circuit breaker interiors) and high-arc-resistance switchgear components.

Testing and Documentation Requirements

Acceptance Testing

MIL-I-24768 divides tests into:

Certified Test Reports (CTR)

A CTR from the laminate manufacturer or mill must accompany MIL-I-24768 material and include:

• Material identification (grade, slash-sheet, lot number)
• Nominal thickness and panel dimensions
• Test results for each required property
• Test method references (ASTM D229, D709, D149, etc.)
• Certifying signature and date

QPL (Qualified Products List)

Some military contracts require the laminate to appear on the QPL maintained by the Defense Logistics Agency (DLA) or Naval Sea Systems Command (NAVSEA). Not all commercial laminates meeting the technical requirements are QPL-listed. Verify QPL status with your supplier or the DLA database at www.ql.dla.mil.

How to Read a MIL-I-24768 Specification Call-Out

A typical drawing call-out reads:

MIL-I-24768/14 Type GEF, 0.125 ± 0.010 inch, Class ___

Decoding:

• MIL-I-24768/14 = Slash-sheet 14 (glass-epoxy)
• Type GEF = FR4 (flame-retardant glass-epoxy)
• 0.125 ± 0.010 inch = thickness with tolerance
• Class = may reference a thickness tolerance class defined in the slash-sheet

If the class field is blank or unspecified, use the standard dimensional tolerances in NEMA LI-1 Table 3 for the equivalent commercial grade.

MIL-I-24768 vs. ASTM D709

ASTM D709 covers the same materials as MIL-I-24768 and uses the same NEMA grade designations. The key differences:

For commercial switchgear and industrial HV equipment, ASTM D709 is typically sufficient. For naval, aerospace, or DoD-funded programs, MIL-I-24768 is required.

More related guides

Specifications

• Dielectric Strength Testing of Thermosets
• Arc Resistance vs Track Resistance

Applications

• Thermoset Tubes for Aerospace
• High-Voltage Standoff Materials
• Switchgear Insulation Materials

Materials

• G10 Material Hub
• FR4 Material Hub
• Phenolic-Glass Material Hub