Phenolic Glass Sheet
Phenolic glass sheet combines the proven electrical and mechanical performance of glass woven-fabric reinforcement with a phenolic resin matrix — producing flat laminate panel stock with 2–3× the flexural strength of paper phenolic sheet, higher arc resistance, and a 300°F (150°C) service temperature ceiling suited to high-voltage switchgear, power transformers, and structural insulating panels.
TL;DR — Phenolic Glass Sheet Quick Facts
| Property | Value |
|---|---|
| Standard panel sizes | 36" × 48" and 48" × 96" |
| Thickness range | 1/16" through 2" (standard stock) |
| Thickness tolerance (NEMA LI 1) | ±10% below 1/8"; ±0.015" at 1/4"–1/2" |
| Color | Natural green/olive (G5); black available |
| Lead time | 2–5 business days |
| Price tier | $$$ — above paper and fabric phenolic; below glass-epoxy (G10 and FR4) |
Standard Sizes — Phenolic Glass Sheet
Per NEMA LI 1:
| Thickness Range | Thickness Tolerance | Standard Panels |
|---|---|---|
| Below 1/8" | ±10% of nominal | 36"×48", 48"×96" |
| 1/8" to under 1/4" | ±0.020" | 36"×48", 48"×96" |
| 1/4" to under 1/2" | ±0.015" | 36"×48", 48"×96" |
| 1/2" to under 1" | ±0.020" | 36"×48", 48"×96" |
| 1" and above | ±0.030" | 36"×48", 48"×96" |
Standard thickness increments: 1/16, 1/8, 3/16, 1/4, 3/8, 1/2, 5/8, 3/4, 1, 1¼, 1½, 2 inches.
Glass-phenolic sheet above 2" thick is available on a made-to-order basis. For heavy plate applications, confirm lead time and minimum order quantity at quotation stage.
Properties Relevant to Sheet Form
Glass-phenolic (NEMA G5) sheet delivers a step-change in performance over paper and fabric phenolic flat stock:
| Property | G5 Glass-Phenolic | NEMA XX Paper Phenolic | NEMA C Cotton Phenolic |
|---|---|---|---|
| Flexural Strength (LW) | 40,000–50,000 psi | 12,000–15,000 psi | 15,000 psi |
| Tensile Strength (LW) | 30,000–40,000 psi | 8,000–10,000 psi | 10,000 psi |
| Compressive Strength (flatwise) | 45,000–55,000 psi | 25,000–30,000 psi | 36,000 psi |
| Dielectric Strength | 400–500 V/mil | 350–450 V/mil | 200 V/mil |
| Arc Resistance | 120–180 sec | 60–120 sec | 40–80 sec |
| Continuous Use Temperature | 300°F (150°C) | 230°F (110°C) | 250°F (121°C) |
| Density | 1.70–1.85 g/cc | 1.30–1.35 g/cc | 1.36 g/cc |
Key application advantages of glass-phenolic sheet over paper phenolic:
- 3× higher flexural strength for load-carrying structural insulating panels
- 300°F service temperature vs. 230°F — critical for transformer and motor winding applications
- Higher arc resistance (120–180 sec) for arc-gap and switchgear applications
- Greater moisture resistance — glass cloth absorbs less water than paper ply
For complete datasheet, see Phenolic Glass properties.
Typical Applications — Sheet Form
Glass-phenolic sheet targets high-performance electrical insulation and structural panel applications that exceed the limits of paper or fabric phenolic flat stock:
High-voltage switchgear arc barriers and shields — G5 glass-phenolic sheet 1/8" to 1/2" thick installed as arc-interrupting barriers between phases in high-voltage air-break switches and bus compartments. The 120–180 second arc resistance exceeds the arcing duration in most fault-clearing events, preventing barrier burnthrough.
Power transformer structural insulating panels — Structural barrier boards inside dry-type and oil-filled power transformers, supporting winding clamping hardware and separating primary and secondary winding structures. The 300°F service temperature accommodates the transformer thermal class while maintaining dimensional stability under sustained compression.
Motor stator slot separators and phase barriers — G5 glass-phenolic sheet cut into phase barrier strips and inter-phase separator plates in large AC motors (above NEMA frame 400 series) where winding temperatures under overload conditions approach or exceed the cotton phenolic thermal limit.
High-voltage test equipment insulating panels — Structural and insulating panels in high-voltage test boards, resonant test transformers, and hipot test equipment frames where the panel must support mechanical loads and provide isolation at voltages up to 100 kV and beyond.
Aerospace and defense structural insulators — G5 glass-phenolic sheet in aircraft electrical system panel boards, avionics bay insulating panels, and military equipment structural electrical isolation plates to MIL-I-24768 specifications.
Chemical-resistant electrical panels — Glass-phenolic sheet provides better resistance to common industrial chemicals (acids, bases, solvents) than paper phenolic — making it the choice for electrical panel boards in chemical plant environments where vapor-phase chemical attack is possible.
Machining Notes — Sheet
Glass-phenolic sheet is significantly more abrasive than paper or fabric phenolic — tooling must be upgraded accordingly:
Sawing: Carbide-tipped blade (60–80 tooth triple-chip). Diamond blade preferred for production volume cutting — the glass cloth quickly dulls carbide at high production rates. Flood coolant is not recommended; use air blast and dust extraction.
Routing and milling: Solid-carbide spiral router bit or carbide-insert milling cutter. Do not use HSS tooling — it will dull within inches in glass-phenolic. Climb milling preferred at panel edges to minimize fiber fraying. Depth of cut: 0.030"–0.050" per pass at router speed.
Drilling: Solid-carbide drill, 400–600 SFM, moderate feed with peck drilling for holes deeper than 2× diameter. Glass cloth causes rapid wear on drill cutting lips — replace drills at the first sign of dullness. Backer board required at exit face to prevent fiber blow-out.
Grinding (thickness): Silicon-carbide or diamond wheel; dry grinding with high-airflow dust extraction. Glass-phenolic grinds significantly faster than paper phenolic — aggressive wheel loading can occur.
Dust and fiber hazard: Glass fiber particulate from cutting glass-phenolic is a serious respiratory hazard. HEPA-rated dust extraction, NIOSH P100 respirator, and enclosed or well-ventilated machining area required. Skin contact with glass fiber dust causes dermal irritation — wear long sleeves.
See Phenolic Glass Machining Guide.
Standards and Compliance
| Standard | Scope |
|---|---|
| NEMA LI 1 | Grade G5 — glass-fabric melamine-phenolic laminate sheet |
| MIL-I-24768/4 | Type GSG — glass-cloth phenolic, general purpose |
| MIL-I-24768/5 | Type GSG-P — glass-cloth phenolic, specific electrical applications |
| ASTM D709 | Standard specification for laminated thermosetting materials |
| ASTM D149 | Dielectric strength test method |
Request a Certificate of Conformance citing NEMA LI 1 Grade G5 on every order. For MIL-spec applications, confirm manufacturer QPL qualification at quotation.
Get a quote on phenolic glass sheet — NEMA Grade G5, standard panels or cut-to-size
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