Phenolic Glass Silicone Rod
NEMA G7 silicone-glass rod is the highest-temperature-rated rod in the commercial phenolic laminate family — a woven glass cloth reinforced with a silicone resin binder that provides a 180°C (356°F) continuous service temperature rating, arc resistance exceeding 180 seconds, and exceptional electrical stability at elevated temperatures far beyond the reach of epoxy-based or standard phenolic-resin-based laminates.
TL;DR — NEMA G7 Rod Quick Facts
| Property | Value |
|---|---|
| Typical diameters | 1/2" – 3" (standard stock; larger by special order) |
| Standard length | 4 ft (48") |
| OD tolerance (NEMA LI 1) | +0.025" / −0.000" up to 1½" |
| Color | Natural cream/white (silicone resin gives lighter color) |
| Lead time | 3–7 business days |
| Price tier | $$$$ — premium specialty laminate |
Standard Sizes — NEMA G7 Rod
Per NEMA LI 1:
| Nominal Diameter | OD Tolerance | Standard Length |
|---|---|---|
| 1/2" – 1½" | +0.025" / −0.000" | 4 ft |
| Over 1½" – 3" | +0.031" / −0.000" | 4 ft |
G7 rod is produced in smaller diameter ranges than fabric phenolic grades — typical maximum stock diameter is 3". For diameters above 3" in a high-temperature laminate rod, consult for special-order G7 or consider whether G9 (melamine-glass) satisfies the application at a lower cost point.
Properties Relevant to Rod Form
The silicone resin in NEMA G7 is what sets it apart from all other laminate grades in thermal performance. Silicone resins are organopolysiloxanes that retain mechanical integrity and electrical properties at temperatures where epoxy (G10 and FR4) and standard phenolic resins undergo rapid degradation.
| Property | NEMA G7 | NEMA G5 (Glass-Phenolic) | NEMA G10 (Glass-Epoxy) |
|---|---|---|---|
| Flexural Strength (LW) | 35,000–45,000 psi | 40,000–50,000 psi | 60,000 psi |
| Tensile Strength (LW) | 25,000–35,000 psi | 30,000–40,000 psi | 45,000 psi |
| Compressive Strength (flatwise) | 35,000–45,000 psi | 45,000–55,000 psi | 65,000 psi |
| Dielectric Strength | 400–500 V/mil | 400–500 V/mil | ~500 V/mil |
| Arc Resistance | 180–240 sec | 120–180 sec | 60–120 sec |
| Continuous Use Temperature | 356°F (180°C) | 300°F (150°C) | 266–284°F (130°C) |
| Density | 1.70–1.85 g/cc | 1.70–1.85 g/cc | 1.85 g/cc |
180°C continuous service — the defining advantage of G7. This aligns with IEC 60085 Thermal Class H rating, making G7 the only standard laminate rod grade suitable for Class H motor and transformer winding insulation systems.
Arc resistance 180–240 seconds — the highest arc resistance of any common laminate grade, making G7 rod the choice for arc-gap components in high-voltage switching equipment where arcing time cannot be predicted and the insulator must survive extended arcing.
G7's mechanical strength is modestly lower than G5 glass-phenolic or G10 glass-epoxy — the silicone resin bonds less strongly to glass fiber than epoxy or phenolic resins. For applications where maximum mechanical strength is the driver and temperature is secondary, G5 or G10 may be more appropriate.
For the complete datasheet, see Phenolic Glass Silicone properties.
Typical Applications — Rod Form
G7 silicone-glass rod is specified when temperature or arc resistance requirements exceed what G5 glass-phenolic rod can meet:
Class H motor insulating rods and spacers — IEC 60034 Class H insulation systems (180°C rated) in large motors and generators require all insulating components to carry the Class H thermal rating. G7 rod qualifies; G5 glass-phenolic rod (Class F, 155°C) does not.
Power transformer insulating support rods — Large oil-filled power transformers operating at elevated thermal loading use G7 rod as structural clamping rods and insulating support members in the winding assembly. The 180°C rating provides adequate thermal margin against hot-spot winding temperatures.
High-voltage arc-gap insulator rods — Rod-type insulators in disconnect switches, load break switches, and recloser assemblies where the rod face sees arcing during interruption. G7's 180–240 second arc resistance prevents surface carbonization that would create conductive tracking paths.
Aerospace and military avionics rods to MIL-I-24768/7 — G7 is cross-referenced to MIL-I-24768/7 (Type GSG-S) for high-temperature aircraft electrical applications: bonding cables' isolation rods, antenna support rods, and structural insulating members in high-temperature aircraft zones.
Industrial furnace and kiln electrical isolation — Insulating rods and standoffs in industrial furnace electrical systems, heat treatment equipment wiring, and kiln car electrical isolation where ambient temperatures approach or exceed 150°C.
Downhole oil and gas electrical isolation — G7 rod as insulating elements in downhole measurement-while-drilling (MWD) and logging-while-drilling (LWD) tools where borehole temperatures can reach 150–180°C.
Machining Notes — Rod
G7 silicone-glass rod machines similarly to G5 glass-phenolic with slightly modified parameters due to the silicone resin:
Tooling: Solid-carbide or PCD tools mandatory. The silicone resin wears tooling at a similar rate to standard glass-phenolic — glass fiber is the primary abrasive factor regardless of resin type.
Cutting parameters (turning):
- Surface speed: 450–650 SFM (carbide); 650–950 SFM (PCD)
- Feed rate (roughing): 0.003–0.007 IPR
- Feed rate (finishing): 0.001–0.002 IPR
- Coolant: Dry or light air mist; flood coolant will not damage G7 but creates glass-fiber-laden slurry for disposal
Surface behavior: G7 silicone-glass machines to a slightly rougher surface finish than G10 glass-epoxy at the same parameters — the silicone resin is less rigid and supports less clean chip formation. Expected surface finish: Ra 32–63 μin on a turned OD.
Interlaminar shear: G7 has lower interlaminar shear strength than G10 due to the silicone resin's lower adhesion to glass fabric. Avoid heavy interrupted cuts or severe unsupported overhang that could induce interlaminar stress.
Dust and fiber control: HEPA exhaust ventilation; NIOSH P100 respirator; enclosed work area. Silicone-glass rod dust contains glass fiber and silicone decomposition products — both respiratory concerns.
See Phenolic Glass Silicone Machining Guide.
Standards and Compliance
| Standard | Scope |
|---|---|
| NEMA LI 1 | Grade G7 — silicone-glass laminate rod |
| MIL-I-24768/7 | Type GSG-S — silicone-glass phenolic, high temperature |
| IEC 60085 | Thermal Class H (180°C) |
| ASTM D709 | Standard specification for laminated thermosetting materials |
| ASTM D495 | Arc resistance test method |
Request a Certificate of Conformance citing NEMA LI 1 Grade G7 on every order. For aerospace and military applications, confirm QPL qualification and IEC thermal class certification at quotation.
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