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D2768-95 – Standard Test Method Technical Guide
📐 Scope and Material Composition
ASTM D2768 – 95 (Reapproved 2001) covers a weather and ozone-resisting crosslinked compound intended for use as the outer covering or jacket on insulated wires and cables. The polymer component of this material consists substantially of ethylene-propylene copolymer (EPM) or ethylene-propylene terpolymer (EPDM). The standard specifies inch-pound units as the standard, except in cases where SI units are more appropriate.
💡 Tip: The crosslinked nature of the EPM/EPDM compound provides outstanding resistance to heat, ozone, and weather, making it ideal for outdoor cable applications and high-temperature environments.
⚡ Electrical and Weathering Requirements
When used on single conductor nonshielded cables rated 2001 through 5000 V phase-to-phase, the jacket must meet strict electrical and weatherability criteria. All testing is performed in accordance with Test Methods D 470 unless otherwise instructed.
| ⚡ Property | 🎯 Requirement | 📏 Test Method |
|---|---|---|
| Surface Resistivity (min) | 200,000 MΩ | D 470 / D 257 |
| U-Bend Discharge (150 V/mil, 6 h) | No cable failure or jacket cracks | D 470 |
| Weatherability Retention (720 h Dual Carbon-Arc) | ≥ 80% (Tensile & Elongation) | D 1499 / G 23 |
For sunlight and weather resistance, the jacket specimens must retain a minimum of 80% of their unaged tensile strength and elongation after a 720-hour exposure in a dual carbon-arc apparatus, performed using Method 1 of Practice G 23.
⚠️ Critical Requirement: For single conductor nonshielded cables rated 2001 through 5000 V, the U-bend discharge test is mandatory. Applying 150 V/mil (6 kV/mm) for a minimum of 6 hours must not result in any jacket cracking or cable failure.
📊 Key Physical Properties and Test Methods
The crosslinked jacket must conform to the prescribed physical property requirements. Sampling and testing are conducted in accordance with the procedures defined in Test Methods D 470.
| 🔍 Property | 🎯 Requirement | ⚙️ Test Method |
|---|---|---|
| Unaged Tensile Strength, min | 1500 psi (10.3 MPa) | D 470 |
| Unaged Elongation at Break, min | 250% | D 470 |
| Heat Aging Retention (121°C, 7 Days) | ≥ 70% (Tensile & Elongation) | D 470 |
Adherence to these physical property values ensures the jacket can withstand the mechanical stresses of installation and the thermal stresses of operational service over its intended lifespan.
❓ Frequently Asked Questions
🔍 What material is the jacket made of?
The jacket is a crosslinked compound consisting substantially of ethylene-propylene copolymer (EPM) or ethylene-propylene terpolymer (EPDM), formulated specifically for weather and ozone resistance.
⚡ What are the specific electrical requirements for medium voltage cables?
The jacket must have a surface resistivity of at least 200,000 MΩ. It must also pass the U-bend discharge test at 150 V/mil (6 kV/mm) for a minimum of 6 hours without failure or cracking.
💡 How is weather resistance verified under this standard?
Weather resistance is evaluated by exposing specimens for 720 hours in a dual carbon-arc apparatus per Practice D 1499 and Method 1 of Practice G 23. The material must retain at least 80% of its unaged tensile strength and elongation.
📌 Which test methods are referenced for the entire qualification?
Unless otherwise instructed in the standard, all physical testing, electrical testing, and sampling is performed according to ASTM Test Methods D 470, which governs crosslinked insulations and jackets for wire and cable.