technical of CROSS ARM
1. Ultimate Tensile Strength (UTS)
1. UTS refers to the maximum load-bearing capacity of the cross arm before mechanical failure.
2. It must support:
I. Conductor load
II. Insulator load
III. Wind and ice load on conductors
3. Strength depends on:
I. Material type
II. Section modulus
III. Arm length (usually 1.2 m to 1.8 m)
2. Working Load / Safe Working Load (SWL)
1. Cross arms are designed to carry:
I. Vertical load (insulator + conductor)
II. Transverse load (wind)
III. Occasionally longitudinal load (tension angles)
3. Conductor Diameter Compatibility
1. While conductors do not contact the cross arm directly, the spacing and mounting points of cross arms determine the suitable conductor size and phase spacing.
2. Cross arms are compatible with:
I. Conductor OD: 6 mm – 30 mm
II. Phase spacing of 600 mm to 1200 mm depending on voltage level.
3. Holes and brackets are aligned to fit:
I. 11–33 kV pin/post insulators
II. 33 kV tension hardware
4. Slip Strength:
1. Applicable at the clamp-to-conductor interface, especially during fault currents or vibrations
2. Good quality clamps (brass/bronze) with serrated grips ensure slip resistance > 3–5 kN, depending on conductor size
3. Ensures joint integrity in mechanical and electrical bonding
5. Temperature Withstanding Capacity:
1. Continuous operation: –40°C to +85°C
2. During fault current (lightning or surge): withstands 250°C to 500°C depending on fault duration and conductor mass
3. Annealed copper or CCS withstands short-circuit currents without insulation breakdown (when bare)
6. Corrosion Resistance:
1. Steel cross arms are Hot-Dip Galvanized (HDG)
I. Coating: ≥ 85 µm (610 g/m²)
II. As per IS 2629 / ASTM A123
2. FRP cross arms are naturally corrosion-resistant, ideal for:
I. Coastal regions
II. Industrial zones
III. High-humidity areas
7. Impact & Fatigue Strength:
1. Must endure:
I. Wind-induced vibration
II. Conductor galloping
III. Mechanical shocks during installation or maintenance
IV. Bird hits or branch impacts
2. Fatigue life: >10⁶ cycles (especially in FRP)
3. No crack, breakage, or deformation under long-term mechanical vibration
8. Mechanical Endurance:
1. Designed for 25–40 years of continuous service
2. Rust-proof if galvanization is intact
3. Withstands high-load zones like strain poles
4. Lightweight, UV-stable, corrosion-proof
5. Long fatigue life and minimal maintenance
9. Design Safety Factors:
1. Design Parameter
Safety Factor
2. UTS vs SWL
≥ 2.5 to 3×
3. Bolt Load Bearing Strength
≥ 2× of load per hole
4. Anti-buckling (bending)
>1.5× max expected moment
5. Galvanization Life
≥ 25 years (HDG)
6. UV Resistance (FRP)
Yes (tested to >2000 hrs)
Top 5 Quality Assurance
1. Compliance with Applicable Standards
a. Cross arms are manufactured and tested as per:
I. IS 802 (Part 1/Sec 2) – Code of practice for use of structural steel in overhead line towers.
II. IS 2062 – For structural steel material.
III. Utility-specific drawings and technical specifications.
b. Ensures proper strength, safety, and compatibility with other line fittings.
2. Raw Material Quality Verification
a. Fabricated from mild steel angle sections (MS angles) or channel sections.
b. All incoming steel is tested for:
I. Chemical composition (carbon, manganese, etc.)
II. Mechanical properties – tensile strength, yield strength, elongation.
c. Material Test Certificates (MTCs) are maintained for traceability.
3. Dimensional and Structural Accuracy
a. Each cross arm is checked for:
I. Correct length, hole diameter, and spacing as per drawing.
II. Straightness, angle cuts, and hole alignment using calibrated tools.
b. Ensures accurate fitment with insulators and pole brackets during installation.
4. Hot-Dip Galvanizing and Corrosion Protection
a. All cross arms are hot-dip galvanized as per IS 2629 / ISO 1461 for corrosion resistance.
b. Zinc coating is tested for:
I. Minimum thickness (≥85 microns for outdoor use).
II. Adhesion test
III. Salt spray test, if specified.
c. Ensures long service life in harsh outdoor environments.
5. Final Inspection, Marking & Documentation
a. 100% inspection for:
I. Welding quality (where applicable), burrs, cracks, bends, and coating uniformity.
b. Marked with:
I. Manufacturer’s logo, batch number, and sometimes size/type.
c. Delivered with:
I. Inspection reports
II. Certificate of Conformance (CoC)
III. Galvanizing and mechanical test certificates