Wooden cross arms
1993 Edition

IS 2203 (1993 Edition) classifies timber suitable for wooden cross arms into three groups based on their bending strength or modulus of rupture (MOR) when tested in green condition (moisture content above 25%). The groups are:

• Group A: Very strong timber with MOR ≥ 85 N/mm², exemplified by Sal (Shorea robusta).
• Group B: Strong timber with MOR between 65 and 85 N/mm², such as Teak (Tectona grandis).
• Group C: Moderately strong timber with MOR ranging from 45 to 65 N/mm², including Chir (Pinus roxburghii).

This classification assists engineers in choosing the right timber species that fulfill strength and durability requirements for overhead line supports.

According to IS 2203, certain wood defects are strictly prohibited, including cross breaks, reaction wood, decayed or unsound knots larger than 50 mm, full-length splits, and pitch seams. However, some defects such as minor sound knots smaller than 50 mm and slight surface checks are permissible provided they do not compromise the structural integrity. All defects must be measured following IS 3364 (Part 2): 1976 to maintain consistency in quality assessment.

IS 2203 mandates that wooden cross arms made from non-durable timber species and sapwood of durable species undergo preservative treatment as per IS 401:1982, the same standard used for wood poles. Untreated cross arms must receive prompt prophylactic treatment to prevent insect and fungal deterioration. Additionally, both treated and untreated cross arms should be stored on creosoted or similarly treated sleepers elevated at least 150 mm above ground level to ensure adequate air circulation and minimize decay. Each treated cross arm must be marked with species, treatment year, and related manufacturing details to ensure traceability.

IS 2203 prescribes that wooden cross arms for 11 kV overhead lines have a cross-sectional size of 100 mm by 76 mm, with lengths varying according to the span and application:

• For spans up to 106.7 meters: 1.07 meters length with 12 mm diameter holes positioned at 305 mm and 457 mm from the center on each side.
• For spans between 106.7 and 152.4 meters: 1.37 meters length with similar hole diameters and adjusted hole placement.
• Dead end and guarding cross arms: 1.68 meters length with holes of 12 mm and 17.5 mm diameters.

Dimensional tolerances are ±5 mm for external measurements and ±2 mm for holes and slots, with bevelled corners on the best edge to distinguish the upper and lower halves.

The standard requires mechanical strength testing via a two-point bending test. A sample of 1% of the batch (up to 5 cross arms) is subjected to bending load for 10 minutes. If no failures occur, the batch is accepted. If failures are detected, an additional 2% (minimum 10 cross arms) are tested. Should any cross arms fail in this second round, the entire lot is rejected. This procedure ensures the cross arms possess adequate bending strength and durability under load.