Code of practice for design, fabrication and maintenance of nail-jointed timber posts from small dimensional timber for overhead electric distribution lines for low voltage

IS 7683: Scope & Key Specifications for Overhead Electric Distribution Posts

• Scope Reference:
  Applies to small dimensional timber posts for overhead electric distribution (Clause 2.1 refers to IS 707-1968 for timber definitions).

• Key Planning Data (Clause 3.1):
  For efficient design, obtain:

  • Timber species and grading (strength and durability)
  • Moisture content (affects strength and shrinkage)
  • Pre-treatment details (preservatives, protection methods)

• Rounding Off (Clause 0.6):
  Final calculated/test values must be rounded per IS 2-1960, maintaining the same significant figures as specified values.

• International Coordination (Clause 0.4):
  The standard aligns with global practices while addressing local conditions.

Summary Table: Essential Inputs for Design

If you need formulas or design tables (e.g., allowable stresses, sizes), refer to IS 7683 Annexures or IS 707 for timber grading details.

IS 7683 - Definitions Summary

• Reference for Definitions: Clause 2.1 states that all definitions in IS 7683 refer to IS 707-1968. So, key terms related to this standard must be understood as per IS 707-1968.

• Rounding Off Values: As per Clause 0.6, all calculated or observed values must be rounded off according to IS 2-1960. The precision (significant figures) should match the specified values in IS 7683.

Important Notes:

• IS 7683 aligns with international standards and Indian practices (Clause 0.4).
• For exact definitions, consult IS 707-1968 (e.g., terms like "bearing capacity," "load," "foundation types").

Quick Reference:

If you need specific definitions or formulas, please refer directly to IS 707-1968 or ask for particular terms.

IS 7683: Key Information for Planning & Design of Timber Electric Posts

1. Necessary Information (Clause 3.1)

For efficient design of overhead electric distribution posts using small timber:

• Timber Species & Grading: Identify species and grading to determine strength properties.
• Moisture Content: Affects timber strength; must be measured or specified.
• Pre-treatment Details: Chemical or preservative treatment to enhance durability.

2. Wind Pressure (Clause 5.3)

Wind loads on posts and conductors must comply with:

• IS 802 (Part I)-1973 or
• Indian Electricity Rules

These specify wind pressure values based on location and height.

3. Design Principles

• Follow IS 7683 in conjunction with IS 707-1968 (definitions) and IS 802 for wind loads.
• Design timber posts considering bending, compression, and shear stresses.
• Use appropriate safety factors as per timber grading and treatment.

Typical Wind Pressure Values (IS 802 excerpt)

flowchart TD
    A[Start: Planning] --> B[Collect Timber Data]
    B --> C{Species & Grade}
    B --> D{Moisture Content}
    B --> E{Pre-treatment Details}
    C --> F[Determine Strength]
    D --> F
    E --> F
    F --> G[Calculate Wind Loads (IS 802)]
    G --> H[Design Timber Post]
    H --> I[Check Stresses & Safety Factors]
    I --> J[Finalize Design]

This ensures a safe, durable, and code-compliant overhead timber post design.

IS 7683: Timber Selection & Specifications Summary

Key Requirements (Clause 3.1 & 4.1)

• Timber species and grading must be identified.
• Moisture content must be controlled.
• Pre-treatment details (e.g., preservative treatment) must be documented.
• Minimum plank thickness: 45 mm.

Timber Species Classification

Important Tables (Excerpt)

Additional Notes

• Use sapwood of durable species with preservative treatment.
• Species with high refractoriness resist seasoning defects.
• Refer to IS 7683 Appendix A for full species list.
• Follow IS codes for anti-termite treatment (IS 6313 series).

flowchart TD
    A[Timber Selection] --> B{Species Choice}
    B -->|First Choice| C[Naturally Durable Heartwood]
    B -->|Third Choice| D[Moderately Durable, Treatable]
    C --> E[Minimal Treatment]
    D --> F[Pressure Treatment (≤60 mm thickness)]
    A --> G[Moisture Content Control]
    A --> H[Pre-treatment Documentation]
    A --> I[Minimum Thickness ≥ 

IS 7683 Load Considerations Summary

• Wind Loads:
  As per Clause 5.3, wind pressure on posts, conductors, and wires must follow IS 802 (Part I)-1973 or Indian Electricity Rules.
  Wind load ( p ) is generally calculated by:
  [ p = k_z \cdot k_d \cdot p_0 \cdot C_f ]
  where:

  • ( k_z ) = height factor
  • ( k_d ) = wind direction factor
  • ( p_0 ) = basic wind pressure
  • ( C_f ) = force coefficient (depends on shape)

• Load Types on Timber Posts (Clause 5.1):

  1. Wind load on post surface above ground
  2. Wind load on conductors and ground wires (horizontal span)
  3. Unbalanced longitudinal pull due to wire breakage

• Design Principle (Clause 6.2.2):
  Joints must be designed for combined loads from Clause 5.

Typical Load Calculation Steps:

Design Recommendations:

• Use IS 802 (Part I)-1973 for detailed wind pressure values and factors.
• Consider combined effects for bending and tension on posts and joints.
• Refer IS 707-1968 for timber properties and definitions.

flowchart LR
    A[Wind Load on Post] --> D[Design Bending Moment]
    B[Wind Load on Conductors] --> D
    C[Unbalanced Pull] --> D
    D --> E[Joint & Post Design per IS 7683]

This ensures safe and efficient design of timber posts under combined wind and mechanical loads.

Principle of Design - IS 7683 Key Points

• Design Basis:
  Timber posts are designed as simple cantilevers subjected to bending moments.

• Fundamental Bending Formula:
  [ M = f \times Z ] where:

  • (M) = design bending moment (kg·mm)
  • (f) = maximum allowable bending stress of timber (kg/mm²)
  • (Z) = section modulus of the timber cross-section (mm³)

• Wind Loads:
  Wind pressures on posts, conductors, and wires must comply with IS 802 (Part I)-1973 or Indian Electricity Rules.

• Joint Design:
  Joints must be designed for loads as specified in Clause 5 (loads and forces).

Additional Notes:

• Modulus of Section (Z):
  For rectangular sections,
  [ Z = \frac{b \times h^2}{6} ] where (b) = width and (h) = height of the timber section.

• Ensure timber properties and allowable stresses are taken from IS 707-1968.

flowchart TD
    A[Timber Post Design] --> B[Determine Loads (Wind, Conductors)]
    B --> C[Calculate Bending Moment (M)]
    C --> D[Select Timber Section]
    D --> E[Calculate Section Modulus (Z)]
    E --> F[Check Bending Stress: M ≤ f × Z]
    F --> G[Design Joints per Clause 5 Loads]

This ensures safe and efficient design of timber posts as per IS 7683.

IS 7683: Overhead Posts - Key Points & Specifications

1. Dimensions & Marking (Clause 10.2.2)

• On a level platform/ground, mark:
  • Post dimensions including flanges and web.
  • Position and thickness of splice plates.

2. Protection (Clause 14.3)

• Fit a G.I. sheet cap at the top of the post to prevent moisture/rainwater ingress.

3. Timber Selection (Appendix A)

• Use Indian structural timbers that:
  • Resist warping, twisting, splitting.
  • Have naturally durable heartwood.
  • Are treatable chemically.
  • Have sufficient sapwood for effective treatment.

4. Load Consideration (Clause 5.2)

• Timber posts at angle locations with guy-rope stay experience vertical downward loads.

Typical Dimensions & Splice Plate Details (Example)

Protection Detail

flowchart TB
    PostTop[Post Top]
    GI_Cap[G.I. Sheet Cap]
    Rainwater[Rainwater/Moisture]
    
    Rainwater -->|Blocked by| GI_Cap
    GI_Cap --> PostTop

Summary:

• Mark all dimensions and splices on ground before assembly.
• Use durable, treatable Indian timbers.
• Protect post tops with G.I. caps.
• Consider vertical loads especially at guy-rope supported posts.

IS 7683: Preservative Treatment Key Points

1. Recommended Preservatives (Clause 8.1, Table 1)

• Types of Treatment: Pressure process or hot & cold process.
• Preservatives:
  • Creosote or Creosote Fuel Oil Mixture (50:50)
  • Copper-Chrome Arsenic (CCA) Composition
• Applicable for sawn timber used in outside locations.

2. Species and Treatability (Appendix A & Table 4)

• Timber classified as Third Choice: Moderately durable, treatability improved for sizes ≤ 60 mm thickness.
• Examples of species:
  • Anogeissus acuminata (yon) – High refractoriness
  • Altingia excelsa (jutili) – High refractoriness
  • Artocarpus chaplasha (chaplash) – Moderate refractoriness
  • Elacocarpus tuberculatus (rudrak) – Scarcely treatable but preferred
• Treatability affects preservative penetration and effectiveness.

3. Design Considerations

• Foundation design for timber posts per IS 4091-1967.
• Moisture content and pre-treatment details must be known for design (Clause 3.1).

Summary Table: Preservative Treatment Types and Timber Choice

Additional Notes:

• Follow IS 401-1970 for preservative composition details.
• Ensure timber moisture content is appropriate before treatment for best results.
• Pressure treatment enhances preservative penetration, especially for less durable species.

flowchart TD
    A[Timber Species] --> B{Durability & Treatability}
    B -->|Highly Durable| C[Minimal Treatment]
    B -->|Moderately Durable| D[Pressure Treatment]
    B -->|Non

IS 7683: Foundation Details of Timber Posts

Key Specifications (Clause 9.1)

Two accepted foundation methods for timber posts:

• Method A:

  • Posts elevated above ground on cement concrete dwarf pillars.
  • Fixed using MS channels and bolts for stability.

• Method B:

  • Posts buried in ground.
  • Encased with cement concrete dwarf foundation for protection and support.

Related IS Codes and Practices

• Anti-termite treatment: As per IS 6313 (Part II)-1971 to protect the foundation soil.
• Foundation design: Follow IS 4091-1967 for timber post foundation design.

Foundation Design Notes:

• Dwarf pillars/foundations typically have minimum dimensions depending on soil bearing capacity; commonly:
  • Width & Length: 300 mm to 500 mm
  • Depth: At least 450 mm below ground level or as per soil conditions.
• Concrete mix generally 1:2:4 (cement:sand:aggregate).

Summary Table for Foundation Types

flowchart LR
    A[Timber Post] --> B{Foundation Type}
    B --> C[Raised on Dwarf Pillar]
    B --> D[Buried & Encased]
    C --> E[Concrete Pillar]
    C --> F[MS Channels & Bolts]
    D --> G[Concrete Encasement]
    D --> H[Direct Soil Contact]

Ensure:

• Anti-termite treatment per IS 6313
• Proper concrete curing
• Compliance with IS 4091 for structural safety

Fabrication of Timber Posts (IS 7683 Key Points)

1. Timber Selection & Preparation

• Use structural timbers (coniferous or hardwood) with durable heartwood and treatable sapwood (Appx A).
• Minimum plank thickness: 45 mm (Clause 4.1).
• Pre-treatment and moisture content must be known (Clause 3.1).

2. Protection

• Fit one G.I. sheet cap at post top to prevent moisture ingress (Clause 14.3).

3. Design Formula for Bending (Clause 6.1.1)

Timber posts behave as cantilevers under load:

[ M = f \times Z ]

Where:

• (M) = design bending moment (kg·mm)
• (f) = max allowable bending stress (kg/mm²)
• (Z) = section modulus (mm³)

4. Section Modulus for Rectangular Section

[ Z = \frac{b \times d^2}{6} ]

Where:

• (b) = width of plank (mm)
• (d) = depth/thickness of plank (mm)

Summary Table: Timber Post Specifications

flowchart TD
    A[Select Timber Species] --> B[Check Moisture Content]
    B --> C[Pre-treatment]
    C --> D[Fabricate Planks (≥45mm thick)]
    D --> E[Assemble Post]
    E --> F[Fit G.I. Sheet Cap]
    F --> G[Design as Cantilever (M = f × Z)]

This ensures durability and structural adequacy of fabricated timber posts per IS 7683.

IS 7683 - Setting and Erection Precautions Summary

1. Fabrication Tolerances (Clause 10.3 & Fig. 2)

• Deviation limits for electric posts fabricated on level platform:
  • ±15 mm at the top (from straight line joining endpoints)
  • ±15 mm at the center (from straight line joining endpoints)
  • ±10 mm at the bottom end (from straight line joining bottom and ⅓ length point)

2. Setting of Foundation (Clause 11.1)

• Foundation setting must follow Clauses 11.1.1 to 11.1.4 (details include):
  • Accurate marking of post dimensions on level ground (Clause 10.2.2)
  • Marking flange, web, and splice plate positions on ground before erection
  • Laying a dwarf concrete base of specified cement concrete mix (Clause 11.1.4)

3. Additional Precautions

• Fabricate posts straight on a leveled platform.
• Ensure proper marking and storage of timber posts before erection (Clause 12).

Quick Reference Table: Allowed Deviations on Electric Post Fabrication

flowchart TD
    A[Start: Fabricate Post on Level Platform] --> B[Mark Dimensions on Ground]
    B --> C[Mark Flanges, Web, Splice Plates]
    C --> D[Check Deviations (±15mm top/center, ±10mm bottom)]
    D --> E[Set Foundation as per Clauses 11.1.1 - 11.1.4]
    E --> F[Lay Dwarf Concrete Base]
    F --> G[Store Timber Posts Properly (Clause 12)]
    G --> H[Erect Post]

Note: Always verify concrete mix specifications and foundation dimensions as per project requirements and IS code details.

IS 7683 - Marking of Timber Posts: Key Points

• Marking Location: At 2.5 m from the bottom end of the post (Clause 12.1).
• Marking Details:
  • Clearly indicate top and bottom ends to avoid erection errors.
  • Specify species of timber.
  • Indicate year of preservative treatment.
• Additional Marking (Clause 10.2.2):
  • On a level platform, mark post dimensions, including flanges and web.
  • Mark position and thickness of splice plates.
• Protection (Clause 14.3): Fit a G.I. sheet cap at the top to prevent moisture ingress.
• Appendix A (Timber Choice): Use species with:
  • Naturally durable heartwood.
  • Good resistance to warping, twisting, splitting.
  • Sapwood amenable to preservative treatment.

Summary Table for Marking

flowchart LR
    A[Timber Post] --> B[Mark Bottom & Top Ends]
    B --> C[Mark Species & Treatment Year @ 2.5m]
    A --> D[Mark Dimensions & Splice Plates on Ground]
    A --> E[Fit G.I. Sheet Cap on Top]

This ensures correct identification, quality control, and durability of timber posts before erection.

IS 7683: Erection Techniques for Electric Timber Posts

Key Specifications (Clause 10.3 & 13.1)

• Fabrication: Posts must be fabricated straight on a leveled platform.
• Limits of Deviation (Fig. 2):
  • ±15 mm at top & center from the straight line joining end points.
  • ±10 mm at bottom end from the line joining bottom and 1/5th length point.

Erection Stages (Clause 13.1 & Fig. 3)

• Manual lifting with two small timbers tightened at the bottom for bodily lifting.
• Positioning post vertically near dwarf concrete base.
• Placing post between two steel channels.
• Preboring holes for bolting post with MS channels.
• Embedding post in ground encased with cement concrete (without steel channels).

Fabrication Notes (Clause 10.2.3.2)

• Flanges and web fabricated separately.
• Use top and bottom splice plates for lengthening.
• Nail central row of lengthening joints during web-to-flange connection.

Summary Table: Limits of Deviation

Visual Summary (Erection Sequence)

flowchart LR
    A[Fabricate Post on Level Platform] --> B[Mark Flanges, Web & Splice Plates]
    B --> C[Manual Lifting with Timbers]
    C --> D[Position Vertically near Concrete Base]
    D --> E[Place Between Steel Channels]
    E --> F[Prebore Holes & Bolt]
    F --> G[Embed in Ground with Concrete]

This ensures straightness, stability, and durability of timber electric posts during erection.

IS 7683: Painting and Maintenance of Timber Posts

Key Specifications (Clause 14.1)

• Painting system: Decorative-cum-protective painting is mandatory.
• Coats:
  • One priming coat: Use aluminium wood primer, white lead primer, or genuine pink primer.
  • For timbers with substances retarding oil paint drying, aluminium wood primer is mandatory.

Timber Treatment Notes

• Timber species vary in durability and treatability (see Table 4 for third choice species).
• Pressure treatment improves treatability for small sizes (up to 60 mm thick).

Maintenance Tips

• Regular inspection for paint deterioration.
• Reapply primer and finishing coats as needed to prevent moisture ingress and decay.
• Ensure proper erection and embedding of posts (see Fig. 3 in IS 7683 for erection techniques).

Summary Table: Painting Coats

Erection & Maintenance Flow (Fig. 3 Overview)

flowchart TD
    A[Post Being Erected by Manual Labour] --> B[Post Kept Vertical Near Concrete Base]
    B --> C[Two Small Timbers Tightened for Lifting]
    C --> D[Post Placed Between Steel Channels]
    D --> E[Preboring and Bolting with MS Channels]
    E --> F[Post Embedded and Encased in Cement Concrete]

Reference:

• IS 7683 Clause 14.1 for painting
• Table 4 for timber species selection
• Fig. 3 for erection technique

IS 7683: Indian Structural Timbers for Fabricated Overhead Electric Distribution Posts

Key Specifications:

• Top Protection: Clause 14.3 mandates fitting a G.I. sheet cap at the post top to prevent moisture ingress.
• Timber Selection (Appendix A):
  • Use Indian structural timbers (both coniferous and hardwood) with:
    • Naturally durable heartwood
    • Surrounding sapwood for effective chemical treatment
    • Resistance to warping, twisting, splitting
  • Timber must be treatable for longevity under weather exposure.

Timber Choice:

• Refer to IS codes for nail-jointed timber posts for low voltage distribution lines for detailed design and fabrication guidelines.
• Prefer species known for durability and good preservative retention.

Commonly Used Indian Timbers (examples):

Protection & Maintenance Tips:

• Apply preservative treatment to sapwood.
• Use G.I. sheet caps or equivalent to prevent water ingress.
• Regular inspection for cracks, splits, decay.

flowchart TD
    A[Select Durable Timber] --> B{Check Properties}
    B -->|Durability| C[Heartwood Quality]
    B -->|Treatability| D[Sapwood Availability]
    B -->|Weather Resistance| E[Warping & Splitting Resistance]
    C & D & E --> F[Fabricate Post]
    F --> G[Fit G.I. Sheet Cap]
    G --> H[Install & Maintain]

This ensures long service life and structural integrity of overhead electric distribution posts.