Code of practice for selection, handling and erection of concrete poles for overhead power and telecommunication lines

IS 7321: Scope - Key Points & Specifications

Scope Summary:

• IS 7321 covers standards related to design, installation, and maintenance of overhead power lines up to 11 kV.
• It ensures compliance through testing and analysis, with results rounded per IS 2-1960 rounding rules.
• Stringing of conductors must be done after foundation concrete has cured and soil is compacted (Clause 5.11).

Important Specifications:

Rounding Off Rule (IS 2-1960):

• Final values must be rounded to the same number of significant digits as specified.
• Example: If specified value = 12.3 (3 significant digits), test result = 12.345 → Rounded to 12.3.

Stringing Conductors Timing:

• After foundation concrete curing (typically 7-28 days depending on mix).
• After soil compaction to ensure stability.

Summary Diagram:

flowchart TD
    A[Foundation Concrete Poured] --> B[Concrete Cured & Hardened]
    B --> C[Soil Compacted]
    C --> D[Stringing of Conductors]

For detailed design, installation, and maintenance procedures, refer to the full IS 7321 document and related codes like IS 5613 (Overhead Lines).

IS 7321 - Definitions and Rounding Off Rules

Key Points from Clause 2.0 and related context:

• Definitions: Clause 2.0 states that all terms used in the standard have specific definitions applicable only within the context of IS 7321.
• Rounding Off (IS:2-1960):
  • Final values (test or analysis results) must be rounded off as per IS 2-1960.
  • The number of significant digits retained in the rounded value must match the number of significant digits in the specified value.

Summary of Rounding Off (IS 2-1960):

Practical Example:

• Specified value: 12.3 (3 significant digits)
• Calculated value: 12.3456 → Rounded to 12.3 (3 significant digits)

This ensures uniformity and accuracy in reporting results complying with IS 7321.

IS 7321: Materials and Standards Referenced – Key Points

1. Rounding Off (Clause 0.3):

• Final test or calculated values must be rounded as per IS 2:1960.
• Retain the same number of significant figures as the specified value.

2. Material Standards Referenced:

• Concrete standards: IS 456 (Plain and Reinforced Concrete)
• Design and maintenance of overhead power lines: IS 5613 (Part I) for lines up to 11 kV
• Other referenced codes for materials and workmanship are consistent with BIS standards.

3. Erection Guidelines (Clause 5.11):

• Stringing of conductors to start only after:
  • Foundation concrete has cured and hardened.
  • Soil is well compacted.

Typical Materials Specifications (from related IS codes):

Summary Diagram: Material Compliance Flow

flowchart TD
    A[Test/Analysis Result] --> B[Rounding as per IS 2:1960]
    B --> C[Compare with Specified Value]
    C --> D{Complies?}
    D -->|Yes| E[Proceed with Installation]
    D -->|No| F[Re-evaluate Material/Test]

For detailed material specs, consult respective IS codes (e.g., IS 456 for concrete, IS 398 for conductors). The standard ensures quality by referencing these established codes and emphasizing proper curing and installation timing.

IS 7321: Handling and Lifting of Concrete Poles

Key Lifting Points (Clause 4.3)

• L = Total length of the pole
• Lifting must be indicated on the pole.
• Applies to both manual and mechanical handling.

Unloading (Clause 4.4.3)

• Use sand-filled sacks or cushioning material at ground level near slanting sleepers/rails.
• Poles should slide down gently onto the cushion to avoid impact damage.
• Shift poles simultaneously to avoid placing poles on junction lines between sleepers and ground.

Diagram: Lifting Points on Pole (for > 500 kg poles)

graph LR
A[Bottom of Pole] -->|1/6 L| B(Lift Point 1)
B -->|1/6 L| C(Lift Point 2)
C -->|1/5 L| D(Lift Point 3)
D -->|1/5 L| E[Top of Pole]

Summary:

• Lift poles at specified fractional lengths.
• Use 2 points for lighter poles, 4 points for heavier poles (>500 kg).
• Cushion poles during unloading to prevent damage.
• Follow these guidelines for safe handling and erection.

IS 7321: Erection of Concrete Poles - Key Points

References:

• Foundation & Erection Details: IS 5613 (Part I/Section 2)-1971
• Pole Erection Method: Direct pole method (manual or tractor-trailer unit)

Key Specifications for Erection (from IS 7321 & IS 5613):

• Foundation Preparation:

  • Excavate hole as per pole base dimension.
  • Ensure firm and level bedding.
  • Backfill with selected soil or concrete for stability.

• Erection Method:

  • Direct Pole Method: Pole is raised directly from horizontal to vertical position using ropes, levers, or mechanical means.
  • Use of tractor-trailer or winch for heavy poles.

• Handling:

  • Avoid impact or shock to poles.
  • Use slings or padded supports; never drag poles on rough surfaces.

Typical Foundation Dimension Table (Indicative):

Important Formulas:

• Embedment Depth (h):
  Typically,
  [ h = 0.1 \times \text{Pole length} \quad \text{(minimum)} ]

• Safe Handling Load:
  Depends on pole weight ( W ) and lifting method; always use safety factor ≥ 2.

Summary Diagram: Direct Pole Erection

flowchart LR
    A[Pole on Ground] --> B[Lifting using ropes/winch]
    B --> C[Pole raised to vertical]
    C --> D[Set into foundation]
    D --> E[Backfill & compact]

For detailed design and erection procedures, always refer to IS 5613 and IS 7321 full texts.

IS 7321: Foundation and Planting Requirements Summary

• Planting Depth (Clause 5.3):
  Poles must be buried at least the minimum specified depth per relevant IS standards (depth depends on pole height and soil).

• Foundation Pit (Clause 5.4):
  Size depends on base plate dimensions and working space. Excavation can be done by earth auger or suitable equipment.

• Concrete Base Plate (Clause 5.6):
  For prestressed concrete poles, provide a concrete base plate at trench bottom.

  • Normal soil: 450 mm × 450 mm × 75 mm thickness
  • Alternative: 1 or 2 baulks of R.C.C or timber, size 800 mm × 300 mm × 100 mm (with approval).

• Concreting in Non-Cohesive Soil (Clause 5.7):
  Concreting from trench bottom up to 150 mm above planting depth to resist overturning.

Typical Base Plate Size Table (Normal Soil)

Planting Depth Concept Diagram

flowchart TB
    A[Ground Surface] --> B[Planting Depth (min) per IS]
    B --> C[Concrete Base Plate / Baulks]
    C --> D[Foundation Pit Bottom]
    D --> E[Soil Bearing]

Note: Always verify planting depth and base plate size based on soil bearing capacity and pole type.

IS 7321: Positioning and Alignment of Concrete Poles

Key Points:

• Major Axis Definition (Clause 2.1):
  The major axis is the geometric axis of the pole's cross-section with the greatest moment of resistance.

• Handling (Clause 4.2):
  Poles should preferably be handled with the major axis horizontal to avoid damage.

• Erection & Positioning (Clause 5.1):
  Poles must be erected so that the major axis aligns with the direction of the power line for maximum structural efficiency.

• Rounding Off Values (Clause 0.3):
  Final calculated or observed values must be rounded per IS 2:1960 rules, retaining the same number of significant digits as specified.

Practical Summary:

Visual Concept:

graph LR
A[Concrete Pole Cross-Section] --> B(Major Axis)
B --> C[Greatest Moment of Resistance]
D[Handling] --> E(Major Axis Horizontal)
F[Erection] --> G(Major Axis Aligned with Power Line Direction)

This ensures optimal strength and durability during handling and service.

IS 7321: Use of Stays and Supports — Key Points

1. Stays Provision (Clause 5.8.1)

• Stays must be provided before tensioning conductors to stabilize poles.
• Essential where poles are subjected to unbalanced loads or high tension.

2. Pole Spacing (Clause 5.2)

• Pole spacing (conductor span) should maintain loading conditions assumed in design.
• Ensures structural safety and performance under expected loads.

3. Concrete Base Plate for Poles (Clauses 5.5.1 & 5.6)

• Required in:
  • Gale, cyclonic storm-prone areas
  • Soil erosion zones (rain/flood)
  • Poor bearing soil conditions
• Standard size for prestressed concrete poles:
  • 450 mm × 450 mm × 75 mm thick
• Alternatives (with approval):
  • One or two RCC or timber baulks of 800 mm × 300 mm × 100 mm

Summary Table: Concrete Base Plate Dimensions

Stay Support Concept Diagram

graph LR
A[Pole] --> B[Stay Wire]
B --> C[Anchor Rod in Ground]
A --> D[Conductor]

• Stay wire transfers lateral loads from pole to anchor.
• Anchors provide counteracting force to maintain pole stability.

Note: Design of stays must consider soil bearing capacity, conductor tension, and environmental loads per IS 7321.

IS 7321: Transportation and Loading/Unloading of Poles

Key Specifications from Clauses:

• 4.4.1: Prefer mechanical loading/unloading (crane or similar). If not feasible, manual loading/unloading is allowed following safe procedures.

• 4.4.2:

  • Use wooden sleepers or steel rails as supports inside vehicle at 3-4 points, placed at a gentle slope against vehicle body.
  • Poles must be dragged one by one, gently and uniformly on these supports using ropes.
  • Once on the vehicle floor, poles should be shifted carefully (crow-bar or similar) to avoid damage.

• 4.4.4:

  • Poles can be loaded flat (minor axis vertical) if fully contained inside vehicle without overhang.
  • In this orientation, wooden battens between layers must be increased by 25%.
  • Provide cushioning (straw, waste gunny) between layers to prevent damage.

Summary Table for Loading Supports and Cushioning

Loading Process Flow (Mermaid Diagram)

flowchart TD
    A[Start Loading] --> B{Mechanical Loading?}
    B -- Yes --> C[Use Crane or Similar]
    B -- No --> D[Manual Loading]
    D --> E[Place sleepers/rails at 3-4 points]
    E --> F[Drag poles gently one by one]
    F --> G[Shift poles carefully on vehicle floor]
    G --> H{Pole orientation flat?}
    H -- Yes --> I[Increase battens by 25% + Cushioning]
    H -- No --> J[Standard arrangement]
    I --> K[Finish Loading]
    J --> K
    C --> K

This ensures safe, damage-free transportation of poles complying with IS

IS 7321: Safety Precautions for Concrete Poles

The code emphasizes safe and proper handling, selection, and erection of concrete poles to ensure serviceability and safety:

Key Safety Specifications (Clause 3.4)

• Dimensional tolerances & uprightness: Must comply with relevant specs.
• Holes: Adequate and suitable holes for cross-arms and fittings.
• Hole plugging: Use hardwood or suitable material for danger/number plates.
• Lifting hooks: Must be provided for safe handling.
• Earthing: Suitable earthing arrangement per relevant standards and engineer's requirements.
• Marking: Poles marked as per IS 785-1964 and IS 1678-1960.
• Quality: Poles free from visible cracks, honeycombing, and have good finish.

Rounding Off Results (Clause 0.3)

• Final test/analysis values should be rounded per IS 2-1960.
• Retain significant figures equal to those in the specified value.

Summary Table: Safety Checks for Concrete Poles

flowchart TD
    A[Concrete Pole] --> B{Safety Checks}
    B --> C[Dimensional Tolerance & Uprightness]
    B --> D[Holes for Cross-arms & Fittings]
    B --> E[Hole

Inspection & Quality Assurance per IS 7321

Key Points:

• Rounding Off (Clause 0.3): Test results must be rounded as per IS 2-1960, matching the significant figures of the specified values.

• Dimensional & Strength Checks (Clause 3.4):

  • Poles must meet length, strength, and dimensional tolerances.
  • Uprightness and hole provisions for fittings must comply with specs.
  • Holes for danger/number plates must be plugged properly.
  • Lifting hooks and earthing arrangements as per relevant standards.
  • Poles must be marked per IS 785-1964 (reinforced concrete poles) and IS 1678-1960 (prestressed poles).
  • Poles should be free from visible cracks, honeycombing, and have a good finish.

• Erection Timing (Clause 5.11): Conductors stringing only after foundation concrete curing and soil compaction.

Summary Table: Pole Inspection Checklist

Rounding Off Rule (IS 2-1960)

If the specified value is 12.3, test results should be rounded to 1 decimal place:

• 12.34 → 12.3

Key Specifications for Stringing of Conductors (IS 7321)

1. Spacing & Loading (Clause 5.2)

• Pole spacing must maintain the loading conditions assumed in design.
• Span length affects sag and tension; design accordingly.

2. Stays (Clause 5.8.1)

• Provide stays before applying working tension to conductors to stabilize poles.

3. Pole Length (Clause 3.2)

• Determined by:
  • Type of line (HV or LV)
  • Required clearance from ground
  • Expected sag (depends on span and tension)
  • Cross-arm position and planting depth
• Refer Indian Electricity Rules (Rule 77 & 133) for clearance.

4. Timing of Stringing (Clause 5.11)

• Conductors should be strung only after:
  • Foundation concrete has cured and hardened
  • Soil is well compacted around poles

Important Formula for Sag (Approximate)

[ S = \frac{w \times L^2}{8 \times T} ]

Where:

• ( S ) = Sag (m)
• ( w ) = Weight per unit length of conductor (N/m)
• ( L ) = Span length (m)
• ( T ) = Horizontal tension in conductor (N)

Summary Table: Stringing Considerations

flowchart TD
    A[Pole Foundation] --> B[Concrete Cured & Hardened]
    B --> C[Soil Well Compacted]
    C --> D[Install Poles & Cross-arms]
    D --> E[Provide Stays]
    E --> F[String Conductors]
    F --> G[Apply Working Tension]

Note: Always cross-check with Indian Electricity Rules and relevant IS codes for exact clearances and safety factors.

IS 7321 - References to Related Standards: Key Points

• Rounding Off Values:
  As per IS 2:1960, all test or calculated results must be rounded off to the same number of significant figures as specified in IS 7321.

• Related Standards Referenced:

  • IS 2:1960 — Rules for rounding off numerical values.
  • Code of Practice for Overhead Power Lines (up to 11 kV) — For design, installation, and maintenance.
  • Code of Practice for Plain and Reinforced Concrete (Second Revision).

• Erection Clause (5.11):
  Stringing of conductors should only commence after:

  • Foundation concrete has fully cured and hardened.
  • Soil beneath is well compacted.

Additional Notes:

• IS 7321 aligns with other IS codes for concrete and power line installation, ensuring consistency in structural and electrical safety.
• For precise calculations, always refer to IS 2 for rounding and IS 7321 for specific material and installation requirements.

flowchart TD
    A[Start: Concrete Foundation] --> B{Has concrete cured?}
    B -- Yes --> C{Is soil compacted?}
    B -- No --> A
    C -- Yes --> D[Stringing of Conductors]
    C -- No --> A

For detailed specifications, consult the full IS 7321 and referenced codes.