Estimation & Costing

Table of Contents
1. Earthing (Earthing and Bonding)
2. Methods of Earthing
3. Inspection and Testing of Wiring Installations
4. Testing Methods
5. Important Considerations Regarding Motor Installation Wiring
6. Estimation and Costing Principles for Earthing and Motor Wiring
7. Testing, Acceptance and Documentation
8. Practical Notes for Estimators
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Earthing (Earthing and Bonding)

Earthing means connecting the neutral point of a supply system or the non-current-carrying metal parts of electrical apparatus (for example, metallic framework, metallic covering of cables, earth terminal of socket outlets, stay wires) to the general mass of earth so that, at all times, any accidental discharge of electrical energy is carried away immediately and without danger.

Purposes of Earthing

• To maintain designed insulation potential - to ensure that no current-carrying conductor rises to a potential relative to the general mass of earth higher than its designed insulation can withstand.
• To protect persons - to avoid electric shock to human beings by providing a low-impedance path to earth for fault currents.
• To reduce fire risk - to avoid fire caused by leakage currents flowing through unintended paths.

How Earthing Provides Safety

If a metallic part of a domestic or industrial appliance comes into direct contact with a bare live conductor (for example, due to insulation failure), the metallic part becomes charged. If that metal is connected to earth, the fault current flows to earth through a low-impedance path. A large fault current will flow immediately and, when the current exceeds the protective device setting, the fuse or circuit breaker will operate and isolate the faulty circuit. Thus, correctly installed earthing ensures rapid fault clearance and minimizes danger to persons and property.

Methods of Earthing

The common methods of earthing are listed below, together with typical applications, advantages and factors to consider when preparing an estimate.

• Strip or wire earthing - Used where the soil is rocky and excavation for other types of electrodes is difficult. A conductive strip (usually GI or copper strip) or wire is buried in a trench and connected to the installation earth. Estimation factors: length of strip, trench excavation and refilling, earth backfill (if required), clamps and connectors, labour.
• Rod (or spike) earthing - Suitable for sandy or loose soils. A conductive rod or spike is driven into the ground to achieve low resistance. Estimation factors: number and length of rods, rod material (GI or copper), welding or clamp connections, backfill material, labour.
• Pipe earthing - A metallic pipe (usually GI or copper) is used as the electrode and is common in many installations. It is often combined with backfill of charcoal and salt to improve contact. Estimation factors: pipe length and diameter, pipe fittings and end caps, backfill materials, excavation, labour, corrosion protection.
• Plate earthing - A metallic plate (commonly copper or galvanized steel) is buried vertically or horizontally in the ground. Plate earthing is used where a larger contact area is required. Estimation factors: plate size and thickness, excavation volume, backfill, clamps, conductor length to equipment earth, labour.

Typical selection guidance (practical considerations): strip earthing is preferred in rocky ground because pipe or plate excavation is difficult; rod earthing is inexpensive and quick in sandy soils; pipe earthing is a common and effective system where soil and moisture conditions permit.

Inspection and Testing of Wiring Installations

Frequency: Internal wiring should be inspected at least once a year. The inspection should cover the items below and must be recorded.

Service Connections

For overhead lines the inspector should ensure the following:

• The lines are terminated at a safe distance from the building.
• Danger notices are displayed where required to warn staff and the public.
• A fuse or suitable overcurrent protective device of correct rating is provided on the phase line.
• The lead-in pipe is properly earthed and bonded, and pipe ends are provided with insulating bushes to protect the conductors from mechanical damage.

For underground cables the inspector should ensure the following:

• The cable terminations are properly sealed and there is no leakage of cable-compound or oil (where applicable).
• There are effective earth connections to armouring or metallic screen of the cable as required.

Tests to be Performed Before Commissioning (New Installation or Addition)

• Insulation resistance between wiring and earth with all fuses and lamps in place and all switches in the 'on' position. The aim is to confirm adequate insulation to avoid leakage to earth.
• Insulation resistance between conductors with all lamps out and all switches in the 'on' position. This verifies conductor insulation integrity.
• Polarity testing of non-linked single pole switches to ensure switch disconnects the phase conductor and not the neutral.
• Earth continuity testing to ensure that all exposed metallic parts and protective conductors are continuous and correctly connected to the earthing system.
• Earth resistance testing to measure the resistance of the earthing electrode and verify that it meets the required value for safety and protective device operation.

The objective of insulation resistance tests is to ensure that the wiring and cables are sufficiently insulated to prevent harmful leakage currents. Earth resistance testing verifies the effectiveness of the earthing arrangement.

Testing Methods

• Insulation resistance measurement - typically done with an insulation tester (megger) between conductor(s) and earth and between conductors.
• Earth resistance measurement - commonly performed by the fall-of-potential (three-point) method, or by clamp-on earth testers for quick checks where applicable.
• Continuity checks - using low-resistance ohmmeters to confirm continuity of protective earth conductors and bonding jumpers.
• Polarity checks - using voltage or continuity tests to confirm correct phase/neutral connections at switches and outlets.

Important Considerations Regarding Motor Installation Wiring

When preparing wiring and estimating for motor installations, observe the following points derived from standard practice and regulatory guidance:

• All equipment used in power wiring should be of ironclad construction and wiring shall be of the armoured cable or conduit type (IE Rule 51).
• Woodwork must not be used for mounting of switchgear.
• Looping of conductors and the use of joints in power running conductors should be avoided wherever possible.
• The length of flexible conduit used for connection between the terminal boxes of motors and starters, switches or motor terminals should not exceed 1.25 metres.
• Every motor, regardless of its size, shall be provided with a switch-fuse placed near it.
• In addition to a switch-fuse, all motors shall be provided with suitable means of starting. Starters are used to limit the starting current to a desirable value. Typical starter practice given in common guidance is:
  • Direct-on-line (DOL) starters for motors of small rating (input guidance given: up to 0.75 kW).
  • Star-delta starters for motors of intermediate rating (input guidance given: above 0.75 kW and below 11 kW).
  • Auto-transformer starters (or rotor-resistance starters for slip-ring induction motors) for larger motors (input guidance given: above 11 kW).
• Preferably the conduit should be laid in covered trenches to facilitate operator movement and for safety.
• Laying of cables should be in separate conduits for separate motors.
• The minimum cross-section of conductor that can be used for power wiring: 2.5 mm² for aluminium conductor cables and 1.25 mm² for copper conductor cables (ISI recommendations).
• Hence PVC/MLPE cable of size lower than 3/0.915 mm copper or 1/1.80 mm aluminium cannot be used for power wiring.
• The current rating of cable for supply to a motor may be based on the normal full-load current of the motor; however, fuse rating should take into account the starting current. In no case should the rating of the fuse be greater than twice the rating of the cable.
• Guidance on cable and fuse selection:
  • For motors of capacity above 12 kW having starting current lower than twice normal full-load current, the cable current rating is based on the normal full-load current and the fuse rating is based on motor starting current.
  • For motors of capacity below 12 kW which have a very large starting current, the fuse should be of a current rating sufficient to carry starting current safely and the cable current rating should not be lower than half the current rating of the fuse.
• The conduit used in power wiring shall be electrically continuous throughout and connected to the frame of the motor. The frame of the motor shall be earthed by the owner by two separate and distinct connections to earth (IE Rule 61).
• The wire used for earthing conductors should be of copper or galvanised material for mechanical and corrosion resistance.

Estimation and Costing Principles for Earthing and Motor Wiring

When preparing an estimate for earthing works and motor wiring, follow a systematic approach so that quantities, specifications and rates are clear and auditable.

1. Carry out a site survey to determine soil type, moisture conditions, location of equipment, access for excavation and safety considerations.
2. Decide the earthing method that is suitable for the site (strip, rod, pipe or plate) and prepare a layout showing electrode positions and conductor routing.
3. Prepare a bill of quantities (items and units) for materials, labour and sundries. Typical items include:
   • Earthing electrodes (strip/rod/pipe/plate) - quantities and material specification (GI, copper, size, length/area).
   • Backfill material (charcoal, salt, bentonite or specialised earthing compound) where specified.
   • Earth conductor (strip, stranded copper/galvanised conductor) - length and cross-section.
   • Connection hardware - clamps, bolts, earthing pits, inspection covers.
   • Excavation and backfilling labour and disposal of surplus material.
   • Protective measures - painting, corrosion protection, protective wrapping.
   • Testing and commissioning charges - instruments, labour and test reports.
   • Contingency, overheads and statutory compliance costs.
4. For motor wiring include items such as:
   • Armoured cable or conduit of specified sizes and lengths.
   • Conduits, accessories, glands and insulating bushes at pipe ends.
   • Switch-fuses, starters (DOL, star-delta, auto-transformer) and protective devices with ratings.
   • Motor starter panels, mounting, earthing conductors and bonding.
   • Trenching, cable laying, supports, saddles and termination materials.
   • Testing and commissioning of motor circuits.
5. Quantity take-off and unit price application: measure lengths, areas and counts. Apply unit rates that include material cost, delivery, installation labour and any testing/inspection charge.
6. Include periodic maintenance or replacement allowances where corrosion or aggressive soil conditions are expected.
7. Prepare the final estimate with summary of costs, taxes, contingencies and completion timeline.

Items to Specify in the Tender/Estimate Documents

• Material specifications and standards (for example, grade of copper, GI, IS/IEC standards applicable).
• Electrode dimensions and acceptable earth resistance limits.
• Type of backfill and volume required.
• Methods of testing and acceptance criteria, and requirement for test reports.
• Safety, excavation and reinstatement obligations.
• Warranty and maintenance period for earthing and motor wiring work.

Testing, Acceptance and Documentation

• All earthing pits and motor supply circuits must be tested on completion and test results recorded in an installation test certificate.
• Earth resistance readings, insulation resistance values, continuity test results and polarity checks must be recorded and retained with the installation documentation.
• Where earth resistance is not satisfactory, remedial measures (additional electrodes, chemical backfill, increased electrode area or parallel electrodes) must be estimated and carried out until acceptance criteria are met.

Practical Notes for Estimators

• Soil resistivity is a key factor affecting earthing design; where resistivity is high, more electrodes, greater buried area or chemical treatment may be required and should be budgeted for.
• Accessibility for maintenance (earth pit covers, inspection chambers) should be included in the layout and costing.
• Corrosion protection and sacrificial earth conductors increase life and may reduce long-term maintenance costs; include these options in the estimate.
• When specifying cables and fuses for motors, ensure coordination between cable rating and protective device rating as noted; consider starting current and continuous duty current.

Summary

Correct earthing and properly installed motor wiring are essential for safety and reliable operation. For reliable estimation and costing, combine a sound site survey with specification of materials, a clear bill of quantities, testing and commissioning requirements and allowances for maintenance and contingencies. Always follow the relevant rules and standards for wiring, earthing and protective devices to ensure safety and regulatory compliance.