In brief
- A low earthing bid often omits rods, excavation, backfill material and continuity testing rather than being genuinely cheaper.
- Earthing resistance depends on soil resistivity, which varies with moisture and salt, so rod count and depth cannot be fixed from an office desk.
- Tie the tender to a measured schedule, a test plan and a resistance figure the contract names before comparing prices.
Contents
Earthing installation underbidding risks start with the tender document, not the price. When a schedule lists electrodes by count and leaves out pits, backfill, conductor routes and testing, bidders fill the gap differently: some price the full system, some price only the rods. A low number can mean a shorter scope rather than a cheaper way of doing the same work, so check each bid line by line against the drawings.
What does a low earthing bid leave out?
Rods, pits, conductor and testing are the items that disappear first. A bill of quantities that says "earthing system, lump sum" tells a bidder nothing about how many electrodes, how deep, how far apart or in what soil. Prices then diverge by the cost of the material actually buried. Watch for allowances that count electrodes but omit excavation, bentonite or similar backfill, inspection chambers, and the conductor run from the electrode to the main earthing terminal.
A second omission is testing. A lump sum can be won without pricing an earth resistance test, continuity checks on protective conductors or a record of results. Ask each bidder for a written method statement covering rod type and length, spacing, burial depth, connection method and the test they will carry out. General practice for rod spacing and depth is to follow the project specification or the engineer's instruction, so confirm those figures against the drawings rather than assuming a number.
How do you spot unrealistic tender allowances?
Compare what the bidder priced with what the drawing shows, item by item. A thin allowance shows up when the contractor installs one rod where the design calls for several, when rods are not driven to the depth the drawing specifies, or when the conductor is clamped with a rebar tie. Electrode resistance must meet the contract target whatever the soil condition, and the materials and labour to reach it are what a low bid has to cover.
Count the electrodes shown, count the ones priced, and check that spacing, depth and route match. Ask for the cable and conductor sizes, the connection detail at the earth bar, and the label or schedule that will be fixed at the main earthing terminal. Then walk a recent site the same contractor finished and look at the earth pit, the clamp and the test record. None of this needs a laboratory; it needs the drawing and an hour on site.
Why does ineffective fault protection follow from this?
A cheap earth path leaves the protective device with nothing to work with. If the electrode resistance is above the value the design assumed, fault current takes the easiest route, which may be through a person or through building services. Underbid work often shows this indirectly: no test results from commissioning, no label at the distribution board, no schedule of circuits and no continuity readings recorded before handover. Ask the electrical subcontractor for the test sheet, not a verbal assurance.
Where soil moisture changes between seasons, a reading taken on a wet day can look better than the same installation in a dry month. Treat any single reading with care and ask when it was taken. Records kept at handover give the client a baseline to compare against later. Confirm acceptable resistance values and test methods with the project engineer or the office that issues the electrical permit, and keep the signed test records with the as-built drawings.
What to do next
Rewrite the earth works section of the bill so it lists electrodes, excavation, backfill, conductor, connections and tests as separate measured items. Ask each bidder to return that schedule priced line by line, and hold a clarification meeting before award to compare method statements and test records. Then ask the project engineer or permitting office what the drawings and specification require for rod spacing, depth and test method before signing.
Frequently asked questions
Can a contractor price earthing from a lump sum clause?
They can, but the number will not be comparable with other bids. Ask for a measured schedule instead so electrodes, pits, conductor and testing are priced separately and omissions become visible.
Who should confirm the earth resistance target for a building?
The project engineer or the office that issues the electrical permit. Take the figure from the drawings or the specification, not from a bidder's assumption.
What test record should be handed over?
A handover should include the test records the contract requires. Confirm the exact documents and their format with the project engineer or the permit office before award.
Local reporting: this article is written for building work in Borama and Somaliland. Ground conditions, prices and rules differ between places; confirm the details for your own site.