Earthing in Lagos Soil: Why Your Earth Resistance Changes Between the Dry and Rainy Season

Many facility managers are surprised when an earth pit that tested well during the rains gives a much higher reading a few months later. It is not a faulty meter. Soil resistance changes with moisture, and in Nigeria the difference between the rainy and dry seasons can be large. Earthing designed only on a wet-season reading may not protect people and equipment when the dry season comes.

Why earthing matters

A good earth gives fault current a low-resistance path back to the source, so protective devices operate quickly and exposed metalwork does not rise to a dangerous voltage. It also gives lightning and surge currents a safe path into the ground, and helps sensitive electronics work reliably.

Why the reading changes

• Moisture: water and the salts dissolved in it carry current through soil. Dry soil conducts far less.
• Soil type: clay holds moisture and usually gives lower resistance. Sand and laterite drain fast and give higher readings.
• Temperature: resistance also rises as soil dries in the heat.
• Water table: in many parts of Lagos the water table rises and falls through the year, changing conditions around the electrode.

What this means in practice

• Test earth resistance at least once a year, and ideally during the dry season, when readings are usually at their worst.
• Record every reading, the date and the weather, so you can see the trend.
• Design for dry-season conditions, not wet-season ones.

How to get a stable, low earth resistance

• Drive electrodes deeper, to reach permanently moist soil.
• Use more electrodes spaced well apart and bonded together, rather than one pit.
• Use an earth enhancement compound around electrodes in sandy or rocky ground.
• Use good materials and connections: copper or copper-bonded rods and properly made joints. Corroded clamps are a common cause of rising readings.
• Fit inspection pits so the connection can be checked and tested without digging.

How earth resistance is tested

The common method is the fall-of-potential test, using an earth tester with test spikes placed at set distances from the electrode under test. For large sites, engineers may also measure soil resistivity before design, so the earthing system is sized for the actual ground conditions.

Don’t forget lightning protection

Lightning protection systems need their own low-resistance earth, bonded to the main earthing system. With frequent storms during the rainy season, tall buildings, warehouses and telecom structures should have both lightning protection and surge protection devices.

Earthing is one of the first things we check in an electrical safety audit.

Talk to Corustar

Corustar Technological Services is an electrical engineering contractor based in Ikeja, Lagos, working across Nigeria. See our Earthing & Lightning Protection service, request a quote, or call +234 816 976 9672. For outages and faults, call our 24/7 emergency line on 0705 739 9297.