Earth electrode testing to BS 7430
Every lightning protection system ends in the ground, and the ground is the part you cannot see. Earth electrode testing measures how easily current flows from each rod or ring into the soil, which is what decides whether a strike is spread safely or hangs around on the building.
BS 7430 is the British code of practice for earthing and sets out how electrodes are installed and measured. For lightning protection, the earth readings are taken at each test clamp during the annual test, and the system as a whole is judged against a recommended 10 ohms.
- Code of practice
- BS 7430 (earthing)
- Lightning earth target
- 10 ohms or less overall
- Tested with
- The lightning protection test
- Small building test
- £200 to £350
When earth testing matters most
- Your last lightning protection test showed a high or rising earth reading.
- You are fitting a new system or extra earths and need a commissioning reading.
- The building has generators, substations, telecoms or sensitive equipment with their own earths.
- Ground works, drainage or new paving have been done near the earth pits.
- The site has rocky, sandy or very dry ground where low readings are hard to reach.
What earth testing costs
| Item | Typical figure |
|---|---|
| As part of a lightning protection test Small building, all earths measured | £200 to £350 |
| As part of a larger site test School or large commercial building | £450 to £800 |
| Extra rods or a ring to lower a reading Quoted after the test, depends on ground and access | Quoted |
Where these figures come from
Test prices are typical ranges from published UK contractor price guides, autumn 2026, excluding VAT, read October and November 2026. Firms are not named. Contract values are copied from Contracts Finder or Find a Tender notices, ex VAT where stated, and show the size of real jobs rather than a rate for yours.
How earth electrodes are measured
Fall of potential
The classic method in BS 7430. The electrode under test is disconnected, and two temporary test spikes are driven into the ground in a line some distance away. The tester passes a current between the electrode and the far spike and measures the voltage at points in between. It needs open ground and takes time, but it gives the truest figure for a single electrode.
Dead earth and clamp methods
Where there is no room for spikes, a tester may measure against a known low-resistance earth, or use a clamp-on meter round the conductor without disconnecting it. Both are quicker but need care to read correctly, and the report should say which method was used.
Earth pits
Most rods finish in a small inspection pit with a lid so the connection can be found and tested. Pits get paved over, filled with soil or lost under car parks. Part of a good first test is finding and marking them on the drawing. Our guide to earth pits covers what to look for.
Bringing a reading down
Extend the rod deeper in sections, add more rods spaced well apart and bond them together, or lay a ring conductor round the building. Soil treatments exist but are a last resort; the design should explain why they are needed.
What the earth readings on your report should show
A single overall figure is not enough to manage an earth system. Look for these.
One reading per electrode
Measured with its test clamp open, so a dead rod cannot hide behind a good one.
The method used
Fall of potential, dead earth or clamp, so next year can be measured the same way.
Ground conditions
Dry, wet or frozen on the day, because soil moisture changes readings a lot.
Last year alongside
So a slow drift upwards gets caught before it becomes a fail.
How earth testing and improvement runs
Find the pits
Locate every test point and earth pit from the drawing, or trace them if there is no drawing.
Measure
Each electrode on its own, then the system overall, by the method the site allows.
Diagnose
Compare with past readings and ground conditions to separate a seasonal rise from a real fault.
Improve and retest
Remake connections, extend or add rods, then measure again and record the new figures.
Questions people ask
What is BS 7430?
The British Standard code of practice for protective earthing of electrical installations. It covers how earth electrodes are designed, installed and measured, and lightning protection testers use its methods for the earth readings in a BS EN 62305 test.
What is a good earth resistance reading for lightning protection?
BS EN 62305 recommends 10 ohms or less for the earth termination as a whole. Individual rods may read higher; what matters is the combined figure and whether readings are stable year to year.
Why has my earth reading gone up?
Dry weather, corrosion at the connection, a rod damaged by ground works, or soil that has settled away from the rod. Testing in a different month helps tell weather from a real fault.
How deep do earth rods go?
It depends on the soil. Rods come in sections and are driven until the reading is low enough, which can be a few metres in damp clay or much deeper in sand or rock.
Is earth testing separate from the lightning protection test?
For lightning protection it is normally part of the same visit. Electrical installation earths for the building wiring are tested by electricians as part of the periodic inspection under BS 7671.
Get earth readings looked at
Three quick screens: the postcode of the building, the work, then your contact details. Most people finish in about two minutes.
We are an introduction service, not a lightning protection contractor. We tell you which specialist covers your building before anything is shared, and no one calls you unless you agree.
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