"What should my earth resistance be?" is one of the most searched questions in electrical work, and the honest answer is that it depends on the installation. A single number does not fit a house, a factory and a substation. This guide explains where the targets come from and what to do when your reading does not meet one.
Why the Target Exists
Earth resistance determines how much a fault current is limited and how high the voltage on an earthed metal part can rise during a fault. Lower resistance means a larger fault current, which trips the protective device faster, and a smaller touch voltage. Higher-risk installations therefore demand lower resistance.
Commonly Quoted Targets
| Installation | Commonly quoted target |
|---|---|
| Generating stations | About 1 Ω or less |
| Large substations | About 2 Ω or less |
| Distribution systems | About 5 Ω or less |
| Domestic and small installations | Often specified between 5 Ω and 10 Ω, or per the supply rules |
| Lightning protection electrodes | Often specified at about 10 Ω or less |
| Sensitive electronics and data rooms | Often specified at 1 Ω to 2 Ω |
Treat these as typical, not definitive
The Resistance of the Electrode Is Not the Whole Story
In many systems what actually trips the protective device is the whole fault loop, including the supply neutral earth and the protective conductors, not just your electrode. A low electrode reading does not guarantee a good loop, and a modest electrode reading may be perfectly safe where the system is well bonded and protected by RCDs. Do not chase a very low number on the electrode alone without understanding the system.
When the Reading Is Too High
- Check the measurement first: spike spacing, spike contact and lead connections.
- Check the connection between the electrode and the installation.
- Add depth, add parallel electrodes or improve the backfill.
- Retest in the driest season.
See earth pit testing and maintenance and earth resistance testing in dry soil for remedies.
Measure It Correctly
Because the target is a hard pass-or-fail number, the measurement must be trustworthy. Use an instrument with adequate accuracy at the resistance you expect, and follow the fall-of-potential method, covered in how to test earth resistance. Browse CIE earth testers in analog and digital forms.
Unsure which target applies to your site? Ask us, and check the project specification.