The lower the earthing resistance, the more safely fault current flows to ground. Everyone knows the "4 ohm" figure, but it is not the right limit for every site. Below are the limits in a table, how earthing is tested and what to do if the resistance is too high.
- In a 380/220 V network, the earthing system of a transformer or generator neutral must be no more than 4 Ω (PUE 1.7.101).
- Repeat earths of the PEN conductor on an overhead line: 10 Ω in total and 30 Ω each (PUE 1.7.103).
- Where soil resistivity ρ exceeds 100 Ω·m, the limits may be raised by a factor of 0.01·ρ, but not more than tenfold.
What is earthing resistance?
It is the resistance that the earthing system (electrodes and connecting conductors) presents to current flowing into the ground. If it is high, a dangerous voltage remains on equipment enclosures when insulation fails, and the protective device may not trip in time. That is why earthing is tested before a site is commissioned and periodically afterwards.
Limits table (installations up to 1 kV)
Under the electrical installation rules used across the CIS, including Azerbaijan (PUE, clauses 1.7.101 and 1.7.103), the limits are:
| Network voltage (3-ph / 1-ph) | Neutral earthing system | Electrode near neutral | Repeat earths on overhead line, total | Each repeat earth |
|---|---|---|---|---|
| 660 / 380 V | 2 Ω | 15 Ω | 5 Ω | 15 Ω |
| 380 / 220 V | 4 Ω | 30 Ω | 10 Ω | 30 Ω |
| 220 / 127 V | 8 Ω | 60 Ω | 20 Ω | 60 Ω |
Where soil resistivity ρ is above 100 Ω·m (dry sand, rocky ground), these limits may be raised by a factor of 0.01·ρ, but not more than tenfold.
The value required for a specific site is set by the design and the network earthing system (TN-C-S, TT, etc.). The table is a general guide.
How is earthing tested?
We test to the GOST R 50571.16-2019 method. There are two main techniques:
- Three-pole (fall-of-potential) method. Two auxiliary electrodes, current and potential, are driven into the ground. The instrument injects current and calculates the resistance. It is the most accurate method but needs space.
- Two-clamp method. An instrument such as the S408 measures with clamps, without disconnecting electrodes. It suits systems with many earths, such as poles or a building's ring electrode.
Soil resistance changes with the season and is worst in dry or frozen ground. That is why the report records the test date and weather.
When should earthing be tested?
- At handover of a new site.
- After repair, reconstruction or equipment replacement.
- At periodic inspections, at intervals set by the operating rules and the site owner.
- After a lightning strike, short circuit or mechanical damage to the earthing.
What if the resistance is above the limit?
- Check the connections first: a rusty bolt or broken conductor is a common cause.
- Add electrodes or use longer ones, since deeper soil is usually damper.
- Space the electrodes far enough apart, or they will shield each other.
- Re-test after the work and get a new report.
What should the test report contain?
An accredited laboratory's report lists the site, test points, method, instrument and its calibration date, measured values, the limit and a conclusion. The Enerji Test Servis LLC laboratory is accredited to AZS ISO/IEC 17025:2020, so our reports are accepted by commissioning panels and regulators.
Earthing tests — Enerji Test Servis
FAQ
What earthing resistance does a private house need?
It depends on the network system. For a house supplied from a 380/220 V overhead line, 30 Ω is often used as the reference for the repeat earth (PUE 1.7.103). The design sets the exact requirement.
Where does the 4 ohm limit come from?
From PUE 1.7.101: in a 380/220 V network, the earthing system of a transformer or generator neutral must be no more than 4 Ω.
Is lightning protection earthing tested separately?
Yes, lightning protection earthing is also tested and included in the report.
Does the power need to be switched off for the test?
Usually not. With the clamp method we test without disconnecting the electrodes. If a shutdown is needed, we agree the schedule in advance.