Earth Rod Selection, TT / TN-S / TN-C-S Systems and Earth Resistance Testing
Earth rod length and material per IEC 60364-5-54, the difference between TT, TN-S and TN-C-S systems, rod resistance estimation and the 3-pole fall-of-potential test.
An earth rod is the vertical electrode that connects the protective conductor (PE) of an installation to the ground. How many rods are needed and which resistance to aim for depend on the system type (`TT`, `TN-S`, `TN-C-S`) and soil resistivity; conductor sizes and bonding rules are set out in IEC 60364-5-54.
> Related Category: Browse testers and connection parts in Earthing Measuring Instruments, Ground Terminal Blocks and Panel Ground Terminal Blocks.
1. TT, TN-S and TN-C-S systems
The first letter describes the source (transformer star point), the second the exposed conductive parts of the installation.
| System | Neutral and protective conductor | Fault current path | Required protection |
|---|---|---|---|
| TT | Source neutral earthed separately; exposed parts on a local electrode | Through the soil; current stays small | Residual current device (RCD) |
| TN-S | N and PE separate from the transformer (5 conductors) | Metallic PE; high current | Fuse or breaker, RCD as additional |
| TN-C | N and PE combined in one conductor (PEN) | Through PEN | Fuse or breaker; RCDs cannot be used |
| TN-C-S | PEN in the network, split into PE and N at the main board | PE → PEN | As TN-S after the split |
- In TN-C-S the PEN is split once at the main board; after that N and PE are never rejoined, otherwise neutral current flows through exposed parts and RCDs trip falsely.
- A PEN conductor must be at least 10 mm² copper or 16 mm² aluminium; below that N and PE run separately.
- Detached houses and buildings fed by overhead lines are mostly TT, where protection relies entirely on the local electrode and the RCD.
2. Required earth resistance in TT systems
To keep touch voltage below 50 V:
R ≤ 50 V / IΔn
| RCD rated residual current | Maximum permitted resistance |
|---|---|
| 30 mA | 1,666 Ω |
| 100 mA | 500 Ω |
| 300 mA | 166 Ω |
These are upper limits. Because resistance varies with season, specifications ask for far lower values; in practice below 10 Ω is commonly targeted for protective earthing and below 2 Ω for transformer system earthing.
3. Rod selection and resistance estimate
| Rod type | Property | Use |
|---|---|---|
| Copper-bonded steel | At least 250 micron copper on a steel core; drives without bending | Building and panel earthing, lightning protection |
| Solid copper | Best corrosion resistance, soft | Aggressive soil, short lengths |
| Hot-dip galvanized steel | Economical; galvanic corrosion in direct contact with copper | Temporary sites, steel tape systems |
Common sizes are 14, 16 and 20 mm diameter in 1.5 m and 3 m lengths, extendable with couplers. Approximate resistance of one vertical rod:
R ≈ ρ / L (ρ: soil resistivity in Ω·m, L: buried length in m)
| Soil | Typical resistivity (Ω·m) | 1.5 m rod | 3 m rod |
|---|---|---|---|
| Marsh, moist humus | 30 | ≈ 20 Ω | ≈ 10 Ω |
| Clayey farmland | 100 | ≈ 67 Ω | ≈ 33 Ω |
| Moist sand | 200 | ≈ 133 Ω | ≈ 67 Ω |
| Dry sand, gravel | 1,000 | ≈ 667 Ω | ≈ 333 Ω |
- If resistance is too high, drive deeper or connect several rods in parallel. Keep spacing at least twice the rod length; closer rods share the same soil and `n` rods will not reach `R / n`.
- Leave the rod head below ground level and make the joint with a clamp or exothermic weld inside an inspection pit.
- On rocky ground use a foundation electrode or buried tape instead of vertical rods.
4. Conductor cross-sections (IEC 60364-5-54)
| Conductor | Minimum size |
|---|---|
| Buried earthing conductor without corrosion protection | 25 mm² copper or 50 mm² galvanized steel |
| Protective conductor (PE), line size S ≤ 16 mm² | S |
| PE, 16 mm² < S ≤ 35 mm² | 16 mm² |
| PE, S > 35 mm² | S / 2 |
| Main protective bonding conductor | At least 6 mm² copper |
Water, gas and heating pipes and structural steel are bonded at the main earthing terminal. For PE colour rules see IEC 60446 Wire Colour Codes.
5. How to measure earth resistance
3-pole fall-of-potential (62%) method with an IEC 61557-5 earth tester:
- Disconnect the electrode under test (`E`) from the installation.
- Drive the current spike (`H`) at least 20–40 m away (five times the electrode dimension or more).
- Drive the potential spike (`S`) in line at 62% of that distance and take a reading.
- Repeat with `S` at 52% and 72%; if the three values agree the result is valid, otherwise move `H` further out.
- Clamp-on (stakeless) testing works only on loops with several parallel electrodes; it cannot measure a single isolated rod.
- In TT installations an earth fault loop impedance test is a practical cross-check.
- Measure when the soil is driest, such as late summer, and repeat periodically.
Frequently asked questions
What should the earth resistance be?
In a TT system the theoretical limit for a 30 mA RCD is 1,666 Ω, but practice targets below 10 Ω for protective earthing and below 2 Ω for transformer system earthing. The project specification governs.
How long should an earth rod be?
The most common length is 1.5 m, yet even in clay a single 1.5 m rod gives about 67 Ω. Reaching 10 Ω needs 3 m or deeper, or several spaced rods.
Can neutral and earth be linked?
Only in TN-C-S, at the single split point in the main board. Never bridge neutral and earth in sub-boards or sockets; it defeats the RCD and can leave exposed parts live.
Copper-bonded or galvanized rod?
Use 250 micron copper-bonded steel for permanent installations and with copper conductors. Zinc on a galvanized rod is consumed quickly by galvanic corrosion when joined directly to copper.
Can a multimeter measure earth resistance?
No. A multimeter cannot measure electrode resistance; an earth tester with current and potential spikes, or a clamp-on earth tester in suitable conditions, is required.