AS/NZS 3000 Earthing Systems — TN-S, TN-C-S, TT, and IT Decoded
Four earthing systems, four different protection philosophies. Most engineers know what TN-S means but not why it matters for protection design. Here's the guide.
February 26, 2026
The Naming Convention
The IEC/AS/NZS naming system uses two or three letters:
- First letter — Relationship of supply to earth: T (direct connection to earth) or I (isolated from earth)
- Second letter — Relationship of exposed metalwork to earth: T (direct connection to local earth) or N (connection to supply neutral/earth)
- Third letter (if present) — Neutral/earth arrangement: S (separate), C (combined), C-S (combined then separated)
TN-S: Separate Neutral and Earth
T = supply earthed | N = metalwork connected to supply earth | S = separate conductors
The supply transformer has its star point earthed. A separate protective earth conductor runs from the transformer to the installation, distinct from the neutral.
Advantages: Clean earth (no neutral current flowing in earth conductor), low earth fault loop impedance, high fault currents for fast disconnection.
Common in: New commercial and industrial installations, dedicated supply transformers.
Protection implication: High fault currents allow overcurrent devices alone to achieve fast disconnection. RCDs are additional protection, not primary.
TN-C-S: Combined Then Separated (PME/MEN)
T = supply earthed | N = metalwork to supply earth | C-S = combined in supply, separated in installation
The supply uses a combined neutral-earth (PEN) conductor from the transformer to the installation origin. At the main switchboard, the PEN splits into separate neutral and earth conductors.
Also called PME (Protective Multiple Earthing) in the UK or MEN (Multiple Earthed Neutral) in Australia.
Advantages: No need for a separate earth conductor in the supply cable (cost saving for utilities).
Risks: If the PEN conductor breaks (open neutral), all exposed metalwork in the installation can rise to supply voltage via the neutral current flowing through the earth path. This is the PEN conductor failure risk.
Protection implication: Equipotential bonding is CRITICAL. The installation must create a local equipotential zone so that even with elevated earth potential, no dangerous touch voltages exist within the building. Main bonding conductors must be sized per AS/NZS 3000 Table 5.1.
TT: Local Earth Electrode
T = supply earthed | T = metalwork connected to local earth electrode
The supply transformer is earthed, but the installation has its own independent earth electrode. No earth conductor from the supply.
Advantages: Immune to PEN conductor failure. Suitable for rural installations with long supply lines.
Challenges: Earth fault loop impedance is high (includes both the supply earth and the local earth electrode resistance). High earth electrode resistance means low fault current, which may be insufficient to trip overcurrent devices within required times.
Protection implication: RCDs are mandatory on virtually all circuits. Overcurrent devices alone cannot achieve fast disconnection due to high Zs. The RCD detects the earth fault current regardless of the earth loop impedance.
IT: Isolated Supply
I = supply isolated from earth | T = metalwork connected to local earth
The supply transformer is NOT earthed (or earthed through a high impedance). A first earth fault produces only a small current through system capacitance — not enough to be dangerous. The system can continue operating.
Advantages: First fault doesn't cause disconnection (high availability). Used in operating theatres, critical process plants.
Requirements: Insulation monitoring device (IMD) must alarm on first fault. Second fault (on a different phase) creates a phase-to-phase fault through earth and must be cleared by protection.
Protection implication: Complex protection design. Requires IMD, fault location system, and protection coordination for second-fault scenarios.
Selection Guide
| Application | Recommended System | Reason |
|---|---|---|
| Commercial buildings (urban) | TN-C-S | Standard utility supply |
| Industrial (dedicated tx) | TN-S | Clean earth, simple protection |
| Rural residential | TT | No reliable supply earth |
| Hospitals (operating theatres) | IT | Continuity of supply |
| Mining | IT or TN-S | Depends on jurisdiction |
Design your earthing: Model earth electrode resistance and fault loop impedance with the Earthing Calculator.
Frequently Asked Questions
What is the target earth electrode resistance?
IEEE 80 recommends <1Ω for substations, <5Ω for commercial, <25Ω for residential. AS/NZS 3000 Clause 5.6.3.3 requires resistance low enough to ensure fault current trips protection within 0.4s (TN) or 5s (TT).
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