Earth Fault Current Paths: TN-S vs TN-C-S vs TT vs IT — Visualized

When a live conductor touches an exposed metal enclosure — a cable fault inside a motor housing, a broken wire touching a steel conduit — fault current must flow somewhere. The path that current takes, how much current flows, and how quickly the protective device disconnects all depend on one fundamental design choice: the earthing system.

Four earthing systems are defined in international standards: TN-S, TN-C-S, TT, and IT. Each creates a different fault current path, produces different touch voltages, and demands a different protection strategy. Understanding these paths is not academic — it determines whether a person touching a faulted enclosure receives a harmless tingle or a lethal shock.

This article traces the actual fault current path in each system, explains why touch voltage depends on the earthing arrangement (not just fault current), and reveals why some systems absolutely require RCDs while others can rely on overcurrent devices.

The IEC notation uses two or three letters:

First letter — Relationship of supply to earth:

• T = one point of the supply (usually the neutral) is directly connected to earth (terre)
• I = the supply is isolated from earth, or connected through a high impedance

Second letter — Relationship of exposed metalwork to earth:

• T = exposed metalwork is connected to a local earth electrode (independent of the supply earth)
• N = exposed metalwork is connected to the supply neutral (which is itself earthed at the supply)

Third letter (for TN systems) — Arrangement of neutral and protective conductors:

• S = separate neutral (N) and protective earth (PE) conductors throughout
• C = combined neutral and protective earth (PEN) conductor
• C-S = combined PEN for part of the system, then separated into N and PE

IEC 60364-1:2005, Clause 312; AS/NZS 3000:2018, Section 5.1; BS 7671:2018, Part 2 Definitions.

In a TN-S system, the supply transformer has its neutral connected to earth at the transformer (the “source earth”). A separate protective earth conductor (PE) runs from the transformer all the way to every socket and appliance in the installation — alongside the line and neutral conductors but separate from the neutral.

When a line-to-earth fault occurs in a TN-S system:

1. Current flows from the transformer winding through the line conductor to the fault point
2. Current passes through the fault into the exposed metalwork (enclosure)
3. Current flows through the protective earth conductor (PE) back to the transformer neutral point
4. The circuit is completed through the transformer winding

The entire fault loop is metallic — copper and steel all the way. The impedance of this loop is low, typically 0.5 to 2 Ω depending on cable lengths and sizes. The fault current is therefore high:

I_fault = U₀ / Z_s — (Eq. 1)

Where U₀ is the nominal line-to-neutral voltage (230 V or 240 V) and Z_s is the total earth fault loop impedance.

A circuit has:

• Supply transformer impedance: 0.05 Ω
• Distributor impedance (line + PE): 0.15 Ω
• Final circuit cable impedance (line + PE): 0.60 Ω
• Total Z_s = 0.80 Ω

I_fault = 230 / 0.80 = 288 A — (Eq. 2)

This is enough to trip a 32 A Type B MCB (magnetic trip at 3–5 times rating = 96–160 A) well within the required 0.4-second disconnection time. In fact, 288 A will trigger the magnetic trip, disconnecting in under 10 milliseconds.

Touch voltage: The voltage a person would experience touching the faulted enclosure depends on the voltage drop across the PE conductor:

V_touch = I_fault × R_PE(circuit) = 288 × 0.30 = 86 V — (Eq. 3)

This is above the 50 V safety threshold (IEC 60364-4-41) but exists only for the brief disconnection time. The protective device must clear the fault within 0.4 seconds for circuits up to 32 A to limit the duration of this dangerous voltage.

BS 7671:2018, Table 41.1; AS/NZS 3000:2018, Table 8.1.

The TN-C-S system uses a combined protective earth and neutral conductor (PEN) for the supply side, which separates into individual N and PE conductors at the installation’s origin. This arrangement is also called Protective Multiple Earthing (PME) because the PEN conductor is earthed at multiple points along its length by the supply authority.

The TN-C-S system has an inherent vulnerability: if the PEN conductor breaks between the supply transformer and the installation, the installation loses its earth reference. Under normal (no-fault) conditions, the neutral current has no return path, and the voltage on the installation’s earthing system can rise to a dangerous level — potentially full line voltage on every exposed metalwork.

This is why TN-C-S systems have specific requirements:

• The PEN conductor must have a minimum cross-sectional area (typically 10 mm² copper or 16 mm² aluminium)
• Multiple earth electrodes are required along the PEN route
• PEN conductors must not be switched or isolated

Under normal conditions and intact PEN, the fault current behaviour is essentially identical to TN-S: high fault current (hundreds of amperes), fast disconnection by overcurrent devices, and relatively brief touch voltage exposure.

Worked Example: With a total loop impedance Z_s of 0.35 Ω (typically lower than TN-S because the PEN conductor is robust):

I_fault = 230 / 0.35 = 657 A — (Eq. 4)

A 32 A MCB will trip magnetically almost instantly.

In a TT system, the supply has its neutral earthed at the transformer, but the installation’s exposed metalwork is connected to a local earth electrode — completely independent of the supply earth. The fault current return path goes through the actual earth (soil).

This system is common in rural areas, in countries where the supply authority does not provide an earth conductor (much of France, parts of Australia, developing countries), and increasingly as a deliberate design choice for specific installations.

The critical difference in a TT system is that the fault loop includes the resistance of two earth electrodes and the soil between them. These resistances are typically:

• Installation earth electrode (R_A): 10 to 200 Ω (depends on electrode type and soil resistivity)
• Supply earth electrode (R_source): 1 to 20 Ω (usually lower because the utility has extensive earthing)

A typical total earth fault loop impedance in a TT system might be 50 to 200 Ω — compared to less than 2 Ω in a TN system.

Installation earth electrode resistance: 20 Ω. Supply earth electrode resistance: 5 Ω. Cable impedance (line conductor): 0.5 Ω. Total Z_s = 25.5 Ω.

I_fault = 230 / 25.5 = 9.0 A — (Eq. 5)

Nine amperes. Not 288 A as in the TN-S example. Not 657 A as in TN-C-S.

A 32 A MCB requires at least 96 A (Type B) or 160 A (Type C) for magnetic trip. At 9 A of fault current, the MCB does not even see the fault as an overcurrent — 9 A is well within the normal operating range of a 32 A circuit. The MCB will never trip on earth fault current.

This is why TT systems must use RCDs (residual current devices) for earth fault protection. An RCD detects the imbalance between line and neutral currents — if 9 A goes out on the line but only comes back on the neutral (because 9 A is going through the earth), the RCD detects the 9 A imbalance and trips.

A 30 mA RCD will trip at 9 A in under 40 milliseconds — far faster than any MCB.

Touch voltage in the TT system:

V_touch = I_fault × R_{A(installation)} = 9.0 × 20 = 180 V — (Eq. 6)

Without an RCD, this 180 V would persist indefinitely — clearly lethal. With a 30 mA RCD, the fault is cleared in under 40 ms, limiting the energy exposure to safe levels.

AS/NZS 3000:2018, Section 5.1.3; BS 7671:2018, Regulation 411.5; IEC 60364-4-41:2005, Clause 411.5.

Here is a seeming contradiction: TT systems have low fault current (which sounds good — less heating, less damage) but higher touch voltage (which is bad — more dangerous). The low fault current is actually the problem, not a benefit, because it means overcurrent devices cannot detect the fault. The system relies entirely on RCDs for safety.

However, TT systems have one significant advantage: they are immune to the “open PEN” failure mode that threatens TN-C-S systems. Since the installation’s earth is independent of the supply, a broken neutral in the supply cable does not compromise the installation’s earthing.

In an IT system, the supply transformer is either unearthed or connected to earth through a high impedance (typically 1,000 to 2,000 Ω). Exposed metalwork is connected to earth either individually or collectively.

The defining feature of the IT system is that a first earth fault does not require automatic disconnection. The fault current through the high-impedance earthing (or cable capacitance) is so small — typically less than 1 A — that it presents no significant shock hazard and causes no damage.

An insulation monitoring device (IMD) raises an alarm when the first fault is detected, but the system continues to supply power. This is the reason IT systems are used in:

• Operating theatres (power interruption during surgery is unacceptable)
• Mining (unexpected shutdowns in underground environments are dangerous)
• Continuous process industries (chemical plants, semiconductor fabs)
• Emergency lighting circuits

If a second fault occurs on a different phase before the first fault is repaired, the situation changes dramatically. Now there is a low-impedance path between two line conductors (through the two fault points and the interconnected exposed metalwork), and the fault current can be as high as a line-to-line short circuit:

I_{fault(second)} = U_LL / Z_loop = 400 / (0.5 to 2.0) = 200 to 800 A — (Eq. 7)

This must be detected and cleared by overcurrent devices. The disconnection requirements for a second fault in an IT system are the same as for a TN system.

Supply voltage: 400 V (3-phase), transformer earthed through 1,500 Ω impedance.

I_fault(first) = V_phase / Z_earth = 230 / 1500 = 0.153 A (153 mA) — (Eq. 8)

Touch voltage with a 20 Ω installation earth electrode:

V_touch = 0.153 × 20 = 3.1 V — (Eq. 9)

This is well below the 50 V safety limit. The person touching the faulted enclosure experiences essentially no shock. The IMD records the fault, an alarm sounds, and maintenance can be scheduled without interrupting the power supply.

IEC 60364-4-41:2005, Clause 411.6; AS/NZS 3000:2018, Section 5.1.4.

The external earth fault loop impedance, Z_e, is the impedance of the fault loop external to the installation — from the supply transformer to the installation’s main earthing terminal. It is the one parameter the installation designer cannot control; it is determined by the supply authority’s infrastructure.

In TN systems, Z_e is typically 0.20 to 0.80 Ω and must be measured or obtained from the supply authority. The maximum permissible Z_e determines the maximum cable length for any given protective device — because the total loop impedance (Z_e plus the circuit impedance Z_s) must be low enough to ensure the fault current exceeds the protective device’s trip threshold within the required time.

In TT systems, Z_e is effectively the sum of the two earth electrode resistances plus soil resistance — typically 20 to 200 Ω. This high Z_e is precisely why overcurrent devices cannot protect TT circuits.

BS 7671:2018, Regulation 313.1 and Tables 41.2–41.4 for maximum Z_s values.

1. In TN systems (TN-S and TN-C-S), the earth fault loop is entirely metallic, producing high fault currents (hundreds of amperes) that overcurrent devices can detect and clear quickly.
2. In TT systems, the earth fault loop includes soil resistance, limiting fault current to as low as 5–20 A. This is why TT systems must use RCDs — overcurrent devices simply cannot see these faults.
3. Touch voltage depends on the earthing system, not just fault current magnitude. A TT system with only 9 A of fault current can produce 180 V touch voltage — more dangerous than a TN-S system with 288 A producing 86 V.
4. IT systems tolerate a first fault without disconnection, making them essential for critical applications where power continuity saves lives.
5. The external earth fault loop impedance Z_e is the critical parameter that links the supply system to the installation’s protective device selection. In TN systems, it must be low; in TT systems, it is inherently high and drives the mandatory use of RCDs.

Standards referenced: IEC 60364-1:2005, Clause 312; IEC 60364-4-41:2005, Clauses 411.4–411.6; AS/NZS 3000:2018, Section 5.1; BS 7671:2018+A2, Chapter 41 and Part 2 Definitions.