2.1 TN-S Earthing System & Earth Loop Impedance (Zs, Ze)

2.1 TN-S Earthing System & Earth Loop Impedance (Zs, Ze)

In low-voltage electrical distribution systems throughout the Emirate of Dubai, earthing and protective conductor architecture is governed strictly by the Dubai Electricity and Water Authority (DEWA) Wiring Regulations and Dubai Municipality (DM) Building Codes, which are anchored in the British Standard BS 7671 (IET Wiring Regulations). A fundamental pillar of electrical safety in Dubai is the implementation of the TN-S earthing system.

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Architecture of the TN-S Earthing System

The designation TN-S is derived from French standard terminology:

• T (Terre): Direct connection of the power source neutral (star point) to Earth.
• N (Neutre): Direct connection of exposed conductive parts of the installation to the earthed point of the supply system.
• S (Séparé): Protective earth (PE) and neutral (N) functions are provided by separate, distinct conductors throughout the entire electrical network.

In a DEWA-supplied TN-S system, the 11kV/400V step-down distribution transformer located in the local substation has its low-voltage neutral point solidly grounded to an earth electrode matrix. From the substation to the consumer's premises, DEWA provides four main supply lines for a three-phase supply (L1, L2, L3, N) along with a dedicated continuous metallic earth path, typically formed by the heavy lead sheath or armored wire layer of the underground service cable, or a dedicated copper earth cable.

Crucially, the neutral (N) conductor and protective earth (PE) conductor are never combined or bonded together anywhere within the consumer's installation. The neutral carries single-phase return load current under normal operating conditions, whereas the PE conductor carries current only during an insulation breakdown or earth fault event.

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Main Earth Terminal (MET) & Earth Electrode Requirements

Every electrical installation supplied by DEWA must feature a centralized Main Earth Terminal (MET) (also called the Main Earthing Bar), positioned adjacent to the Main Distribution Board (MDB) inside the dedicated electrical switchroom.

Functions of the MET

The MET acts as the central potential equalization node for the entire building. The following key conductors must be solidly connected to the MET using individual, labeled disconnectable terminals or bolted copper lugs:

1. Main Earthing Conductor: Connects the MET to the consumer's earth electrode system.
2. DEWA Earth Cable / Armor Termination: Connects the MET to the utility supply earth protective conductor (PE).
3. Main Protective Bonding Conductors: Connects the MET to extraneous conductive parts (incoming metallic water mains, structural steelwork, central HVAC chilled water pipes).
4. Circuit Protective Conductors (CPCs): Connects the MET (via earth bars in sub-boards) to exposed conductive parts of electrical equipment.

Earth Electrode Systems & Inspection Pits

To complement the utility supply earthing and provide localized reference earthing, DEWA mandates the installation of a dedicated earth electrode system. This system consists of:

• High-Conductivity Copper Earth Rods: Typically 15mm or 20mm diameter molecularly bonded copper-clad steel rods (minimum 250-micron copper coating thickness) driven into the ground to a depth of 3 meters or more.
• Earth Inspection Pits: Concrete or high-density plastic inspection chambers installed at the top of earth rods, fitted with heavy-duty covers, allowing periodic testing of earth electrode resistance ($R_A$).
• Earthing Plates / Grid Matrices: In deep foundation structures or high-resistivity soil conditions common in desert ground, extended copper earth plates (minimum $900\text{mm} \times 900\text{mm} \times 3\text{mm}$) or bare copper tape loops embedded in concrete foundations (ufer earthing) are specified.
• Maximum Resistance Limit: DEWA Regulations for Electrical Installations require the main earth electrode resistance to not exceed $1.0\ \Omega$ for each incoming DEWA supply/MDB; the resistance from any point of the earth continuity conductor to the main earth electrode shall not exceed $0.5\ \Omega$.

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Earth Fault Loop Impedance ($Z_s$) Theory & Calculation

When an insulation breakdown occurs between a line conductor (L) and an exposed metal enclosure (earth fault), an earth fault loop circuit is completed. The magnitude of the fault current ($I_a$ or $I_{pf}$) is limited entirely by the total impedance of this loop, known as the Earth Fault Loop Impedance ($Z_s$).

$\text{Earth Fault Current } I_f = \frac{U_0}{Z_s}$

Where $U_0$ is the nominal line-to-earth voltage (230V in Dubai's standard 400V/230V 50Hz supply system).

The $Z_s$ Formula

The total earth fault loop impedance $Z_s$ measured at any point or load in an electrical installation is given by the fundamental formula:

$Z_s = Z_e + (R_1 + R_2)$

Where:

• $Z_e$ (External Earth Loop Impedance): The impedance of the supply side of the loop external to the consumer's installation. This includes the HV/LV transformer secondary winding impedance, the line conductor from transformer to main switchboard, the DEWA supply earth return path, and the MET connection.
  • BS 7671 Declared Value: For the TN-S supply arrangement that DEWA mandates, the distributor-quoted typical maximum external earth fault loop impedance $Z_e$ is $0.80\ \Omega$ (the $0.35\ \Omega$ figure applies to TN-C-S/PME supplies, which DEWA prohibits in Dubai).
• $R_1$ (Line Conductor Resistance): Resistance of the phase conductor from the MDB to the furthest point of the final circuit under test.
• $R_2$ (Circuit Protective Conductor Resistance): Resistance of the CPC from the furthest point of the final circuit back to the MDB earth bar.

Temperature Correction Factors

Conductor resistances ($R_1+R_2$) are measured cold during initial dead testing (typically at $20^\circ\text{C}$). However, during normal full-load operation or fault conditions, copper conductors heat up, increasing their electrical resistance. To ensure that protective devices will still operate within mandatory disconnection times when conductors are operating at their maximum permitted temperature (e.g. $70^\circ\text{C}$ for PVC or $90^\circ\text{C}$ for XLPE cables), a temperature correction factor of 1.2 (multiplier of 0.8 on maximum permitted $Z_s$ tables) is applied:

$Z_{s(\text{measured at } 20^\circ\text{C})} \le 0.8 \times Z_{s(\text{tabulated max limit})}$

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Automatic Disconnection of Supply (ADS) & BS 7671 Limits

The primary measure for fault protection (indirect contact protection) in Dubai installations is Automatic Disconnection of Supply (ADS). Under ADS rules, when a fault of negligible impedance occurs between a line conductor and exposed metalwork, the protective device (MCB, MCCB, or RCD) must disconnect the circuit rapidly enough to prevent dangerous touch voltages from persisting and causing electrocution.

Mandatory Disconnection Times (BS 7671 & DEWA)

For a nominal system voltage $U_0 = 230\text{V}$ to earth:

To achieve disconnection within 0.4 seconds using miniature circuit breakers (MCBs), the fault current $I_f$ must be large enough to activate the instant magnetic trip element of the breaker. The maximum allowable $Z_s$ values for standard 230V MCBs operating within 0.4s are summarized below:

If the calculated or measured $Z_s$ exceeds the maximum permitted table limit, the circuit breaker will fail to trip within 0.4 seconds, presenting a severe shock hazard. In such cases, installing a Residual Current Device (RCD) is mandatory to enforce fast disconnection.