13.4 Automatic Disconnection of Supply (ADS) & Maximum Zs

13.4 Automatic Disconnection of Supply (ADS) & Maximum Zs

In electrical installation design, Automatic Disconnection of Supply (ADS) is the primary and most widely applied protective measure against electric shock. Defined in BS 7671 Section 411, ADS operates on a fundamental electrical engineering principle: whenever an insulation breakdown occurs between a line conductor and an exposed-conductive-part, the resulting earth fault current must be sufficiently high to trigger a protective device (circuit breaker, RCBO, or fuse) to disconnect the supply automatically before ventricular fibrillation or death can occur.

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The Dual Requirements of ADS: Basic Protection & Fault Protection

Under BS 7671 Regulation 411.1, the protective measure ADS requires the coordinated provision of two distinct layers of defense:

                    AUTOMATIC DISCONNECTION OF SUPPLY (ADS)
                               (Section 411)
                    ┌─────────────────┴─────────────────┐
                    ▼                                   ▼
             BASIC PROTECTION                    FAULT PROTECTION
      (Protection under Normal Conditions) (Protection under Single-Fault Conditions)
      • Basic Insulation of live parts     • Protective Earthing (CPCs & MET)
      • Barriers & Enclosures:             • Protective Equipotential Bonding
        - IP2X / IPXXB general             • Automatic Disconnection:
        - IP4X / IPXXD top surfaces          - TN systems: 0.4 s (≤32A)
                                             - TT systems: 0.2 s (≤32A)

1. Basic Protection (Protection Against Direct Contact)

Basic protection prevents humans and livestock from coming into direct physical contact with live electrical conductors under normal, healthy operating conditions. It is achieved through:

• Basic Insulation: Non-conductive insulation surrounding live copper conductors (e.g. polyvinyl chloride PVC or cross-linked polyethylene XLPE), capable of withstanding the electrical, thermal, and mechanical stresses of normal operation.
• Barriers and Enclosures: Protective enclosures housing live electrical terminations (e.g. consumer units, junction boxes, switch plates). Under BS 7671 Section 416, accessible outer surfaces of enclosures must provide a minimum degree of ingress protection of IP2X or IPXXB (preventing entry of fingers or objects $>12\text{ mm}$ diameter). Furthermore, readily accessible horizontal top surfaces of enclosures (such as the top lid of a consumer unit or distribution board) must achieve at least IP4X or IPXXD (preventing ingress of small wires or tools $>1.0\text{ mm}$ diameter).

2. Fault Protection (Protection Against Indirect Contact)

Fault protection prevents danger if basic insulation fails, causing exposed metalwork to become energized. Fault protection requires:

• Protective Earthing: Interconnecting all exposed-conductive-parts via Circuit Protective Conductors (CPCs) to the Main Earthing Terminal (MET).
• Protective Equipotential Bonding: Linking extraneous services (water, gas, steel) to eliminate touch voltages.
• Automatic Disconnection: A circuit breaker, fuse, or residual current device that disconnects the line conductor within a strictly mandated time limit.

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Maximum Permitted Disconnection Times (BS 7671 Table 41.1)

Human physiological tolerance to electric shock is inversely proportional to duration. A shock voltage of $230\text{ V}$ sustained for several seconds is lethal, whereas disconnection within fractions of a second prevents fatal heart fibrillation. BS 7671 Table 41.1 specifies maximum permitted disconnection times for circuits operating at nominal AC voltage $U_0 = 230\text{ V}$ to earth:

Why TT Systems Demand Faster Disconnection ($0.2\text{ s}$ vs $0.4\text{ s}$)

A classic City & Guilds examination concept is understanding why TT systems require a faster disconnection time ($0.2\text{ s}$) than TN systems ($0.4\text{ s}$):

• In a TN system, the earth fault return path is formed by a low-resistance metallic conductor (cable sheath or PEN conductor). During an earth fault, the impedance of the line conductor ($R_1$) and the CPC ($R_2$) are roughly comparable. Consequently, the supply voltage ($230\text{ V}$) divides approximately equally across the loop, so the touch voltage ($V_t$) appearing on exposed metalwork is approximately $115\text{ V}$.
• In a TT system, the earth return path passes through the consumer's earth electrode ($R_A$) and the surrounding soil. The electrode resistance ($21\ \Omega$ to $100+\ \Omega$) dwarfs the sub-ohm resistance of the internal copper circuit cables. Under Ohm's voltage divider law, virtually the entire $230\text{ V}$ supply voltage appears across the exposed appliance casing as touch voltage.
• Because a touch voltage of $230\text{ V}$ triggers ventricular fibrillation far more rapidly than $115\text{ V}$, the disconnection time on TT systems is halved to $0.2\text{ seconds}$ to ensure life safety.

The Earth Fault Loop Impedance Path ($Z_s$)

The earth fault loop is the complete, closed conductive circuit traversed by fault current when a line-to-earth breakdown occurs at an electrical accessory or appliance.

The Fundamental $Z_s$ Formula

Total earth fault loop impedance ($Z_s$) is calculated using the fundamental equation: $Z_s = Z_e + (R_1 + R_2)$

Where:

• $Z_s$ (Total Earth Fault Loop Impedance): The total impedance of the fault path in ohms ($\Omega$) measured at the furthest, most vulnerable point of the circuit.
• $Z_e$ (External Earth Fault Loop Impedance): The impedance of the supply network outside the installation origin in ohms ($\Omega$), comprising the substation transformer secondary winding, the incoming DNO phase conductor, and the external earth return path (lead cable sheath in TN-S, PEN conductor in TN-C-S, or mass of earth in TT).
• $R_1$: The resistance of the line conductor from the distribution board to the furthest point of the circuit ($\Omega$).
• $R_2$: The resistance of the circuit protective conductor (CPC) from the distribution board to the furthest point ($\Omega$).
• $(R_1 + R_2)$: The combined internal circuit loop resistance ($\Omega$).

Calculating Earth Fault Current ($I_f$)

By Ohm's Law, the prospective earth fault current ($I_f$) flowing through the loop during a fault is determined by: $I_f = \frac{U_0}{Z_s}$ Where $U_0$ is the nominal line-to-earth AC voltage ($230\text{ V}$).

For an overcurrent device (such as an MCB or RCBO) to disconnect within $0.4\text{ seconds}$, the fault current $I_f$ must be large enough to activate the circuit breaker's instantaneous electromagnetic trip mechanism:

• Type B MCB (BS EN 60898): Instantaneous magnetic tripping occurs between $3 \times I_n$ and $5 \times I_n$. To guarantee tripping within $0.1\text{ s}$ to $0.4\text{ s}$, the current must reach $I_a = 5 \times I_n$.
• Type C MCB (BS EN 60898): Instantaneous magnetic tripping occurs between $5 \times I_n$ and $10 \times I_n$. Tripping requires $I_a = 10 \times I_n$.
• Type D MCB (BS EN 60898): Instantaneous magnetic tripping occurs between $10 \times I_n$ and $20 \times I_n$. Tripping requires $I_a = 20 \times I_n$.

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Maximum Tabulated $Z_s$ Values (BS 7671 Table 41.3)

Under BS 7671, the maximum permitted earth fault loop impedance for a circuit breaker is derived from the formula: $Z_s \le \frac{U_0 \times C_{\text{min}}}{I_a}$ Where:

• $U_0 = 230\text{ V}$.
• $C_{\text{min}} = 0.95$ is the minimum voltage factor BS 7671 uses to allow for variation in the supply voltage ($230\text{ V} \times 0.95 = 218.5\text{ V}$).
• $I_a$ is the current causing automatic operation of the protective device within specified time ($5 \times I_n$ for Type B, $10 \times I_n$ for Type C).

Example: Deriving Maximum $Z_s$ for a 32 A Type B MCB

$I_a = 5 \times 32\text{ A} = 160\text{ A}$ $Z_s \le \frac{230\text{ V} \times 0.95}{160\text{ A}} = \frac{218.5\text{ V}}{160\text{ A}} = \mathbf{1.3656\ \Omega \approx 1.37\ \Omega}$

BS 7671 Table 41.3 Maximum $Z_s$ for Type B Circuit Breakers at 0.4 s

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The 0.8 Rule-of-Thumb Temperature Correction Factor

A critical practical and examination requirement is applying the 0.8 temperature correction factor (a rule of thumb given in IET guidance such as the On-Site Guide and Guidance Note 3).

The Engineering Rationale

• The maximum $Z_s$ values published in BS 7671 Table 41.3 are calculated assuming that circuit conductors are operating at their maximum permitted conductor operating temperature—which is $70^\circ\text{C}$ for standard thermoplastic (PVC) insulated cables, or $90^\circ\text{C}$ for thermosetting (XLPE) cables.
• However, when an electrician conducts initial verification or periodic inspection on site, the installation is unenergized or lightly loaded, meaning the conductors are cold at ambient room temperature (standardized as $20^\circ\text{C}$).
• Because copper has a positive temperature coefficient of resistance ($\alpha = 0.00393 / ^\circ\text{C}$), a cold copper conductor has significantly lower resistance than a hot conductor operating at $70^\circ\text{C}$ under full load.
• If a measured $Z_s$ reading taken at $20^\circ\text{C}$ is barely below the tabulated maximum value, once the circuit is loaded to its full capacity and warms up to $70^\circ\text{C}$, the conductor resistance will rise, causing the true operating $Z_s$ to exceed the permitted limit and preventing disconnection within $0.4\text{ s}$!

Applying the Rule-of-Thumb Formula

To ensure compliance when testing at ambient temperatures, the measured earth fault loop impedance must not exceed $80\%$ ($0.8$) of the tabulated value in BS 7671: $Z_{s(\text{measured})} \le 0.8 \times Z_{s(\text{max tabulated})}$

Worked Example

A ring final circuit wired in $2.5\text{ mm}^2 / 1.5\text{ mm}^2$ cable is protected by a $32\text{ A}$ Type B MCB to BS EN 60898. During commissioning testing at $20^\circ\text{C}$:

• Measured external loop impedance: $Z_e = 0.28\ \Omega$.
• Measured circuit loop resistance: $(R_1 + R_2) = 0.76\ \Omega$.
• Total measured earth loop impedance: $Z_s = 0.28 + 0.76 = \mathbf{1.04\ \Omega}$.

Compliance Check:

1. From BS 7671 Table 41.3, maximum tabulated $Z_s$ for a $32\text{ A}$ Type B MCB = $1.37\ \Omega$.
2. Apply the 0.8 rule-of-thumb temperature correction factor: $Z_{s(\text{test limit})} = 1.37\ \Omega \times 0.8 = \mathbf{1.096\ \Omega \approx 1.10\ \Omega}$
3. Compare measured value against the 0.8 limit: $\text{Measured } Z_s\ (1.04\ \Omega) \le 1.10\ \Omega \implies \mathbf{COMPLIANT}$