5.1 Fundamentals of Electric Shock: Basic Protection and Fault Protection

5.1 Fundamentals of Electric Shock: Basic Protection and Fault Protection

Quick Summary: Electric shock occurs when electric current passes through the human body, causing physiological injury ranging from muscular paralysis to fatal cardiac ventricular fibrillation. Under Singapore Standard SS 638 (Code of Practice for Electrical Installations), protection against electric shock is structured into two fundamental safeguards: Basic Protection (preventing direct contact with live parts under normal conditions) and Fault Protection (preventing indirect contact with exposed conductive parts during insulation breakdown). The standard statutory touch voltage threshold limit is $U_L = 50\text{ V AC}$ for dry indoor locations, reduced to $U_L = 25\text{ V AC}$ for wet or conductive environments.

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1. Physiological Effects of Electric Shock & Human Body Impedance

Electric shock is the pathophysiological effect resulting from an electric current passing through human body tissue. When a person comes into simultaneous contact with two conductive points at different electrical potentials—such as a live phase conductor ($230\text{ V AC}$) and an earthed metallic structure or moist floor—an electrical circuit is completed through the body.

Factors Determining Severity of Shock

The physiological impact of electric shock depends on four primary parameters:

1. Magnitude of Current ($I$): Current density flowing through vital organs dictates cellular and muscular trauma.
2. Duration of Current Flow ($t$): The time exposure relative to the vulnerable T-wave phase of the cardiac cycle.
3. Path of Current Through the Body: Trajectories traversing the myocardium (heart muscle), brain stem, or spinal cord carry the highest risk of fatal complications.
4. Frequency of the Supply: Alternating current (AC) at $50\text{ Hz}$ (Singapore standard utility frequency) is three to five times more dangerous than direct current (DC) at identical voltages because $50\text{ Hz}$ AC severely interferes with human nerve transmission and cardiac pacemaker signals.

                    [ ELECTRIC SHOCK PATHWAYS ]

     Hand-to-Hand Path                 Hand-to-Foot Path
 (Crosses Chest / Heart)             (Traverses Myocardium & Legs)

       [Live Phase]                       [Live Phase]
            |                                  |
            v                                  v
     Left Hand -> Chest -> Right Hand    Left Hand -> Heart -> Feet
            |                                  |
            v                                  v
     [Earthed Metal]                     [Earthed Floor]

Current Threshold Effects at 50 Hz AC (rms)

Based on IEC 60479-1 and SS 638 guidelines, the biological response of a healthy adult to $50\text{ Hz}$ AC current is categorized into distinct threshold zones:

Human Body Impedance ($Z_b$) Parameters

The human body acts as a complex non-linear impedance network consisting of internal tissue resistance (blood vessels, muscle tissue, internal organs) in series with skin contact resistance at entry and exit sites:

• Internal Tissue Resistance: Relatively constant, ranging between $500\ \Omega$ and $800\ \Omega$.
• Skin Resistance: Highly variable depending on moisture, sweat, contact surface area, applied voltage, and skin pressure. Dry, unbroken skin offers $2000\ \Omega$ to $> 5000\ \Omega$ at low voltages.

Under $230\text{ V AC}$ contact conditions or when skin is moist with perspiration, the skin's outer stratum corneum layer experiences electrical breakdown (electroporation). Under SS 638 design criteria, total human body impedance is conservatively standardized as:

$R_b = 1000\ \Omega$

Applying Ohm's Law ($I_b = \frac{U}{R_b}$), touching a live $230\text{ V}$ phase conductor yields an internal body current of:

$I_b = \frac{230\text{ V}}{1000\ \Omega} = 0.230\text{ A} = 230\text{ mA}$

Because $230\text{ mA}$ is nearly five times the $50\text{ mA}$ ventricular fibrillation threshold, accidental direct contact with $230\text{ V}$ AC is almost instantly lethal unless automatic protective devices disconnect the supply within milliseconds.

Statutory Touch Voltage Threshold Limits ($U_L$)

To prevent dangerous body currents, SS 638 defines maximum safe conventional touch voltage limits ($U_L$):

• Standard Dry Indoor Environments: $U_L = 50\text{ V AC}$ (rms) or $120\text{ V}$ ripple-free DC. At $50\text{ V AC}$ across $1000\ \Omega$, the body current is $I_b = 50\text{ mA}$, representing the upper safe limit before deterministic cardiac fibrillation occurs for brief exposures.
• Wet, Damp, or Highly Conductive Locations: $U_L = 25\text{ V AC}$ (rms) or $60\text{ V}$ ripple-free DC. Enforced in hazardous locations such as construction sites, agricultural units, and outdoor damp areas where skin resistance is permanently compromised.
• Submerged Locations (Swimming Pools / Fountains): $U_L = 12\text{ V AC}$ (rms) SELV under SS 638 Section 702.

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2. Basic Protection (Protection Against Direct Contact)

Basic Protection (formerly termed Protection Against Direct Contact) comprises all protective measures intended to prevent persons from touching live, uninsulated electrical parts under normal operational conditions.

+-------------------------------------------------------------------------+
|                         BASIC PROTECTION MEASURES                       |
|                  (Protection Against Direct Contact)                    |
+-------------------------------------------------------------------------+
         |                                           |
         v                                           v
+-----------------------------------+     +-------------------------------+
|    INSULATION OF LIVE PARTS       |     |    BARRIERS & ENCLOSURES      |
| - Permanent, non-removable cover  |     | - Top surface: min IP4X/IPXXD  |
| - Withstands electrical/thermal   |     | - Side surfaces: min IP2X/IPXXB|
|   stresses without breakdown      |     | - Requires tool/key to open   |
+-----------------------------------+     +-------------------------------+

Key Basic Protection Mechanisms under SS 638

1. Basic Insulation of Live Parts: Live conductors must be completely covered with solid insulation that can only be removed by destruction (e.g. PVC, XLPE, EPR cable sheathings). Paints, varnishes, lacquers, and enamels do not meet the standard for basic insulation.
2. Barriers and Enclosures: Live parts must be housed inside switchboards, distribution boards, trunking, or conduit boxes providing defined Ingress Protection (IP) ratings:
   • General Accessible Surfaces: Minimum IP2X or IPXXB (prevents insertion of a standard jointed test finger of $12\text{ mm}$ diameter and $80\text{ mm}$ length).
   • Top Horizontal Surfaces of Enclosures: Minimum IP4X or IPXXD (prevents insertion of a $1.0\text{ mm}$ diameter rigid wire or tool).
   • Access Control: Enclosure doors must require a key or tool (e.g. panel key, screwdriver) to open, or feature an interlocked switch disconnector that automatically de-energizes internal live parts prior to panel opening.
3. Obstacles: Placing physical guardrails or mesh screens to prevent inadvertent contact in specialized industrial sub-stations accessible only to authorized Licensed Electrical Workers (LEWs).
4. Placing Out of Reach: Positioning bare overhead conductors out of normal reach. Accessible standing surfaces must maintain a vertical clearance of at least $2.5\text{ m}$ or a horizontal clearance of $1.25\text{ m}$ from uninsulated live parts.

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3. Fault Protection (Protection Against Indirect Contact)

Fault Protection (formerly termed Protection Against Indirect Contact) comprises protective measures designed to protect persons against electric shock when touching exposed conductive parts (e.g. metallic enclosures of washing machines, distribution board casings, motor frames, steel conduits) that have become energized due to an internal insulation breakdown.

  [ PHASE CONDUCTOR FAULT TO METALLIC ENCLOSURE (INDIRECT CONTACT) ]

       Live Phase (230V)
            |
            +---> [Insulation Breakdown]
                        |
                        v
          +---------------------------+
          | Exposed Conductive Frame  | <--- Person touches frame!
          +---------------------------+
                        |
                        v
           Fault Current Flow (If)
                        |
                        +---> Protective Earthing Conductor (CPC)
                        |     (Triggers Automatic Disconnection)
                        |
                        +---> Human Body (Shock hazard if ADS fails!)

Primary Fault Protection Strategy: Automatic Disconnection of Supply (ADS)

Automatic Disconnection of Supply (ADS) is the universal fault protection measure deployed across Singapore LV installations. ADS relies on three mutually dependent architectural provisions:

1. Protective Earthing: Connecting all exposed conductive parts of electrical equipment to the Main Earthing Terminal (MET) via Circuit Protective Conductors (CPCs).
2. Protective Equipotential Bonding: Interconnecting metallic building services (water pipes, structural steel) to MET to maintain near-zero potential differences across metalwork.
3. Automatic Disconnection via Protective Devices: Deploying overcurrent protective devices (MCBs, MCCBs, fuses) or Residual Current Devices (RCDs) configured to isolate the fault automatically within statutory time limits.

Statutory Disconnection Time Limits under SS 638 (Table 41.1)

To limit touch duration before ventricular fibrillation occurs, SS 638 mandates maximum automatic disconnection times based on supply voltage ($U_0 = 230\text{ V AC}$ nominal phase-to-earth) and circuit classification:

Alternative Fault Protection Methods (Without ADS)

• Class II Equipment (Double or Reinforced Insulation): Construction featuring non-conductive supplementary insulation or reinforced single insulation. Symbolized by concentric squares ($\square$), Class II appliances do not require an earth connection.
• Electrical Separation: Supplying a single load circuit via an ungrounded isolating transformer ($230\text{ V} : 230\text{ V}$). An earth fault on the secondary side cannot complete a path back to neutral, preventing shock.
• Extra-Low Voltage Systems (SELV & PELV): Safety Extra-Low Voltage (SELV) and Protective Extra-Low Voltage (PELV) operating at nominal voltages $\le 50\text{ V AC}$ rms or $120\text{ V}$ DC.