3.2 Electrical Hazards & Electric Shock Physiology

3.2 Electrical Hazards & Electric Shock Physiology

Electric shock occurs when an electric current passes through the human body. The human body is an electrochemical system; its nervous system, muscular contractions, and cardiac rhythm operate on minute milli-volt electrical impulses. When an external electrical voltage is applied across the body, it overrides these internal bioelectric signals, leading to muscular spasms, respiratory failure, cardiac arrest, and severe thermal burns. Understanding the exact biophysical mechanisms of electric shock is fundamental to designing protective systems in accordance with BS 7671 (The IET Wiring Regulations) and preserving life on site.

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The Physics of Electric Shock: Voltage, Resistance, and Current

The severity of an electric shock is determined not primarily by voltage alone, but by the magnitude of the current ($I$) flowing through the vital organs, the pathway it takes across the body, and the duration of contact ($t$). In accordance with Ohm's Law:

$I = \frac{U}{Z_{body}}$

Where:

• $I$ is the electric current flowing through the casualty (in Amperes, A)
• $U$ is the touch voltage applied across contact points (in Volts, V)
• $Z_{body}$ is the total electrical impedance of the human body pathway (in Ohms, $\Omega$)

Touch Voltage Thresholds

In BS 7671, the conventional safe touch voltage limit ($U_L$) is defined as 50 V AC (or 120 V ripple-free DC) for dry indoor environments. Under normal dry conditions, human skin resistance is sufficiently high that a potential difference of 50 V AC will not drive a lethal current across the thoracic cavity. However, in harsh, damp, or agricultural environments, the touch voltage limit is reduced to 25 V AC.

Human Body Impedance Components

Human body impedance comprises two elements in series:

1. Skin Contact Resistance: The stratum corneum (the thin, dry, non-living outer layer of the skin) provides the majority of the body's electrical resistance. When clean, dry, and calloused, skin resistance can range from 10,000 $\Omega$ to over 100,000 $\Omega$. However, if the skin becomes damp, wet, or sweat-soaked, skin resistance collapses to 1,000 $\Omega$ or less, because perspiration contains dissolved ionic salts that conduct electricity readily.
2. Internal Body Resistance: Internal fluids, blood vessels, muscles, and organs have high water and electrolyte concentrations, presenting a low internal resistance of approximately 500 $\Omega$ to 1,000 $\Omega$ between extremities (hand to hand, or hand to foot).

   Equivalent Electrical Model of the Human Body
   ┌──────────────────────────────────────────────────────────────┐
   │                                                              │
   │   Contact Point A (Hand)                                    │
   │        │                                                     │
   │      ┌─┴─┐                                                   │
   │      │   │ Skin Resistance A (Dry: 10k-100kΩ / Wet: <1kΩ)   │
   │      └─┬─┘                                                   │
   │        │                                                     │
   │      ┌─┴─┐                                                   │
   │      │   │ Internal Tissue Resistance (~500 - 1000 Ω)        │
   │      └─┬─┘                                                   │
   │        │                                                     │
   │      ┌─┴─┐                                                   │
   │      │   │ Skin Resistance B (Foot / Earth Contact)         │
   │      └─┬─┘                                                   │
   │        │                                                     │
   │   Contact Point B (Earth / Neutral Return)                   │
   └──────────────────────────────────────────────────────────────┘

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Physiological Effects of 50 Hz Alternating Current

Alternating current at 50 Hz (the UK standard grid frequency) is particularly dangerous to human physiology. The 50 Hz cyclic alternation coincides directly with the natural vulnerability window of the cardiac cycle (the vulnerable T-phase of the electrocardiogram). As current increases, the physiological impact progresses rapidly from minor tingling to immediate lethality.

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Direct Contact vs Indirect Contact (BS 7671 Terminology)

In electrotechnical theory and BS 7671, the terms direct contact and indirect contact define how a person touches a dangerous potential:

1. Direct Contact (Basic Protection)

Direct contact is the contact of persons or livestock with live parts (conductors or conductive parts intended to be electrically energized in normal service).

• Examples: Touching an uninsulated copper terminal inside a consumer unit, a cracked light switch exposing live screws, or a cut in a flexible extension lead.
• Protective Measures (Basic Protection): Insulation of live parts, barriers or enclosures providing an ingress protection rating of at least IP2X or IPXXB (preventing ingress of a standard test finger).

2. Indirect Contact (Fault Protection)

Indirect contact is the contact of persons or livestock with exposed-conductive-parts which have become live under fault conditions.

• Examples: Touching the metal casing of an electric cooker, motor, or metallic conduit system that has become energized because internal basic insulation failed and a line conductor touched the casing.
• Protective Measures (Fault Protection): Automatic Disconnection of Supply (ADS) achieved via circuit protective conductors (CPC), protective equipotential bonding, earthing systems (TN-S, TN-C-S, TT), overcurrent protective devices (MCBs, fuses), and RCDs.

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High Voltage, Arc Flash, Arc Eye & Electrical Burns

Beyond direct shock current, electrical hazards present devastating thermal and blast risks:

High Voltage (>1000 V AC or >1500 V DC)

High voltage systems can jump across air gaps without direct physical contact (known as flashover or dielectric breakdown). Electricians must observe strict safety working distances and never approach uninsulated high-voltage conductors.

Arc Flash and Arc Blast

An arc flash is an explosive electrical discharge through ionized air resulting from a short circuit (e.g., dropping a metallic tool across live three-phase busbars). Temperatures in an arc flash can exceed 20,000 °C (four times hotter than the surface of the sun). This instantaneous thermal release vaporises copper conductors, expanding them up to 67,000 times in volume and generating an arc blast pressure wave that can collapse lungs, rupture eardrums, and propel shrapnel at ballistic speeds.

Arc Eye (Photokeratitis)

Arc eye is an acute, painful superficial burn of the cornea caused by intense ultraviolet (UV) radiation emitted during electrical arcing or welding. Symptoms typically develop 6 to 12 hours after exposure, including severe burning pain, extreme photophobia (light sensitivity), excessive tearing, and a feeling like sandpaper under the eyelids. Appropriate UV-filtering goggles or arc-rated face shields must always be worn.

Electrical Burns

When high current passes through tissue, electrical energy converts to thermal energy in accordance with Joule's Law ($Q = I^2 R t$). Electrical burns produce distinct entrance and exit wounds with extensive, unseen deep muscle and tissue necrosis beneath unblemished skin.

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Emergency Electric Shock Rescue & First Aid Procedures

When discovering an electric shock victim on site, every second counts, but hasty action without personal protection can create multiple fatalities.

   Emergency Electric Shock Rescue Protocol
   ┌────────────────────────────────────────────────────────┐
   │  1. ASSESS DANGER: Do NOT Touch Casualty Directly!     │
   │                                                        │
   │  2. BREAK CIRCUIT: Switch Off Isolator / Pull Plug     │
   │     (If not possible: Push clear with DRY wooden pole) │
   │                                                        │
   │  3. CALL 999 / 112: Request Ambulance & AED            │
   │                                                        │
   │  4. CHECK BREATHING:                                   │
   │     ├── Normal: Recovery Position                      │
   │     └── Absent/Abnormal: Start CPR (30:2) + Apply AED │
   └────────────────────────────────────────────────────────┘

Phase 1: Safe Separation

• NEVER touch the casualty with bare hands while they are still in contact with the electrical supply. The current will flow directly through your body, causing muscle tetany and incapacitating you.
• Isolate the supply immediately: Turn off the main switch, trip the circuit breaker, or pull the plug from the socket.
• If isolation is impossible: Stand on dry insulating material (rubber mat or dry wooden plank) and use a clean, dry, non-conductive object (such as a wooden broom handle, dry timber battens, or a specialized fibreglass rescue hook) to push the casualty's limbs clear of the live conductor, or loop a dry belt around their foot to drag them away.

Phase 2: Medical Assessment & Resuscitation

• Call 999 or 112 immediately. State that the incident is an electrical shock casualty and request an ambulance with an Automated External Defibrillator (AED).
• Primary Survey (DR ABC):
  • Danger: Ensure the area is safe and power is off.
  • Response: Gently shake shoulders and ask loudly: "Are you alright?"
  • Airway: Tilt the head back and lift the chin to open the airway.
  • Breathing: Look, listen, and feel for normal breathing for no more than 10 seconds.
• Cardiopulmonary Resuscitation (CPR): If the casualty is unresponsive and not breathing normally:
  • Place the heel of one hand in the centre of the chest, interlock the other hand on top, and deliver 30 chest compressions at a rate of 100–120 compressions per minute to a depth of 5–6 cm.
  • Deliver 2 rescue breaths.
  • Continue cycles of 30 compressions to 2 breaths without interruption.
• Deploy the AED: Switch on the AED as soon as it arrives. Attach the self-adhesive defibrillator pads to the casualty's bare chest as illustrated on the packaging. Follow the automated voice prompts. If the device detects ventricular fibrillation, ensure nobody touches the casualty and press the shock button.
• Treatment of Burns: Once resuscitation is achieved or if the patient is conscious, cool electrical burns immediately with clean, cold running water for at least 20 minutes. Cover the burns loosely with sterile non-adherent dressings or clean plastic cling film. Never apply butter, ointments, or burst blisters.