11.1 Physiological Effects of Current & Shock Factors

Why Shock Physiology Matters on the Regulations Exam

Safe isolation and personal safety are not only procedural slogans. They rest on a clear physical fact: electric current through the human body can injure or kill, and the outcome depends on more than “was it 230 V?” EWRB candidates are expected to reason about why isolation, RCDs, earthing, barriers, and test-before-touch exist—not merely recite device names.

IEC/TS 60479 (effects of current on human beings and livestock) is the international reference often cited for qualitative awareness of thresholds. You are not expected to memorise full IEC tables for the open-book regulations exam, but you must hold the order of severity and the factors that move a person from a mild tingle to a life-threatening arrhythmia.

Physiological Effects: Perception → Let-Go → Fibrillation

As residual or contact current through the body increases (and as time under that current lengthens), typical progressive effects include:

Key teaching points:

• Current and time drive risk. A lower current for a longer duration can still be dangerous; a higher current for a short time can also kill. Design and regulations therefore attack both residual magnitude and disconnection time.
• Ventricular fibrillation is the classic life-threatening cardiac effect associated with AC shock in the household-frequency region. That is why fast clearance of residual-current faults is treated as human-safety engineering, not a convenience feature.
• Effects are path-dependent. Current across the chest (e.g. hand-to-hand or hand-to-foot involving the torso) is more hazardous than a path that does not involve the heart region.

Do not invent precise milliamp “exam magic numbers” beyond what your permitted materials support. Use qualitative IEC/TS 60479 awareness: more current + worse path + longer time = worse outcome.

Factors That Determine Shock Severity

Voltage

Voltage is the driving force that pushes current through body impedance. Extra-low voltage reduces risk but does not make every ELV situation “safe to ignore”—wet conditions, damaged equipment, and secondary sources still matter. Low-voltage mains (e.g. 230/400 V systems) are fully capable of causing lethal shock under ordinary site conditions.

Current path through the body

• Hand-to-hand or hand-to-foot paths that cross the chest raise cardiac risk.
• Contact that includes earth (active–earth) or neutral (active–neutral) while the person completes the circuit is classic hazardous contact.
• Standing on a conductive, earthed floor or wet concrete can complete a path even when the second hand is free.

Duration of contact

Longer contact increases energy delivered and the chance of fibrillation or burns. Protective devices that disconnect quickly (overcurrent for high fault currents; RCDs for residual currents) exist to cut duration. A person who cannot “let go” effectively extends duration involuntarily.

Wet or broken skin

Dry intact skin has relatively high resistance. Wet skin, sweat, cuts, or abrasions lower contact resistance so the same voltage drives more body current. Damp situations, bathrooms, outdoor work, and construction sites are higher-risk environments for that reason—not only because of water near equipment, but because body impedance falls.

Frequency

Power-frequency AC (50 Hz in NZ) is in a range where cardiac effects are a serious concern. DC and other frequencies have different physiological profiles; for regulations-exam purposes, treat mains AC shock as the default high-yield scenario and do not assume “it is only DC so it is harmless.”

Simultaneous Active–Neutral or Active–Earth Contact

A frequent exam and site trap is focusing only on “one wire touched.” The body becomes dangerous when it bridges potentials:

• Active to neutral: the body is across the supply voltage of the circuit.
• Active to earth / exposed earthed metal: the body provides a residual path; earthing and RCD behaviour become critical.
• Touching an active while leaning on an earthed switchboard frame, pipe, or wet floor is a classic combined path.

Simultaneous contact is why barriers, IP integrity, insulated tools, one-hand techniques where appropriate, and isolation before work matter. Never assume neutral or “earth metal” is a free place to rest your other hand.

Peak Voltages and Insulation Considerations

Supply systems include peaks above RMS values, and switching or inductive events can produce higher transient stresses. From a personal-safety angle:

• Insulation systems are designed and selected for rated voltages and conditions; damaged, aged, or wrong-rated insulation is a direct path to contact.
• Unknown or temporary wiring, extension leads, and opened equipment raise the chance of exposed live parts at full system potential.
• Treat conductors and terminals as live until proven dead by a competent prove–test–prove process (next section). Peak and RMS debates do not replace isolation discipline.

Why 30 mA RCDs and Fast Disconnection Matter

NZ residential and similar practice under AS/NZS 3000 centres 30 mA residual operating current as personnel shock protection on final subcircuits (sockets, lighting, and related contexts in your edition). ESR criteria and Wiring Rules disconnection requirements also stress time (e.g. classic RCD performance numbers such as 300 ms at IΔn and 40 ms at 5× IΔn, and ADS disconnection times such as 0.4 s / 5 s in their proper contexts).

Map physiology to hardware:

RCDs are additional protection. They do not replace isolation, earthing, or insulated construction. But when residual current flows through a person, a correctly selected and functioning 30 mA device with fast trip is often the difference between a survivable event and a fatal one.

Exam habits

1. Prefer answers that combine path + duration + residual magnitude, not voltage alone.
2. Link wet/broken skin and active–earth contact to higher body current, not “admin risk.”
3. Defend 30 mA + fast disconnection as human-safety design, consistent with RCD and ADS chapters.

Study note

Flag physiological factors next to your RCD and isolation notes. On stems about “why isolate” or “why RCD,” answer with body current, path, and time—then name the control (prove–test–prove, LOTO, 30 mA RCD, ADS).