Human Electrical Impedance: A Technical Review of Variability, Determinants, and Safety
Implications Based on IEC 60479-1 by Bob Preston
Quick Navigation
Human electrical resistance is not a fixed value. IEC 60479-1 provides experimentally derived impedance data demonstrating that total body impedance varies with voltage, moisture, contact area, pressure, age, and current path.
Under certain conditions, particularly damp or large-area contact, touch voltages as low as 5– 15 V can produce currents within physiologically hazardous ranges.
This paper summarises the relevant impedance data, physiological eƯ ect zones, and the implications for low-voltage shock risk.
Introduction
IEC 60479-1 EƯ ects of current on human beings and livestock – Part 1: General aspects is the principal international standard governing the scientific understanding of how electric current interacts with the human body. It provides the foundational framework used by electrical engineers, safety regulators, medical researchers, and standards bodies to quantify shock risk and establish protective measures. The document consolidates decades of experimental work, including controlled studies on living subjects and impedance measurements performed on cadavers, to produce statistically robust models of human electrical impedance andphysiological response.
A central contribution of IEC 60479-1 is its detailed characterisation of the components of human impedance, skin impedance, internal impedance, and total body impedance and the factors that influence them. These include moisture, contact area, mechanical pressure, temperature, applied voltage, and the anatomical path of current flow.
By presenting impedance values across a range of realistic contact conditions, the standard demonstrates that human electrical resistance is highly variable rather than a single fixed number.
In addition to impedance data, IEC 60479-1 defines the internationally recognised time/current eƯ ect zones (AC-1 through AC-4), which describe the probability of physiological outcomes such as involuntary muscular contraction, loss of voluntary control (“let-go”), respiratory interference, and ventricular fibrillation.
These zones form the scientific basis for modern electrical safety requirements, including the designation of 50 V AC as a hazardous voltage, the use of 30 mA residual-current protection devices, and the development of safe-touch voltage limits in power distribution systems.
By integrating empirical impedance measurements with physiological response thresholds, IEC 60479-1 provides a comprehensive and quantitative framework for assessing electric shock risk across a wide range of voltages and environmental conditions. This makes it the authoritative reference for understanding how even relatively low touch voltages can produce harmful or potentially fatal currents when human impedance is reduced by moisture, pressure, or large-area contact with conductive surfaces.
Impedance Components Defined in IEC 60479-1
Internal Impedance (Zi)
IEC 60479-1 identifies internal body impedance, comprising muscle, blood, and internal tissue as relatively stable across individuals.
- Typical value: ~300 Ω at 50/60 Hz.
Skin Impedance (ZS)
Skin impedance is the dominant variable component of total body impedance.
IEC 60479-1 identifies the following influencing factors:
- Moisture leve
- Contact area
- Contact pressure
- Temperature
- Age (via skin thickness)
- Applied voltage
Skin impedance decreases significantly with increased moisture, increased contact area, and increased pressure.
Total Body Impedance (ZT)
Total body impedance is the sum of internal and skin components. IEC 60479-1 provides statistical values for diƯ erent contact areas and conditions at 50/60 Hz.
Statistical Impedance Ranges (IEC 60479-1)
Large Contact Area (e.g., whole hand on radiator, pipe, tap)
- Dry: 1𝑘Ω − 3𝑘Ω
- Damp/sweaty: 500Ω − 1𝑘Ω
- Wet / high pressure: 300Ω − 500Ω
Medium Contact Area (e.g., palm, firm grip)
- Dry: 2𝑘Ω − 5𝑘Ω
- Damp: 1𝑘Ω − 2𝑘Ω
- Wet: 500Ω − 1𝑘Ω
Small Contact Area (e.g., fingertip)
- Dry: 5𝑘Ω − 10𝑘Ω
- Damp: 2𝑘Ω − 5𝑘Ω
- Wet: 1𝑘Ω − 2𝑘Ω
These values reflect the statistical ranges published in IEC 60479-1.
Voltage Dependence of Impedance
IEC 60479-1 emphasises that human impedance is non-linear with respect to applied voltage:
- At low voltages, skin impedance dominates, producing higher total impedance.
- At higher voltages, skin impedance decreases due to dielectric breakdown, and total impedance approaches the internal impedance (300Ω).
This non-linearity explains why low touch voltages can produce hazardous currents under wet or high-pressure contact conditions.
Physiological EƯ ect Zones (IEC 60479-1)
IEC 60479-1 defines time/current zones for 50/60 Hz AC:
- AC-2: Involuntary muscular reactions
- AC-3: “Let-go” threshold (~10–15 mA)
- AC-4: Increasing probability of ventricular fibrillation, particularly above ~30–50 mA
These zones form the basis for widely adopted safety thresholds, including:
- 50 V AC as a dangerous voltage
- 30 mA residual-current protection
- 10–15 mA as a conservative intervention threshold
Touch Voltage and Resulting Current: Examples Using IEC Impedance Values
Dry, small-area contact (5𝑘Ω typical)
- 50 V → 10 mA
- 75 V → 15 mA
- 150 V → 30 mA
Damp/sweaty hand, large-area contact (1𝑘Ω typical)
- 10 V → 10 mA
- 15 V → 15 mA
- 30 V → 30 mA
Damp/Wet or skin-broken contact (300Ω − 500Ω typical)
- 3–5 V → 10 mA
- 5–8 V → 15 mA
- 10–15 V → 30 mA
These values follow directly from Ohm’s law applied to IEC 60479-1 impedance ranges.
Age-Related Variation
IEC 60479-1 identifies skin thickness, moisture, temperature, contact area, and contact pressure as primary determinants of human electrical impedance.
These factors influence the skin component of total body impedance, which is the most variable part of the human electrical pathway.
Because IEC 60479-1 lists skin thickness and moisture as determinants of impedance, age-related diƯ erences in skin structure may influence shock current for a given touch voltage.
Children typically have thinner skin and higher skin moisture, while older adults often exhibit age-related thinning of the epidermis.
Both conditions can reduce skin impedance under pressure or moisture.
Reduced skin impedance decreases total body impedance, meaning that children and older adults may experience higher current for the same touch voltage compared with a typical healthy adult.
This inference follows directly from the factors identified in IEC 60479-1 but is not presented as a separate age-specific table within the standard.
Contact Pressure as Defined in IEC 60479-1
IEC 60479-1 lists pressure as a key factor aƯ ecting impedance.
In this context, pressure refers to mechanical compression of the skin against a conductive surface
- Low pressure: Light fingertip touch; minimal skin deformation; higher impedance
- Medium pressure: Normal grip; moderate skin flattening; reduced impedance
- High pressure: Firm grip or body weight; increased contact area; moisture expression; impedance approaching 300Ω − 500Ω
IEC’s lowest impedance category corresponds to large-area, high-pressure contact.
Real-World Examples of High-Pressure, Large-Area Contact
Examples consistent with IEC’s “large surface area” category include:
-
Leaning both hands on a metal sink while washing dishes
(Large wet contact area; continuous pressure; low impedance conditions.) -
Resting a forearm and palm on a metal towel rail after a shower
(Moisture + large surface area + moderate pressure.) -
Holding a metal shower valve or mixer tap with a full grip
(Wet conditions; high pressure; full-hand contact.) -
Leaning chest, arms, or torso against metal ducting or pipework
(Very large contact area; sustained pressure.) -
Holding a metal tool cabinet or machine frame while standing on a conductive surface
(Palm and forearm contact; pressure increases contact area.) -
Bracing yourself with both hands on a metal surface while slipping or losing balance
(Sudden high pressure; large-area contact.) -
Kneeling on a damp floor while touching a metal appliance casing
(Large-area contact at both hands and knees; moisture reduces impedance.) -
Leaning your full body weight on a metal appliance while reaching or stretching
(Large-area, high-pressure contact.)
These conditions can reduce impedance to the lower end of IEC ranges. All of these scenarios involve one or more IEC-listed factors that reduce impedance:
- Large contact area (palm, forearm, torso)
- High mechanical pressure (gripping, leaning, supporting body weight)
- Moisture (sweat, water, condensation, damp clothing)
- Low insulation (bare feet, wet floors, conductive surfaces)
Under these conditions, total body impedance can fall into the 300–500 Ω range documented in IEC 60479-1, meaning that touch voltages as low as 5–15 V can produce currents in the AC-3 and AC-4 physiological eƯ ect zones.
Low-Voltage Hazard Scenarios and Diverted Neutral Currents
IEC 60479-1 does not address PME diverted neutral currents directly. However, the impedance data allow quantitative assessment:
- If metalwork rises to 5–15 V above true earth, a moist or high-pressure contact can produce 10–15 mA.
- If metalwork rises to 20–30 V, currents may exceed 30 mA, entering the AC-4 fibrillation-risk zone.
This is a significant safety defect.
Expanded Human Impedance and Touch-Voltage Table
| Condition | IEC Impedance Range | Touch Voltage for 10 mA | Touch Voltage for 15 mA | Notes |
|---|---|---|---|---|
| Dry, small contact (fingertip) | 5–10 kΩ | 50–100 V | 75–150 V | Best-case scenario; high variability |
| Dry, medium contact (palm) | 2–5 kΩ | 20–50 V | 30–75 V | More realistic than fingertip contact |
| Dry, large contact (whole hand) | 1–3 kΩ | 10–30 V | 15–45 V | Common when gripping radiators, taps |
| Damp/sweaty hand (medium area) | 1–2 kΩ | 10–20 V | 15–30 V | Typical in warm domestic environments |
| Damp, large-area contact (radiator, pipe) | 500–1,000 Ω | 5–10 V | 7.5–15 V | Very common in kitchens/bathrooms |
| Wet hand (medium area) | 500–1,000 Ω | 5–10 V | 7.5–15 V | Skin barrier partially compromised |
| Wet, large-area contact (full hand on metal) | 300–500 Ω | 3–5 V | 5–8 V | Matches IEC’s lowest impedance category |
| High-pressure contact (leaning, gripping hard) | 300–500 Ω | 3–5 V | 5–8 V | Pressure increases contact area; impedance collapses |
| Skin-broken / internal contact | ~300 Ω | 3 V | 4.5 V | Represents internal impedance only |
| Child (inference: thinner skin, higher moisture) | 500–2,000 Ω | 5–20 V | 7.5–30 V | Based on IEC factors; not a separate IEC table |
| Older adult (inference: thinner epidermis) | 500–2,000 Ω | 5–20 V | 7.5–30 V | Same reasoning as above; IEC lists factors only |
| Barefoot on conductive floor + hand contact | 500–1,500 Ω (hand-to-foot path) | 5–15 V | 7.5–22.5 V | Common in bathrooms, kitchens, gardens |
| Wet clothing contact (forearm, torso) | 300–800 Ω | 3–8 V | 5–12 V | Large-area + moisture = low impedance |
| Two-hand contact on metalwork (both palms) | 500–1,500 Ω | 5–15 V | 7.5–22.5 V | Hand-to-hand path; high fibrillation relevance |
| Hand on metal + knee on damp floor | 300–1,000 Ω | 3–10 V | 5–15 V | Very common during appliance work |
| Sweaty hand + metal tool handle | 1–2 kΩ | 10–20 V | 15–30 V | Tool handles often increase pressure |
| Condensation-cooled metal surface | 500–1,000 Ω | 5–10 V | 7.5–15 V | Cold metal increases moisture at skin interface |
The 15 mA region is reached at surprisingly low voltages.
Under realistic domestic conditions (damp hands, large contact area, moderate pressure):
- 15 mA occurs at 7.5–15 V
- This is well within the range of diverted neutral faults on PME systems.
The “dry fingertip” scenario is the least relevant in real homes. IEC’s high-impedance values (5– 10 kΩ) apply only to:
- small contact area
- dry skin
- low pressure
These conditions rarely occur when touching radiators, taps, boilers, or pipework.
Large-area or high-pressure contact collapses impedance. IEC’s lowest impedance category (300–500 Ω) is triggered by:
- full-hand contact
- leaning or gripping
- moisture or sweat
- cold metal surfaces
This is why 3–8 V can produce 10–15 mA.
Children and older adults may have lower skin impedance. IEC lists the factors (skin thickness, moisture, pressure) but does not provide age tables.
Applying those factors:
- Children: thinner skin + higher moisture → lower impedance
- Older adults: age-related thinning → lower impedance
Both groups can reach hazardous currents at lower voltages.
Conclusion
IEC 60479-1 provides clear evidence that human electrical impedance is not a fixed quantity but a variable physiological parameter influenced by multiple interacting factors, including moisture, contact area, mechanical pressure, applied voltage, and the anatomical current path.
The standard’s impedance data demonstrate that the skin component of total body impedance is highly sensitive to environmental and contact conditions, while internal impedance remainscomparatively stable.
As a result, total body impedance can vary by more than an order of magnitude between dry, low-pressure fingertip contact and wet, high-pressure, large-area contact.
The non-linear voltage dependence described in IEC 60479-1 further shows that impedance decreases as voltage increases, particularly once the skin barrier begins to break down. Under conditions involving moisture, sweat, condensation, or firm contact with conductive surfaces, total impedance can fall into the 300–500 Ω range documented in the standard.
In this impedance band, touch voltages as low as 5–15 V are suƯ icient to produce currents of 10–15 mA, placing the exposure within the AC-3 “let-go” region and approaching the AC-4 ventricular fibrillation risk zone at higher voltages or longer durations.
These findings have direct implications for low-voltage shock risk assessment.
They indicate that hazardous physiological eƯ ects are not limited to nominal mains voltages but can occur at significantly lower touch voltages when environmental and contact conditions reduce human impedance, including situations where a conductor or main earth is assumed to be “dead” but is elevated in potential due to diverted neutral currents, bonding faults, or other rise-of-earth-potential conditions.
This is particularly relevant in domestic and industrial settings where conductive metalwork, such as radiators, pipework, taps, boiler casings, or appliance enclosures, may become energised due to diverted neutral currents, bonding faults, or other low-voltage rise-of-potential scenarios.
The impedance ranges and physiological eƯ ect thresholds presented in IEC 60479-1 therefore support the need for systematic evaluation of low-voltage touch potentials, especially in environments where moisture, large-area contact, or body-weight pressure are likely.
They also reinforce the rationale behind conservative intervention thresholds used in electrical safety practice, including the treatment of currents above 10-15mA as potentially dangerous under certain conditions.
Overall, IEC 60479-1 demonstrates that accurate assessment of human impedance and realistic contact scenarios is essential for preventing harmful or potentially fatal electric shockoutcomes, even at voltages traditionally considered low.