Touch and step
Permissible touch voltage UTp vs fault duration (BS EN 50522)
- where
- permissible touch voltage (V)
- tabulated curve of UTp against fault duration
- fault duration (s)
- EPR limit for compliance condition C2 (V)
- EPR limit for condition C3 with specified measures (V)
Explain this equation
Gives the BS EN 50522 permissible touch voltage for a fault duration, with the EPR levels at which the standard accepts the design without further touch voltage checks.
UTp comes from the curve tabulated in BS EN 50522 Table B.3, which assumes bare hand-to-feet contact, the body impedance exceeded by 95 % of people and the c2 fibrillation curve of IEC 60479-1. The calculator interpolates between table points on log scales. 2UTp and 4UTp are the EPR limits for compliance conditions C2 and C3 in clause 5.4.2.
The limit falls steeply between 0.2 s and 1 s because the heart's vulnerable period makes short shocks much less dangerous. Beyond a few seconds the curve flattens to around 80–85 V.
At 1 s UTp is 117 V and at 0.5 s 220 V. Halving the clearance time near these durations roughly doubles the permitted touch voltage.
Watch out: footwear and surface layers raise the prospective limit considerably, which is why ENA TS 41-24 quotes 233 V at 1 s for shoes on soil.
The BS EN 50522 permissible touch voltage curve for bare hand-to-feet contact, as used for HV substations in the UK.
Enter your values to see the result.
Good to know
In the working, t1, U1 and t2, U2 are the table points either side of the fault duration (capped at 10 s), and UTp is interpolated on log–log axes between them. Values tabulated in BS EN 50522:2010 Table B.3 (the curve of Figure 4; Figure 8 in EN 50522:2022): 0.05 s 716 V, 0.1 s 654 V, 0.2 s 537 V, 0.5 s 220 V, 1 s 117 V, 2 s 96 V, 5 s 86 V, 10 s 85 V. Between points this uses log–log interpolation. Clause 5.4.2: an installation meets the touch voltage requirement if EPR ≤ 2 UTp (C2), or ≤ 4 UTp with specified measures (C3); otherwise touch voltages must be calculated or measured. Footwear and surface layers raise the prospective limit (Annex A/B, and the ENA TS 41-24 Table 1 values).
Common mistake
Comparing a calculated EPR directly with UTp. UTp is a touch-voltage limit; the EPR may be several times higher where the grid grades the surface potential.
Limits
Bare hands and feet, no additional resistance, 5 % body impedance and the c2 fibrillation curve of IEC 60479-1. Interpolation between table points is an approximation of the published curve (worst near 0.2–0.5 s, where it falls steeply). Values below 0.05 s are not given.
Sources
- BS EN 50522 Earthing of power installations exceeding 1 kV a.c. (2010; superseded by EN 50522:2022)
- ENA TS 41-24 Issue 2 Guidelines for the design, installation, testing and maintenance of main earthing systems in substations (consultation text, Tables 1 and 2)
- IEC 60479-1:2018 Effects of current on human beings and livestock, Part 1: General aspects
For competent high-voltage engineers. Results are calculations from the published methods shown and depend entirely on the data you enter. They are not a substitute for a system study, manufacturer data, the applicable standards and Engineering Recommendations, or your organisation’s safety rules. Never use a result to justify working on or near live HV equipment.