Conventional touch voltage limit UL — 50V and 25V

When assessing automatic disconnection of supply (see the §413 guide and the earthing system guide), everything revolves around one central concept: the conventional touch voltage limit UL. This is the voltage that, under prolonged exposure, is considered the maximum acceptable level before a hazardous situation for a person arises — and the value against which disconnection times and protective measures in IEC 60364-4-41 (and hence NEN 1010) are ultimately tested.

The standard limit: 50V AC / 120V DC

For healthy adults, under normal (dry) conditions, a conventional touch voltage limit of 50V alternating voltage (AC, 1–1000 Hz) or 120V direct voltage (DC) applies for prolonged exposure (longer than approximately 3 seconds). Below this limit, the risk of a life-threatening situation is considered sufficiently low; above this limit, rapid disconnection is required.

The reduced limit: 25V AC / 60V DC

For children, livestock and other vulnerable situations (and in practice also applied in damp/wet environments with elevated risk), the standard applies a reduced limit: 25V AC or 60V DC respectively. This is, for example, the background to the PELV limit value: a PELV circuit with a rated voltage that does not exceed 25V AC/60V DC is considered protected by a protective conductor.

Why this matters: the basis behind disconnection times

The maximum disconnection times prescribed by NEN 1010 for automatic disconnection of supply (for example in the event of an earth fault in a TN system) are not chosen arbitrarily — they are derived from how long a body can withstand a voltage above UL before the risk becomes unacceptable. The higher the actual touch voltage during a fault rises above UL, the shorter the required disconnection time.

Note: the exact disconnection-time tables (for example 0.4s/5s for different voltage levels in TN systems) follow from the standard's own tables and differ per earthing system — this article explains the underlying principle (the UL limit), not the full disconnection-time table.

Practical application

PELV/SELV circuits are deliberately designed below the 25V/60V limit so that even in a fault situation no touch hazard arises (see also the protection class guide for class III/SELV equipment).
In agricultural/horticultural environments with damp, conductive conditions (moisture, livestock, metal greenhouse structures), the reduced 25V limit is applied more often than the standard 50V limit, given the elevated risk of that environment.

Common mistakes

Confusing UL with a "safe voltage" in an absolute sense — UL is a conventional limit for prolonged exposure in an existing fault scenario, not a guarantee that every voltage below it is completely risk-free in every circumstance.
Applying the same 50V limit in damp/agricultural environments without considering that the reduced 25V/60V limit may be more appropriate there given the elevated risk.
Viewing disconnection times in isolation from the UL limit — the table values for maximum disconnection time exist precisely to guarantee that a touch voltage above UL does not persist longer than permissible.