The current-time curve (IEC 60479-1) — why 30 mA and 5 seconds are not arbitrary numbers

The RCD guide (§531) cites 30 mA as the standard value for additional protection, and the automatic disconnection guides cite maximum disconnection times that depend on touch voltage. This article covers the question underlying both: why these particular numbers? The answer comes from IEC 60479-1 — the technical specification describing the physiological effects of current through the human body as a function of both current magnitude and duration.

Four zones, not one fixed threshold

IEC 60479-1 divides the effects of alternating current (15–100 Hz, hand-to-feet path) not into a single hard threshold, but into four zones that together form a curve on a current-time diagram: the longer the current flows through the body, the lower the current level at which it can already become dangerous.

Zone	Characteristic	Typical physiological effect
AC-1	Below the perception threshold	Usually no reaction or sensation at all
AC-2	Above the perception threshold, below the let-go threshold	Perceptible, usually muscular reactions, generally no harmful physiological effect
AC-3	Above the let-go threshold	Painful muscular contractions, possible breathing difficulties, usually reversible effects without organ damage
AC-4	Further increasing, duration-dependent	Increasing probability of pathophysiological effects, including ventricular fibrillation; subdivided into AC-4.1/4.2/4.3 by increasing fibrillation probability

The two key thresholds

Perception threshold (upper limit of zone AC-1, curve a): for 50/60 Hz alternating current about 0.5 mA — the point at which a person first feels the current (tingling); the individual value differs from person to person.
Let-go threshold (curve b, boundary between AC-2 and AC-3): the point beyond which muscular contraction makes it impossible to voluntarily release a gripped conductor. As a practical rule of thumb, roughly 10 mA is used; the individual value differs from person to person.
From roughly 40 mA, sustained for more than 2 seconds, the risk of ventricular fibrillation — a life-threatening cardiac arrhythmia — increases noticeably. This is precisely why the curve, rather than a single fixed current value, is decisive: the same current magnitude can fall within zone AC-3 at a shorter exposure and within zone AC-4 at a longer one.

The link to 30 mA and the maximum disconnection time

This is the physiological basis behind two commonly used values in NEN 1010:

30 mA additional protection (§411.3.3, see the RCD guide): an RCD with a rated residual current of 30 mA disconnects well before the fault current through a person reaches the level at which ventricular fibrillation becomes a real risk — with a substantial safety margin relative to the roughly 40 mA/2 s zone.
Maximum disconnection times for automatic disconnection of supply (§411): the higher the expected touch voltage, the shorter the permitted disconnection time — precisely because the current-time curve shows that a higher current can remain within an acceptable zone for a shorter exposure duration, while the same current falls into the danger zone at a longer duration.

Note: the curve also explains why an RCD trip-time measurement (see the RCD trip-time measurement guide) checks not only the tripping current, but also the time within which this happens: an RCD that does trip at 30 mA but takes too long to do so provides, in practice, less protection than the standard assumes.

Practical relevance

When explaining — or answering an exam question about — why certain threshold values apply in NEN 1010/3140 (30 mA, maximum disconnection times), it is not enough to know only the number; the underlying reason is the current-time relationship of IEC 60479-1. This also prevents a common misunderstanding: that a current below 30 mA is by definition "safe" — the curve shows that lower currents too can fall into a higher-risk zone given a sufficiently long exposure.

Common mistakes

Assuming there is one fixed, duration-independent danger threshold — the IEC 60479-1 curve is explicitly duration-dependent; current magnitude and duration together determine the zone.
Confusing the perception threshold with the let-go threshold — the perception threshold (felt) lies well below the let-go threshold (muscular contraction prevents release); both are separate, distinct limits.
Treating 30 mA as an absolute safety guarantee rather than a practically chosen value with a substantial safety margin relative to the fibrillation zone — the margin exists precisely to absorb uncertainty in individual sensitivity and measurement accuracy.
Not factoring in a protective device's disconnection time when assessing the safety of an installation, and looking only at the tripping current.