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IEC
IEC 60364-4-41 · Edition 2005+A1:2017

§411.3 Protection by Automatic Disconnection of Supply — General Requirements EB SUMMARY

Automatic disconnection of supply applies when touch voltage exceeds the safety threshold: UL ≤ 50 V AC or 120 V DC under normal conditions. Protection is achieved by coordinating the fault loop impedance Zs, the protective device characteristic, and the operating current Ia. For TN systems the condition Zs × Ia ≤ U₀ must be met, where U₀ is the nominal voltage to earth (typically 230 V). The protective device must disconnect the circuit within the time given in Table 41.1 — from 0.1 s to 5 s depending on voltage level and distribution system type. For final circuits up to 32 A in fixed installations the maximum disconnection time is 0.4 s at U₀ = 230 V. Requirements also apply to IT systems with insulation monitoring devices (IMD) and to TT systems where an RCD serves as the primary protective measure.
EB-authored summary for design convenience — NOT normative IEC text. Official standard IEC 60364-4-41:2005+A1:2017 available at webstore.iec.ch →.
💡 Explain in plain words
In plain words
AI-generated — may contain inaccuracies

This clause sets the general rule for automatic disconnection: protection must operate before the touch voltage exceeds the safe limit of 50 V AC. To achieve this, three things are coordinated — the fault loop impedance Zs, the characteristic of the protective device, and its operating current Ia. For TN systems, the main condition is simple: Zs × Ia ≤ U₀, meaning the loop must be sufficiently low-impedance so that the short-circuit current is high enough for the device to operate quickly. The standard specifies maximum disconnection times (Table 41.1): for final circuits up to 32 A at 230 V — no more than 0.4 s. In practice, this is why it is important to measure the loop impedance during commissioning — to ensure that the circuit breaker will actually disconnect in time.

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