§445 — Protection against unintended automatic restart after undervoltage

§445 of IEC 60364-4-45 (and therefore NEN 1010) covers a risk that is easy to overlook: what happens after the supply returns following an interruption or dip, rather than what happens during the interruption itself. While most safety articles focus on what happens when the supply is lost, §445 addresses the moment the supply comes back: if a machine then restarts by itself, without any human intervention, this can be a direct hazard to anyone working on the machine at that moment.

The problem: a mechanically latching switch

A simple toggle switch or a non-latching isolator that is left "on" does not treat a supply interruption as a distinct event — as soon as the supply returns, the load simply resumes exactly as it was before the interruption. For a lamp that is harmless. For a running machine where someone is, at that exact moment, cleaning, adjusting or freeing a moving part, an unannounced restart is a genuine injury risk.

The built-in solution: the self-holding circuit of a contactor

In practice, the most common solution is not a separate, dedicated undervoltage-protection device, but something already built into the standard wiring of a direct-on-line (DOL) starter: the coil of the contactor is made self-holding through its own auxiliary contact. As soon as the supply drops, the coil loses its excitation and the main contacts open — that happens regardless of the switching principle used. The crucial difference from a plain toggle switch is what happens when the supply returns: the self-holding circuit is now broken (the auxiliary contact that kept the coil energised has also opened), so the contactor stays open until an operator deliberately presses the start button again. This latching principle is itself already a form of undervoltage protection, built into the standard circuit — no extra device required, but it is a deliberate design choice, not something automatically true for every type of switching device.

Note: this is precisely why a DOL starter with a contactor is inherently safer than a solution that drives a machine directly with a mechanically latching circuit-breaker or isolator without a self-holding control circuit in between — the latter may not automatically satisfy §445.

When automatic restart is actually permitted

§445 does not universally forbid automatic restart — the requirement applies specifically where an unexpected restart would create a danger. For an unattended, inaccessible installation without a mechanical-maintenance hazard (for example a closed circulation pump or a fan without accessible moving parts during operation), automatic restart after supply is restored can be acceptable — provided that is explicitly justified in the installation's risk assessment, not assumed by default.

Difference from the §537 maintenance-switching function

The guide on the four functions of §537 covers the maintenance switching function — a switching device an operator actively operates to prevent a machine from unexpectedly restarting during mechanical maintenance. §445 is distinct from that: it is not about an action an operator performs, but about the automatic behaviour of the control circuit itself after a supply loss outside the operator's control — for example a network fault, a protective device operating elsewhere on the network, or a generator fault. Both articles serve the same ultimate purpose (preventing an unexpected restart), but through a fundamentally different mechanism: one is a deliberate, manual switching action; the other is an intrinsic property of the control circuit that is always active, even if nobody is thinking about it.

Practical relevance during commissioning

During commissioning of a motor drive, it is worth explicitly testing what happens during a simulated supply interruption: switch the supply off briefly and back on, and verify that the machine does not restart by itself but waits for a new, deliberate start command — unless a documented risk assessment for that specific machine explicitly permits automatic restart.

Common mistakes

Driving a machine directly with a mechanically latching switch (no self-holding control circuit) without assessing whether automatic restart after supply is restored creates a danger.
Assuming every contactor automatically has a self-holding circuit — with some control concepts (for example PLC-driven start/stop logic), the no-automatic-restart behaviour must be explicitly secured in software, not implicitly assumed from the hardware alone.
Confusing §445 with the §537 maintenance-switching function — the former is an automatic, always-active control-circuit behaviour; the latter is a deliberate, manual switching action by an operator.
Allowing automatic restart without a documented risk assessment, purely because the process is "unattended anyway" — the exception in §445 requires an explicit, documented judgement, not an implicit assumption.