Undervoltage release

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Undervoltage releases are electromagnetic or electronic assemblies that respond when the supply voltage falls below a minimum . They are used to switch off electrical devices, machines and systems when the voltage is too low and to prevent them from restarting automatically after a power failure. Undervoltage releases can, for example, be integrated in motor protection switches or attached to the main switch and line protection switch .

If the mains voltage drops , they trigger the switch, which must be switched on again manually after the voltage has returned. Switching on without sufficient supply voltage is not possible. This prevents an unwanted and uncontrolled restart of a machine after the supply voltage has returned. They also protect under load current electric motors against overloading due to reduced performance and prevent at by batteries powered plants a deep discharge of the battery.

The tripping voltage can be set for most undervoltage releases. Undervoltage releases have not been so widespread since the introduction of monitored electronic controls in industry.


  • Alfred Hösl, Roland Ayx, Hans Werner Busch: The electrical installation in accordance with regulations, residential construction, commercial industry. 18th edition, Hüthig Verlag, Heidelberg, 2003, ISBN 3-7785-2909-9