inrush

Inrush, in electrical engineering, refers to the transient surge of current that occurs when an electrical device or system is energized. This initial surge often exceeds the steady-state operating current and can last from a few milliseconds to a few seconds, depending on circuit design and components. Inrush current is a normal phenomenon for many loads, but it can cause voltage dips, nuisance tripping of protective devices, and electrical stress on components such as cables, fuses, switches, transformers, and power supplies.

Common causes of inrush include the charging of input capacitors and EMI filters in power supplies, rectifiers, and converters; the magnetizing current of unloaded transformers; and the starting behavior of induction motors, where low initial resistance enables a higher current until the motor reaches operating speed. Other factors include cold resistance of conductors, which is lower when components are at room temperature, and non-linear loads that draw current abruptly at energization. The magnitude of inrush is typically expressed as a multiple of the device’s steady-state current and is highly dependent on impedance in the supply and within the device.

Mitigation strategies aim to limit or control inrush without compromising performance. These include soft-start or ramped-current designs, inrush current limiters such as negative temperature coefficient thermistors, pre-charge resistors, staggered or sequential starting, and selecting protective devices with appropriate instantaneous trip characteristics. Accurate measurement and modeling of inrush are important in power supply design, distribution planning, and safety analysis.