Switchgear for switch mode power supplies - Inrush current limitation

Blog post06. March 2025 by Dr. Hendrik Köpf - Reading Time 2 min.

To make device installation and replacement as easy as possible for the users, we will answer some frequently asked questions regarding the challenges of high cut-in currents or so-called “inrush currents” in the following. "Inrush currents" occur in applications with inductive loads such as transformers and motors, but also in applications with switch mode power supply units.

More and more customers are asking for switch mode power supply units without internal inrush current limiting for lamps, medical devices, automation products, etc. The reason: Many switching devices are not suitable for these application fields, which leads to premature device failure.

In the following, we explain which E-T-A circuit breakers with which rated current and which characteristic curve enable safe operation for these applications.

1.1 Which is the correct circuit breaker for overcurrent protection?

Switch mode power supply units for LED lamps, X-ray devices, etc. with capacitive intermediate circuit storage are switched and protected by circuit breakers as a standard. Overload protection is provided by thermal circuit breakers such as E-T-A’s 3120.

The “inrush currents” that occur when such devices are switched on are used to charge the capacitors. When designing the overcurrent protection, it must be ensured that no nuisance tripping occurs when switching on.

The thermal limit load integral of the fast tripping curve I²t_trip must therefore be greater than the thermal limit load integral of the current pulse I²t_Charge

I²t_Charge< I²t_trip

This example is intended to test whether a current pulse of 70 A can be switched on for 0.5 ms by a 12 A device.

In the current range below 300 ms, the following calculation can be made based on the characteristic curve:

I²t_Charge = (70 A)² * 0,5 ms = 2,45 A²s < 3,53 A²s = (84 A)² * 0,5 ms = I²t_trip

A 12 A circuit breaker is therefore suitable for the application.

1.2 Which is the correct circuit breaker for overload and short circuit protection?

If short circuit protection is required in addition to overload protection, a suitable switching device must be selected. A fairly slow characteristic curve is often selected in such applications to ensure capacitive connection without tripping, which is at the expense of selectivity and device protection. For this reason, E-T-A offers switching devices that have been specifically developed for such applications. Product groups 8345-F and 8365-T offer pulse-delayed characteristic curves. These suppress tripping at high pulse charging currents and still guarantee adapted fast tripping characteristics to protect the devices.

DC 110 V – 100 A

DC 220 V – 100 A

DC 400 V – 100 A

DC 400 V / 800 V - 100 A

AC 230 V - 100 A

AC 230 V/400 V - 100 A

1.3 Our solution for every application

Please contact us! As your technical partner, we will help you select the correct E-T-A circuit breaker for operational switching and protection of your equipment.