Switchgear for switch mode power supplies in DC 400 V / 800 V industrial networks
External overcurrent protection and optional disconnection on the input side are just as important as protection on the output side. “Inrush currents” occur when the switch mode power supply units are switched on despite active current limiting. In the following article, we explain which E-T-A circuit breakers can be used to safely operate and selectively protect such applications in AC and DC networks.
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
1.1 Wiring in industrial DC networks 400 V / 800 V
Converters, such as DC/DC-converters have capacitive intermediate circuit buffers. In DC systems, these devices are often switched at all poles using 2-pole switches. The following diagram shows a simplified equivalent circuit diagram of a DC power distribution unit with 8365-T circuit breakers and a comb busbar.
The “inrush currents” that occur when such devices are switched on are used to charge the capacitors.
1.2 What must be considered in case of limited “inrush currents?
The device that must be protected is equipped with an “inrush current limitation” or a “soft start module” according to ETSI 300 132-3. In order to prevent nuisance tripping of the protective element, the endurance and amplitude of the current are limited.
The “inrush current” is classified according to the endurance T50. This is the time between half the peak value of the rising and falling current level.
The power level of the impulse is specified with the following I2t value: I2p T50
1.3 Which circuit breaker is suitable for overcurrent protection?
Switch mode power supplies, such as DC/DC converters, with capacitive intermediate circuit buffers are often switched and protected during operation via circuit breakers with a disconnecting function. Overload and short circuit protection must be provided by the user. Therefore, the I2ttrip thermal limit load integral of the trip curve must be greater than the I2tCharge thermal limit load integral of the current pulse.
I2tCharge < I2ttrip
This example is intended to calculate whether a DC/DC converter with a thermal limit load integral of 0.1 A2s and a rated current of 1 A is suitable to be connected. In the current range below 300 ms, the following calculation can be made based on the characteristic curve:
I2tCharge = 0,1 A2s < 0,18 A2s = (3 A)2 * 0,02 ms = I2ttrip
An 8365-T with 3 A and a pulse-delayed fast G1 characteristic is ideally suited for the application.
1.4 Which circuit breaker is suitable for overload and short circuit protection?
If short circuit protection and overload protection are required, suitable switchgear must be selected. It is not uncommon to select a slow characteristic curve for these applications to ensure capacitive connection without tripping, often at the expense of selectivity and device protection. For this reason, E-T-A offers switchgear designed specifically for such applications. The 8345-F and 8365-T product groups provide pulse-delayed, temperature-independent characteristic curves. They suppress tripping at high pulse charging currents and still guarantee well-adjusted fast tripping characteristics to protect the devices.
1.5 Our solution for every application
Get in touch with 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.
