Reducing high inrush currents

Many components in an electrical installation experience a brief but very high power surge at the moment of operation, which can amount to many times the rated current during operation of the components. Causes can include, for example, the PTC (Positive Temperature Coefficient) characteristic of a component or the magnetic behaviour of a transformer. Many installations use primary-switched power supplies, and these can also show high inrush currents.

With a primary-switched power supply, the input side storage capacitor (diagram, in blue) is decisive in determining the inrush current (diagram, in green). Regardless of the unit – whether a low-power, PCB-mounted power supply or a high-performance unit for built-in equipment or top-hat rail mounting – the peak inrush current cannot necessarily be derived from the rating of the power supply.
 

With excessive inrush currents in existing installations, an inrush current limiter (ICL) represents a fast and economical solution for keeping inrush currents to below a specified value. An ICL is particularly advisable for new installations. The solutions from MEAN WELL, for example, work not only with a simple NTC circuit, but with a timed bypass circuit, and have been developed to handle both capacitive and inductive loads:

• Models for top-hat rail installation and built-in versions for equipment
• Overvoltage Category III compliant and EN62368-1 certified
• Integrated thermal circuit breaker monitors the temperature of the ICL.

When choosing the right limiter, a few points have to be taken into account:

• The sum of the inrush currents from the connected power supplies is irrelevant.
• In every case, peak currents are limited to exactly what is specified in the datasheet.
• The maximum capacitive load and maximum possible rated current restrict the number of power supplies that can be connected.
• The rated current is usually the decisive factor, with the ICLs able to handle capacitive loads up to 2500µF (MEAN WELL ICL-16 series). Anticipated in the third quarter of 2019, MEAN WELL is releasing its ICL-28 series, with a higher maximum capacitive load of 6000µF.
• The input capacity of the power supplies is largely determined by primary-side storage capacitors.
• Operating temperature range is from -30°C to +70°C.
• The function of the ICLs also allows smaller and faster circuit breakers to be used.

The ICL must be installed

• at the sub-distribution level (ICL-16R or ICL-28R, as applicable) or
• upstream of the loads (ICL-16L or ICL-28L, as applicable), e.g. for LED lighting in suspended ceilings,
and switched together with the load.

The ICL limits the current for the specified time until the capacitors of the connected power supply units have charged completely or to the point where they can no longer cause the upstream fuse to trip (=Internal Relay Limiting Time).

Switching a Mean Well ICL-16 current limiter together with the standard 100W ELG-100 LED power supply series increases the number of power supplies on a standard B16 circuit breaker from 3 to a maximum of 21 units with the limiter.
The higher the inrush currents of the power supplies, the clearer the increase in the usable number of power supplies becomes when using an upstream ICL.

Schukat electronic offers an extensive range – from device fuses to power supplies to inrush current limiters – all ex stock. Customers can obtain advice and support for the products and how to use them from Schukat's technical power supply sales team.