SFB technology

Usually additional loads, such as sensors or actuators, are connected to a power supply unit parallel to the controller. In order to minimize downtimes, each of these current paths should be protected individually.

Therefore, if a short circuit occurs, only the faulty path of the power supply will be disconnected and the other loads will continue operating without interruption.

At present standard circuit breakers represent the most cost-effective solution for protecting a circuit. They can be tripped electromagnetically or thermally via a bimetal.

To ensure that they trip within a few milliseconds, the current for the integrated solenoid must always be significantly higher than the nominal current of the circuit breaker.

The short-circuit currents required for electromagnetic tripping are usually specified for alternating current (AC) by manufacturers. Users must therefore make sure that the DC values are 1.2 times higher.

Circuit breakers are available with various tripping characteristics; characteristic B or C circuit breakers are mostly used in industrial applications.

For characteristic B, the following currents are required in order to trip the circuit breaker:

• AC applications: three to five times the nominal current
• DC applications: three to six times the nominal current

Therefore, under the most unfavorable conditions, 150 A is required to trip a 25 A characteristic B circuit breaker within a few milliseconds.

For characteristic C circuit breakers, the following currents are required:

• AC applications: five to ten times the nominal current
• DC applications: five to twelve times the nominal current

In the event of an error, long cable paths limit the required tripping current. This can delay or even prevent circuit breakers being tripped.

If power supplies provide a lower power reserve, this means that thermal tripping may take several seconds or minutes.

In this case troubleshooting is very easy as you can see which circuit breaker has been tripped. However, during this time the 24 V DC voltage of the power supply has already been interrupted and the controller has failed.

In the worst-case scenario, the current supplied by the power supply unit is so low or it just supplies a brief power reserve of a few seconds meaning that the fuse is not even tripped. As a result troubleshooting is extremely time-consuming and expensive.

With SFB technology, the devices in the QUINT series therefore supply up to six times the nominal current. The circuit breakers are tripped magnetically with this pulse.

Example scenario:

• A power supply (24 V/20 A) supplies a controller and three other loads.
• Each current path is protected by a circuit breaker (6 A/B characteristic).
• The current paths consist of 25 m long copper cables (2.5 mm² cross section).

In the event of a short circuit in this example, the 20 A power supply unit supplies six times the nominal current for a short period using SFB technology, i.e., a maximum of 120 A. The circuit breaker is always tripped within 3 to 5 ms with ten times the rated current in the magnetic range of its characteristic curve.

The other loads continue operating, the controller is consistently supplied with 24 V DC and continues running without interruption in spite of the short circuit.