Reliable power supplies and commercially viable circuit design

Faults in production are difficult to eliminate in their entirety - for example, short circuits may appear in the wiring or individual consumers may experience malfunctions. Ideally, however, the machine or plant should be able to continue operation without interruption. The fuse protection on the secondary side of the 24 V power supply plays an important role in this. Thanks to the SFB technology provided by Phoenix Contact’s Quint Power power supplies and matching circuit breakers, production can continue unabated.

Parallel to the control system, a number of other consumers, such as sensors and actuators, may be connected to the power supply. In order to minimise downtimes, each circuit should be protected individually. When a short circuit occurs, only the faulty circuit is disconnected from the power supply and the control system continues to operate without interruption.

Commercially available circuit breakers are the most economical solution for this. These can trip electromagnetically or thermally via a bimetallic strip. If it is to trip within just a few milliseconds, the integrated solenoid requires a significantly higher current than the rated current of the circuit breaker.

Manufacturers usually specify AC currents for the short-circuit currents required for electromagnetic tripping. Users must take into account that when compared to AC applications, the DC values are higher by a factor of 1.2. Circuit breakers are available with different trip characteristics, whereby in the industrial environment, circuit breakers with either a B or C characteristic are used the most. Characteristic B requires 3-5 times the rated current of the circuit breaker for AC and 3-6 times the current for DC applications. Therefore, 150 A is required for a 25 A circuit breaker with a B characteristic in order to trip under unfavourable conditions within just a few milliseconds. The C characteristic requires 5-10 times the rated current for AC and 5-12 times the current for DC applications.

Circuit breakers reliably tripped

When power supplies only provide a low power reserve, this causes thermal tripping, which can last several seconds or even minutes. A tripped circuit breaker makes fault finding easy. However, within this time, the 24 VDC voltage of the power supply has already dipped and the control system failed. In the worst case, the power supply supplies a current that is too low or only provides a brief current reserve of just a few seconds. In this case, the circuit breaker does not trip and fault finding turns out to be time-consuming and costly.

Phoenix Contact’s ‘Quint Power’ power supplies feature SFB (Selective Fuse Breaking) technology, which means they trip magnetically. A module with rated values of 24 V and 20 A, for instance, supplies one control system and three additional loads. Each circuit is protected using a 6 A circuit breaker with a B characteristic. The 25 m copper cables have a diameter of 2.5 mm². If, in this example, a short circuit occurs at the display because of a damaged cable, using its SFB technology, the 20 A power supply briefly supplies six times the rated current - a maximum of 120 A. The circuit breaker reliably trips at 10 times the rated current in the magnetic range of its characteristic. The circuit breaker involved trips within 3-5 ms, while the other loads continue to operate uninterrupted. In spite of the short circuit that has occurred, the control system is continuously supplied with 24 VDC and continues to operate without any interruption.

Tripping time depends on cable length and diameter

Whether a circuit breaker trips fast enough depends, among other things, on the length and diameter of the cable that is used to connect the load. In this case, it is not just the high current that the power supplies can provide which is decisive. The high current can only flow during the short circuit and magnetically trip the circuit breaker if the impedance of the faulty circuit is low enough. Which power supply with which cable diameter and which cable length immediately trips the circuit breaker is shown in a table. This simplifies the design of the machine or plant.

New circuit breakers with SFB curve

As a wide range of different consumers is available on the market, it can be a challenging task to find the right type of circuit breaker protection. In practice, circuit breakers are often oversized, as the matching current rating is not offered with the B characteristic. Circuit breakers with a C characteristic are used instead; however, these are significantly slower to respond, and the cable lengths need to be minimised accordingly.

Phoenix Contact’s new CB type circuit breakers are based on the SFB characteristic curve. This characteristic was developed specifically for Quint Power SFB type power supplies in order to trip even more quickly and reliably. The characteristic curve is based on the C characteristic but has a much lower tolerance so that there is less short-circuit current. This lessens the load on the cables and on the connected consumers. Cable distances can also be longer, as the decreased tripping current is not limited by the cable resistance so soon.

The SFB-matched circuit breakers trip reliably even when the cable distances are very large or the cable diameters very low. Compared to conventional circuit breakers with a C characteristic, the cable distances can be up to 30% longer. If, for example, a 2 A circuit breaker with a C characteristic is required, the current caused by a short circuit will be no more than 24 A. To ensure reliable tripping, the distance between the power supply and the consumers should be no more than 14 m with a 1 mm² copper cable. If a 2 A circuit breaker with the new SFB characteristic is used, on the other hand, it takes only 20 A to trip in time. The cable distance can therefore be extended to 18 m. Sensors and actuators are frequently preassembled, with cable diameters as low as 0.22 mm² or 0.34 mm². This greatly limits the available distances, and the necessary protection is usually not given anymore.

The new circuit breakers, which have a component width of just 12.3 mm, feature an integrated signalling contact to report the current machine or plant status. The new system is complemented by the bridging concept of the Clipline Complete range of terminal blocks. Fast and easy connection is ensured through push-in technology.

Conclusion

In order to trigger circuit breakers magnetically, and thus quickly, SFB technology delivers a current that is six times higher than the rated current for 12 ms. Faulty current paths are selectively turned off, the fault is localised and important system components remain operational. Especially when extremely long cables are being used, carefully matched circuit breakers ensure ongoing, commercially viable production.