Figure 1: Circuit-breakers with C characteristic (filled area) require a multiple of tripping currents and therefore large, oversized power supplies (t/s = time in seconds, x IN = multiple of the rated current).

© Phoenix Contact

For functional protection of 24 V (DC) load circuits against overloads and short circuits, it is necessary to take a comprehensive look at the interaction between the power supply, cables and protection elements and to design them correctly. The naturally limited power supply of 24 V (DC) power supplies is not comparable with the power supply that can be provided by 230 V (AC) low-voltage networks. This means that the short-circuit current that a power supply unit can deliver in the event of a fault may not be sufficient to trigger a standard circuit breaker or fuse. This is because both are primarily constructed and designed for short-circuit protection in low-voltage networks that are capable of supplying a multiple of the rated fuse currents.

Regardless of this, the low-cost circuit breakers or even simple fuses familiar from building installations are often still used for protection in 24 V (DC) control circuits. However, using such components means saving at the wrong end. In the event of a short circuit, they sometimes trip too late or not at all - resulting in voltage dips and complete system shutdowns. Circuit breakers with a C characteristic, for example, have an upper tripping limit that is significantly higher than their rated current(see Figure 1). An overcurrent that does not yet cause the circuit breaker to trip, but which exceeds the power output of the power supply, causes a voltage dip at all other loads - such as the controls. This leads to a total failure of the system.

No protection for long cables

Particularly in widely branched systems or elongated production lines, the cable lengths between the 24 V (DC) power supply and the loads in the field are often quite long. In the event of a short circuit, this can lead to the line impedance of the circuit to be protected limiting the short-circuit current to such an extent that far too little current flows through a conventional thermomagnetic circuit breaker to be able to trip at all. Depending on the capacity of the power supply, this results in a long-lasting thermal overload of the defective terminal device and the supply lines. This causes a dangerous system condition and, in the worst case, can even trigger a fire.

Electronic circuit breakers, on the other hand, react far more sensitively and quickly than thermomagnetic circuit breakers or even fuses. Models with an active current limiter(see Fig. 2) are available on the market - such as devices from the CB-E1 and CBM families from Phoenix Contact. These limit the overload and short-circuit current to such an extent that there is no voltage dip. By limiting the residual overcurrent to a value of 1.25 to 2 times the rated current, a fire hazard can also be avoided.

In combination with electronic circuit breakers, the output of a 24 V (DC) power supply can be almost fully utilized for the loads. There is no need to provide a power reserve to trigger thermomagnetic circuit breakers or thermal fuses in the event of a fault by means of multiple rated fuse currents. As a result, smaller or fewer power supplies can be installed. In short: space and costs are saved.

Figure 3: Single-channel electronic circuit breakers of type CB-E1 can be adapted precisely to the required number of circuits to be protected.

© Phoenix Contact

However, the thermal dissipation of enclosures is also an issue as the packing densities of the electronic components installed become ever tighter. In other words, fewer or smaller power supply units reduce the thermal waste heat within the enclosures. Last but not least, the associated energy savings are relevant.

Circuit breakers such as the CB-E1 mentioned above can be adapted precisely to the required number of circuits to be protected in applications with one channel or more(see Fig. 3). The circuit-breakers each consist of two plug-in parts - base element and circuit-breaker plug. The base elements can be pre-wired and only fitted with the appropriate plugs during commissioning. The base elements can also be quickly and easily 'wired' to each other using the standard comb bridge material from the Clipline Complete range of terminal blocks - without the need for complicated and time-consuming cable assembly. Furthermore, the circuit breakers are only 12.3 mm wide and have the potential-free remote signaling contact already integrated. In contrast, miniature circuit-breakers with an auxiliary contact module are twice as wide.

CBM miniature circuit-breakers are available in two configurations - with four and eight channels. The rated current value of each channel can be set individually. Both of these devices have a so-called 'rated current assistant', which makes it possible to precisely adjust the rated current value to the actual conditions during initial commissioning.

Analyzing the flow 'intelligently'

Other electronic circuit breakers use intelligent current measurement methods to evaluate the rate of rise of the flowing load current and switch off selectively within a few milliseconds in the event of an overload or short circuit. This type of current analysis also enables these devices to meet the requirements of the PLC standard DIN EN 61131-2 - namely to limit voltage dips to less than 10 ms. This ensures continuous operation of the control system. Thanks to the intelligent evaluation of the current rise speed, capacitive loads and high starting currents of motors are safely started, while short-circuit and overload currents are safely switched off.

Figure 4: CBMC multi-channel electronic circuit breakers have an IO-Link interface. Preset devices are also available, which can be individually preconfigured according to the desired values.

© Phoenix Contact

The devices in the CBMC family(Fig. 4), for example, use this intelligent current analysis method. These circuit breakers have four channels whose rated currents can also be set individually. There are also variants that can be parameterized and read out via an IO-Link interface. Last but not least, devices are available that are designed and approved for the strict requirements of power limitation - i.e. less than 100 VA in NEC Class 2 circuits.

The electronic circuit breakers are pre-wired without being fixed to specific rated current values and can later be adapted to the system situation during commissioning in the field. Miniature circuit-breakers and fuses, on the other hand, only have fixed rated currents. If the rated current value needs to be adjusted during commissioning, the wiring must first be laboriously disconnected. The circuit breaker must then be replaced with one with a different rated current value and rewired. This time-consuming procedure may have to be repeated if the newly selected rated current value is still not suitable.

In a nutshell: Failures of complete systems are effectively avoided by selectively disconnecting only the defective system parts from the 24 V (DC) power supply very quickly. This also protects the defective terminal devices and the supply lines from thermal overload. The adjustable or, on request, pre-configured rated currents of the circuit breakers keep the parts variance and therefore the qualification and storage costs low.

Author:
Carsten Plattmann works in product marketing for circuit breakers at Phoenix Contact.