The classic structure of a 24 V (DC) power supply and distribution system looks like this: Two or more power sources supply the required energy redundantly. This then goes via decoupling diodes to the supply voltage rail of the power distribution, where protective elements such as fuses or circuit breakers are connected, which pass the currents on to the individual consumers. These can be controllers, sensors, actuators or interface modules. The return lines from the loads are normally brought together via a separate patch panel and fed back to the supplying sources. With this classic approach, the number, tripping characteristics and nominal values of the fuse elements must be defined before they are installed. This is because everything is then hard-wired and can only be changed with great effort. Depending on the number of channels to be protected, two to three rows are quickly occupied in a normal switch cabinet.

In this constellation, the technician receives little or no information about the status of the power supply or the distribution to the load circuits during operation. The only feedback is a status message as to whether an automatic system has tripped. This message is sent via the signal contacts of the circuit breakers, if they are connected at all - so the information only comes "when the child has long since fallen into the well".

Service-friendly design

Service display of the functional states of the individual channels in the intelligent power distribution system.

© E-T-A Electrotechnical Apparatus

The power distribution system offered by E-T-A consists of an aluminum housing that can be mounted on the mounting plate in the control cabinet using four screws. With a width of 482 mm (19 inches) and a height of 133 mm (3 U), the housing is compatible with the 19-inch grid. The decoupling diodes are already accommodated in the housing - together with the necessary heat sinks at the rear, as a certain amount of heat loss already occurs at 80 A. Together with four supply terminals up to 25 mm² (suitable for 80 A) for the two power supplies, this forms the 'power supply side'.

On the load side, there are slots for plugging in the electronic overcurrent fuses to protect the 24 consumer circuits. These include the corresponding connection terminals with push-in technology for the load cables with a cross-section of up to 2.5 mm² or 10 A rated current. Everything is freely accessible from the front, thus minimizing wiring work and installation costs. The fact that the circuit breakers can be plugged in makes subsequent fitting, retrofitting or modification easier. Servicing is also simplified, as maintenance personnel no longer have to loosen any screws or disconnect any wires - everything can be done during operation.

Intelligence on board

This system also contains intelligence via the electronic circuit breakers. These components can be set, remotely controlled and monitored via data bus - typically Profibus-DP or Profinet. Users can define various parameters for the electronic circuit breakers - for example the rated current, the tripping behavior (time behavior) or threshold values for a pre-warning. On the one hand, information is continuously read from each individual overcurrent protection element during operation, which can then be evaluated in higher-level systems or controllers. On the other hand, the device also displays the status directly on site using colored LEDs.

It is also possible to connect to the power distribution system directly on site using a laptop or tablet via a USB interface and clearly display the status of all load circuits on the screen with the help of service software. With the appropriate access authorizations, it is also possible to intervene directly.

In this way, the user has two remote control options: firstly, via the bus, which continuously integrates, monitors and controls the data in the system processes. The second is via the service interface on site, which allows the user to intervene immediately and rectify problems or prevent them from occurring in the first place.

For preventive diagnostics, for example, the current and voltage states as well as the operating temperatures of the element to be protected can be read out. This allows conclusions to be drawn about the status of each individual load branch. If a trip has actually occurred, the user is shown what type of fault is present - short circuit, overload or problems with the supply voltage. An internal event counter provides information on how often a fault has already occurred and whether it may have simply been ignored. A readable oscillogram can be used to view and evaluate the current and voltage curve directly before the last trip. This makes it easier and quicker to identify the cause and ultimately rectify the fault.

Current limitation included

Display of the history memory (oscillogram) of a channel after triggering.

© E-T-A Electrotechnical Apparatus

In addition, the electronic circuit breakers have a current limiter that only allows a short-circuit current slightly above the set rated current in the event of a fault. In this way, the electronics protect connected cables and loads. Particularly with very long load lines, which themselves have a current-limiting effect, this feature leads to the safe
selective tripping of the residual current circuit. This prevents a reduction or even total failure of the 24 V (DC) supply voltage due to a fault on a single load. The other fault-free loads connected to the same 24 V (DC) distribution system remain unaffected.

The system detects impending excessive overloads or short circuits, switches off and prevents any resulting damage. An error message on the device itself or via the communication system also ensures that the operating personnel are informed accordingly. At the same time, the error message can be used to initiate a predefined countermeasure.

Centerpiece controller

The power distribution system contains all components for communication via Profibus-DP or Profinet; the 'CPC10' bus controller is the actual intelligent core of the system. It connects the communication to the higher-level systems in the outside world and communicates with the individual electronic circuit breakers via a special internal bus, the EL bus. It collects all the data from the 24 participants and makes it available to the outside world. The controller forwards configuration or control information from the higher-level system to the participants. All the parameters of each channel are stored in the controller for each slot. When an electronic circuit breaker is replaced, it immediately sends them to the new subscriber.

The data lines of the internal bus and the connecting lines for distributing the large supply currents to the individual slots are routed via a compact circuit board, which also houses all the load connection terminals. This eliminates flying leads and unnecessary terminal points, and sources of error within the system are eliminated.

The wiring outside the power distribution board is user-friendly and all external cables can be clearly routed. For this purpose, a module equipped with jumper brackets is docked on ex works - underneath the distribution board at the level of the connection terminals. This enables the cables to be laid horizontally and generally eliminates the need for deeper cable ducts that are difficult to access.

Author:
Dieter Arenz is an application specialist for industrial system solutions at E-T-A Elektrotechnische Apparate in Aldorf.