Safety-related hard-wired controls

Dipl.-lng. Bernhard Kendel

The mean down time refers to the period during which the plant stands still. It consists of two components: the time consumed in finding the error cause (fault detection time) and the time consumed in replacing a defective module (fault clearance time). Replacing the defective module only takes a few minutes. However, up to now the time consumed in finding the fault in hard-wired controls could not be calculated. And this is exactly where the Planar4 system comes in: the fourth generation of safety-related hard-wired controls. This system is a further development of the Planar F model and comprises all the proven features of its predecessors. The new features are the self-diagnosis of the modules and the capability of communication of the complete control with other systems, i.e. Planar4 combines the classical advantages of a hard-wired control at the highest safety level with the features of a programmable logic control.

Up to the present moment diagnosis and error display of defective modules with the help of a microprocessor were a „by-product“ with safety-related programmable logic controls (PLCs). But now they have been realised, thanks to further development, also for hard-wired programmed controllers:

Behind the front panels in modern „book spine“ design you will find, besides the safety-related hardware layout, the diagnostic and communication module DCM which, with the help of a micro-controller, simulates and compares all the functions of a module. In the event of a defect a red LED with corresponding identification lights up on the front panel of the module in question and thus clearly identifies the defective module. The error status is also signalised via additional separate outputs of the module. These signals can either be recorded via a busbar or can be assembled in any kind of grouping with the help of a potential-free contact output to be displayed at a suitable location (e.g. in the control room). This clearly shows the advantages of Planar4: expensive down-times are limited to the short time it takes to replace the defective module, and the replacement can be carried out without staff requiring specific documents or profound knowledge of the control or the plant.

The safety-related modules of the Planar4 system are TÜV tested both according to DIN V 19250 and IEC 61508, and the system concept is approved for requirement classes 1 to 7, which corresponds to SIL 4. Therefore it meets the requirements of DIN VDE 0116 for the operation of furnaces.

The availability of the safety-related system may be increased according to the redundancy principle as now the modules may be switched in parallel without decoupling. This application has another advantage: the additional material and cost expenditure for equivalence monitoring which is usually required in two-channel systems is not necessary here; this monitoring is integrated by the self-diagnosis and the error display of the system. With input modules having line monitoring the external circuits are monitored to detect possible short circuits or breaks. Any interruptions of electronic power outputs which may be caused e.g. by defective fuses are also detected. These faults are signalised via additional separate outputs and are also displayed by LEDs on the front panel informing the user that the module does not have to he replaced.

To reduce wiring expenditure, a subrack with a busboard at the rear is available for Planar4 as an alternative to standard wiring. With the help of plug-in adapters, the usual wiring methods such as soldering, termi-point, wire-wrap (also for (Ex)i modules) can be applied. Only the functions have to be wired, as the basic wiring for the voltage supply of the modules or error signalling are already included in the busboard; They are protected and require little space. All this is another technical and economic advantage.

The additional application of the communication module which is currently in preparation provides new perspectives: a hard-wired control can communicate with other systems. Error messages from modules or missing modules for example are transmitted to a process control system and represented graphically. All input and output signals within the controls can be transmitted and evaluated, event recording (change of binary signals with date and time) can be realised. Data transmission via Ethernet and also via the HIBUS and MODBUS protocol is being planned.

Hima

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Further information cpp-202