Cable lengths of several hundred metres from the sensors and actuators to the power distribution system, and thus the power supply, are not at all uncommon in chemical plants. As these sensors and actuators are often supplied with 24 V DC, efficient power distribution and selective protection of the 24 V DC control circuits are essential for overall operational reliability.
Author Tim Decker Regional Sales Manager, E-T-A Elektrotechnische Apparate
High uptime and maximum operational reliability are of utmost importance in production facilities in the chemical industry, where complex processes have to be controlled. In the event of a disruption, prompt and effective trouble-shooting is required to eliminate the cause of the failure so that normal operation can continue. Downtime equates to financial loss.
In the interests of overall efficiency, the entire production equipment is fitted with various sensors and actuators to allow early detection of failures and, if appropriate, preventive action. This equipment includes servo motors, servo drives and magnetic valves as well as automation components such as PLCs and fieldbus couplers. The equipment’s large size often necessitates cable lengths of several hundred metres from the sensors and actuators to the power distribution system, and thus the power supply. The operational reliability and availability of the production facilities depend to a very large extent on the complex interaction of the individual components, which must be maintained even in critical situations. The above-mentioned sensors and actuators are often supplied with 24 V DC, and efficient power distribution and selective protection of these circuits is consequently essential. E-T-A Elektrotechnische Apparate offers optimal protection and power distribution of the 24 V DC circuits from one source. The focus is on automation technology for complex equipment like that found in the chemical industry.
Safety first
In complex production plants, it is paramount to have all components supplied with power reliably at all times. However, if the 24 V DC switched-mode power supply is overloaded and therefore reduces the power supply, the “lights go out” in the entire system. In order to prevent such a dangerous situation from arising, the ESS20 electronic circuit breaker protects the 24 V DC loads. The integral current limitation of the ESS20 at typically 1.8 times rated current prevents the 24 V DC supply voltage from collapsing in the event of an overload or short circuit. Selective protection of all 24 V DC automation components, such as electronic sensors, fieldbus couplers, PLCs and drive controls, can be achieved even under unfavourable load conditions (long load line, small cable cross-section) because the device will normally trip at 1.1 times rated current and disconnect the current flow after 3 to 5 s. This key characteristic is already a part of many implementing provisions for electrical equipment and plants.
The European standard EN 60204-1 (Safety of Machinery – Electrical Equipment of Machines) is also of major importance in this connection. Single-pole overcurrent protection against overload and short circuit to EN 60204-1, Section 7.2, has to be provided whenever the current in a control circuit can exceed either the rated value of a component or the ampere capacity of a conductor. The rated current of fuses and circuit breakers should be as low as possible. Thus, the weakest link in this chain must always be the upstream protective device and never the cable. As the ESS20 combines electronic overcurrent protection at rated currents from 0.5 to 10 A with active current limiting and additional electrical isolation, it fully meets these requirements.
Power distribution made easy
It is standard procedure in the chemical industry for a plant to be extended or modernised within a very short time frame. It is therefore vital to retrofit this plant as quickly as possible. Since designing a complex power distribution system is both complicated and time-consuming, more and more customers tend to buy a complete system, tailor-made by E-T-A for their particular application.
The best possible solutions for the entire power distribution and protection of the 24 V DC system are worked out together with a plant designer, for example based on the number of channels to be protected and the type of signalling (e.g. single or group). At the same time, various additional components can be installed, e. g. terminal blocks or switched-mode power supplies. The aim is that these components should fit into the plant operator’s existing system.
In view of the increasing trend towards distributed systems, one top priority for these control cabinets and junction boxes is a very compact design. This is supported by the ESS20’s extremely small width of only 12.5 mm per channel to be protected, including the signal contact. This design can save up to half the normal space on the mounting rail, which can then be utilised to extend the plant. Due to the high complexity involved it is not unusual to develop a complete – customer-specific – protection cabinet. This is often defined as a “factory standard” that significantly simplifies servicing and maintenance work.
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