6.1 Functional Safety (SIL)
6.1.1 Functional Safety
The breakdown and malfunction of process plants and machines can lead to personal, environmental, and material risks. The effects of damage and the probabilities of their occurrence determine the measures required for minimizing risk through fault prevention, fault detection and fault control. The international safety standard IEC/EN 61508 defines the requirements for safety-related systems in a plant. The safety-related systems considered in this standard are based on electrical, electronic and programmable electronics technology. The term "functional safety" describes the part of overall plant safety which depends on the correct functioning of the safety-related systems.
6.1.2 Safety Integrity Level (SIL)
The IEC/EN 61508 standard uses a risk based approach to determine the safety integrity requirements. Each safety function is assigned a SIL (safety integrity level) which specifies the risk reduction required for each defined hazardous event. The remaining residual risk is determined by calculating the probability of failure of all components involved in the safety circuit and their interconnection. There are four different safety levels: from SIL 1 for a low risk to SIL 4 for a very high risk. Two SIL categories are distinguished according to the respective application:
- For safety functions which are activated when required (on demand), the probability of failure to perform the function on demand is specified (Low Demand Mode).
- For safety functions which are continuously active, the probability of a hazardous failure is specified for a given period of time (per hour) (High Demand Mode).
6.1.3 Device Certification
The IEC/EN 61508 standard covers safety-related systems over the entire safety life cycle, i.e. from conception to shutdown or de-installation. In the product development, particular importance is placed on verification. In a series of analyses and tests it must be proved that a device complies with the safety integrity requirement for a certain safety chain. The results form the basis for certification by an independent body. The PolyTrans P 32000, ThermoTrans P 32100, SensoTrans DMS P 32200 and SensoTrans P 32300 transmitters are SIL 2 certified by the TÜV Rheinland. They can achieve up to SIL 3 in redundant configurations.
6.2 Surge Withstand
Fast transients with slew rates of several 1000 V/µs can occur in industrial plants, especially due to switching processes. This kind of overvoltage can lead to damage or destruction of sensitive electronic components.
Newly developed Knick isolation amplifiers are surge-proof according to IEC 255-4. Both the individual circuits (input, output, supply) and the isolation between the circuits are protected against destruction by temporary overvoltages. Testing is performed with positive and negative pulses of 5 kV with 1.2 or 50 µs pulse duration.
6.3 Electromagnetic Compatibility (EMC)
The term “electromagnetic compatibility“ refers to the ability of electrical and electronic equipment to function satisfactorily in its electromagnetic environment without introducing unacceptable electromagnetic interference to other equipment in that environment. The increasing use of all kinds of devices that could emit interference, the constantly increasing use of modern microelectronics in all fields of industry and the interaction of power electronics and microelectronics calls for measures protecting against mutual influence. Electromagnetic interference can cause malfunctions and even complete failure of unprotected electronic devices. The combination of constantly lowered power requirements and higher processing speeds in microelectronics leads to increased sensitivity to electromagnetic interference.
Within the European Union the limit values to be observed with regard to EMC, in addition to other requirements to be met, are regulated by harmonized standards and laws. The Low-Voltage Directive and the EMC standards are relevant for isolation amplifiers. Conformity with the corresponding regulations is shown by the CE labeling of Knick products.
6.4 Protection against Mechanical Loads
Mechanical loads, such as shocks or vibration, cannot be precluded in industrial plants, in vehicles, on ships, etc. The operating reliability of electrical and electronic components must be ensured even in such environments. The VariTrans P 27000, A 26000, P 15000, B 13000 and A 20400 devices were therefore tested according to the rules of Germanischer Lloyd and have been granted the GL approval for environmental category D: Applications with increased heat development and at increased vibration levels, such as devices for installation on combustion engines or compressors.
6.5 IP 68 Protection
With the new M12 field housing series, Knick has considerably simplified decentralization in the field of analog signal transmission. The DuraTrans M12 and IsoTrans M12 devices are protected to IP68 and are installed without an enclosure or additional protective housing directly on the machine. Even harsh ambient conditions are no problem.
The housing material PBT (polybutylene terephthalate) is extremely resistant to mineral lubricants and alcohols and complies with flammability rating V-0 according to UL 94. Vacuum encapsulation reliably protects the electronics against environmental influences. For connection, the tried and tested M12 circular connectors are used.
Knick offers a five year warranty for isolation modules. On account of their widely recognized quality and reliability, they are used widely in industrial measurement and control technology around the world.
The Knick quality management has been certified according to ISO 9001 for all areas of the company and has also contributed to Knick being certified according to KTA 1401 as a recognized supplier of isolation amplifiers for use in nuclear power stations since 1994. Additional corresponding certifications related to individual safety-related areas of nuclear power plants have also been issued.
Intelligent circuits with minimal use of components allow reliability values that were previously considered to be impossible – for example, the MTBF (Mean Time Between Failures) of the VariTrans B 10000 standard-signal isolator is 440 years.
The quality of all Knick products is ensured by computer-supported individual testing. For the isolation modules, this means that each individual device is checked again for perfect functioning and compliance with all data after the functional test, adjustment, encapsulation where necessary, and high-voltage testing. Only then is the device released for delivery.