Safety barriers, intrinsically safe

For intrinsically safe circuits, a basic protection measure is a safe isolation from all other non-intrinsically safe circuits. With the exception of safety barriers a safe galvanic isolation is required. As a rule, this is guaranteed by observance of geometrical distances. It is differentiated between ... 

Safety Barriers. Figure 1 illustrates an application employing intrinsically safe electrical circuits for the demilitarization of ammunition. Three separate areas are required for this application - one area, classified as non-hazardous, to serve as the control and loading area a second area, classified as hazardous, where the actual demilitarization is accomplished and a third area, classified as non-hazardous, is required for the hydraulic pump due to the level of noise produced. 

Associated apparatus is commonly installed in a safe area. Many applications of intrinsic safety in remote control and monitoring instrumentation are assembled in such a way that an intrinsically safe apparatus, e.g. a sensor or actuator in the hazardous area, is connected with an associated apparatus, e.g. a safety barrier or an Ex i-isolator in the safe area (see Fig. 6.196). With that, the associated apparatus takes over the function to safely limit current and voltage in the intrinsically safe circuit to permissible values. 

Due to the galvanic isolation lacking between intrinsically safe and non-intrinsically safe electrical circuits the power-limiting characteristic of components commonly used for it (e.g. small transformers, optocouplers, relays) is lacking as well. So, a robust safety barrier shall be constructed. 

The safety-related parameters U, I, P, C and L can be matched to the corresponding values of the relevant intrinsically safe apparatus (sensors, actuators etc.) by variations of rated voltage and series resistance. Intrinsic safety may be engineered in a very flexible way using safety barriers. After all, even hopeless cases may be put into practice frequently. Additional advice for engineering is given in Section 6.9.5. 

A protection against polarity reversal cannot be realized for the non-intrinsically safe side of a safety barrier without increasing its series resistance which is considered to be frequently disturbed. One solution consists of the implementation of a second fuse so that it can be replaced if the safety barrier has been operated at reversal polarity or supplied with overvoltage (see Fig. 6.205). The internal fuse protecting the Zener diodes as well as the other components shall be guarded against external access. So, safety barriers are encapsulated for the most part. 

If a high quality equipotential bonding system cannot be put into practice or a steady supply voltage cannot be guaranteed, a galvanic isolation between intrinsically safe and non-intrinsically safe electrical circuits is recommended. Such apparatus, e.g. power supplies for transmitters or switching repeaters, are equivalent to safety barriers with respect to the philosophy of intrinsic safety. The additional feature may be seen in the galvanic isolation and functional characteristics which may be practicable. 

Externally, the only difference between this Ex i-isolator (Fig. 6.207) with galvanic isolation and a safety barrier is its increased size. The complex electronics and, of course, the components limiting current and voltage are hidden inside. In this example, a trip amplifier for DIN rail mounting is shown, which is suitable for operating temperature sensors in intrinsically safe circuits. The marking is ... 

In most cases, an intrinsically safe circuit is put into practice so that a safety barrier or an Ex i-isolator or an Ex i-input/output is connected to measuring devices or positioning elements (sensors, actuators) located in a hazardous area via a cable (see Fig. 6.217). Hence, explosion protection cannot be purchased ready for use for the most part, but it has to be put into practice by the planner and electrician. Only in cases where apparatus and cables are... 

Ordinary electrical equipment cannot be installed in zone 0, even when it is flameproof protected. However, many chemical and oil-processing plants are entirely dependent upon instrumentation and data transmission for their safe operation. Therefore, very low-power instrumentation and data-transmission circuits can be used in special circumstances, but the equipment must be intrinsically safe, and used in conjunction with a safety barrier installed outside the hazardous area. Intrinsically safe equipment must be marked Ex ia or Ex s , specially certified for use in zone 0. 

By definition, an intrinsically safe circuit is one in which no spark or thermal effect is capable of causing ignition of a given explosive atmosphere. The intrinsic safety of the equipment in a hazardous area is assured by incorporating a Zener diode safety barrier into the control circuit such as that shown in Fig. 13.6. In normal operation, the voltage across a... 

Intrinsic Safety. Division I areas can also use intrinsically safe barriers to provide an "explosion-proof" installation. These barriers must be located and connected in accordance with the rules for intrinsically safe installations, such as ISA RP12.6. 

The safety profile PROFlsafe defines the connection of intrinsically safe equipment (emergency stops, immaterial barriers, locks) to programmable controllers on PROFIBUS. The specific area of safety, most of whose constituents were previously connected by wire, may benefit from the multiple strengths of open communication on PROFIBUS. 

Applying the barrier model can be complex, and in many cases safely integrity requirements are applied to equipment failure by a simpler means such as a risk classification scheme, as discussed in (Pierce 2005). There is no intrinsic problem with using this method provided that the function and performance requirements for the equipment in question are also shown to be correctly established and it is recognized that safety integrity requirements and function and performance re-qirirements are interdependent (Fowler et al. 2007). 

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