Intrinsic safety isolation

In principle, intrinsic safety - i is a characteristic of an electric circuit. It may run within an autonomous apparatus, e.g. within a hand lamp. The electric circuit, however, may be formed just as well by two (or more) apparatus, e.g. a transmitter which is connected to a limiting stage (in the following described as Ex i-isolator ). 

As a rule, this Ex i-isolator is installed in a safe area and connected to additional apparatus not ex-protected, e.g. to a single loop controller or to a PLC (Programmable Logic Controller). Therefore, in the field of intrinsic safety two types of apparatus are distinguished ... 

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. 

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. 

Currently, remote I/O systems of several manufacturers are available for operation in zone 1 or Division 1. To a large extent, these systems are based upon the technology of intrinsic safety. The power supply only is designed according to another type of protection, e.g. flameproof enclosure - d A remote I/O suitable for hazardous areas may be an integration of input (I) and output (O) assembly units of automation systems and of Ex i-isolators of classic design. The installation may be made in a hazardous area and supersede the classic field distribution box (terminal box). Standardized fieldbus systems may be used for data transmission from/to the automation system. These remote I/O systems for hazardous areas are characterized as follows ... 

For each interconnection of an intrinsically safe circuit with an associated apparatus a proof of intrinsic safety - i shall be performed on the basis of the safety-related maximum values. As far as the simple standard design given in this example (Fig. 6.218) is concerned, it is assumed that the associated apparatus only - the Ex i-isolator - can feed electrical power into the intrinsically safe circuit. Therefore the safety-related parameters are marked as out-parameters with the subscript o. On the other side, the intrinsically safe apparatus may consume electrical power only. Its values are marked as in-parameters, subscript i. ... 

Safety barriers (without galvanic isolation) ensure intrinsic safety only, when connected to the equipotential bonding system. 

Prevention Barrier for dropping object, ventilation, process control, use of barriers [e.g., intrinsic safety (IS)] Quick isolation, maintenance/operating procedure... 

Intrinsic safety ground This is a grounding system with a dedicated conductot isolated from... 

A short discussion on earthing/grounding systems for IS has been covered in Chapter X. Here, the systems will be discussed in greater depth. In most of the cases, reference voltage point of computer and I. C systems and the barrier bus bar are linked, making the earth returns are combined. Combined earthing helps to avoid interference and ensures intrinsic safety. When isolators are used, the barrier bus bar may be omitted and the screens of the field wiring are connected to the 0 V rail of the system. Safety barriers shall be connected to the main electrical system earth or... 

The signaling relay (intrinsic safety) is controlled through a power supply galvanically isolated from the rest of the installation. This insulation helps to reduce the risk of wrongly recharging and activating in the case of a cable mpture. 

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. 

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 ... 

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 ... 

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... 

The redundant computer DIGISAFE XME is based on a redundancy management protocol that allows two coded uniprocessor computers to work as symmetrically as possible. Each computer (called units) is intrinsically safe, and the maintenance of consistency between the execution contexts of the two units allows the switching or transition to the non-redundant (isolated) unit at any time, without requiring special precautions at the application or system level. This last point is extremely important because it significantly reduces system complexity and the associated safety analysis. 

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