Hazardous area classification, Signal isolation, methods of protection in the dangerous atmosphere.

Classification of hazardous areas

The codes used by the different manufacturers subdivide hazardous areas into zones.

Zone 0: In this zone, an explosive gas-air mixture is present continuously or for long periods of time.

Zone 1: In this zone, an explosive gas-air mixture is likely to be in normal operation.

Zone 2: In this zone, an explosive gas/air mixture is unlikely to be in normal operation, and if it does, it will only be for a short period of time.

Signal isolation:

When working in the field of chemical or petrochemical industries, the explosion risk factor must be taken into account.

As a consequence, it is necessary to install a system that exits in the safe zone from the dangerous zone. That means the equipment installed in the panel and in the field respectively. There are basically two types of signal isolation.

Flameproof (In America explosion-proof) enclosure.

Intrinsically safe enclosure.


The first of them is used whenever the conditions of the current, intensity of heat produced by the connected elements, exceed the limits allowed by the CENELEC Standards, or any other that is used in each specific case.

This type of installation requires that the connections be made through a box.

Intrinsic Safety:

It is the most widely used, is based on installing primary and final elements with intrinsic safety certification, that is, that their parameters do not exceed the limits set by the CENELEC Standards (in English: European Committee for Electro technical Standardization), such as:

Voltage: 30 volts

Current:  50 milliamps

Depending on the technology used to develop the engineering of the system, the mentioned elements can be connected through Zener safety barriers or galvanic isolators.

The digital signals are connected through suitably certified serial repeater relays. The following sections describe, in a simplified way, the characteristics of each of the connection types mentioned. Simply put, an intrinsically safe system can be divided into three parts.

Interface, in a safe area, between the non-intrinsically safe equipment located on the panel and the intrinsically safe system located on the ground, an example is the Zener barrier.

Wiring, boxes, and accessories to connect the interface with equipment located in a hazardous area plant located equipment. For example, a 2-wire transmitter with 4 to 20 mA of serial.

Protection methods in dangerous atmospheres.

The CENELEC EN-50014 standards are bound by the features that are common to all protection methods.

In America, the term explosion-proof is used for the same concept as in Europe it is called flameproof. So care must be taken to explain it correctly

The most common protection methods are described below, with the indication of the corresponding symbols.

Pressurization (Ex p): It is a type of protection by which the atmosphere surrounding the enclosure of an appliance is prevented from penetrating inside it, maintaining an inert protection gas inside at a pressure higher than the one that surrounds the box. It is used in analytical equipment and other areas where the use of other techniques would be impossible.

Explosion-proof enclosures (Ex d): It is the flameproof protection technique, in which the parts that can ignite an explosive atmosphere are placed inside an enclosing box capable of withstanding the pressure developed during the internal explosion of the explosive mixture, and avoiding the transition of the explosion to the dangerous atmosphere surrounding the box.

This technique is used for instruments whose power level is very also and cannot be eliminated.

Increased safety (Ex e): This technique obtains safety by applying additional measures to the equipment to increase the protection against the possibility of excessive temperature and the occurrence of arcs or sparks.

In practice, it translates into rugged construction, additional insulation, and new mechanical protection.

Intrinsic safety (Ex i): It is a protection technique whereby devices that contain circuits designed under this concept are incapable of occasionally exploding the surrounding atmosphere, limiting energy and surface temperature. In turn, it has two applicable standards, such as are.

Where security is maintained with up to two failures produced.

Where security is maintained with a single failure.

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