Ex “d” protection mode

Without retracing the whole theory about how an explosion occurs, we just remind you that three factors are necessary: the combustible...

Without retracing the whole theory about how an explosion occurs, we just remind you that three factors are necessary: the combustible material, the combustion agent and the ignition energy. The combustible material and the combustion agent, called explosive mixture, must be present in percentage defined within two limits to be triggered by a spark or an electric arc. Beginning from this theory, all the different modes of protection have been studied. Over time, they led to the various standards on how to design and manufacture safe equipment which can be used in areas with potentially explosive atmospheres. The first mode used since the early years of last century, covers the explosion-proof enclosures. In this mode of protection, the energized electric circuits can be in contact with the explosive atmosphere.

However, they have to be enclosed within an enclosure specially designed to withstand the pressure caused by a possible explosion occurred inside the case itself and to prevent the spread of flames outside the enclosure that could ignite the external explosive atmosphere. This method is based on the concept that it is impossible to prevent a gas from spreading. Hence it is impossible to produce electrical equipment inside an airtight enclosure which prevents the ingress of gas. These enclosures are, therefore, built to allow the ingress of gas, but if it comes in contact with the ignition source (an arc or a spark) the explosion will be contained inside and the combusted gases will escape through the fittings between the parts of the enclosure. These fittings are specifically designed to allow the flame to cool as it escapes, so that only the product of combustion reaches outside the enclosure; and by then, it has cooled down and is unable to ignite the surrounding atmosphere.

This mode of protection was the first to be standardized. There is an Italian standard, the CEI 31-1, born in the late 50s of last century, which was one of the first to establish the construction and testing rules. Today this rule, superseded by international and European laws, lives in the spirit of EN 60079-1, which is currently under review by CENELEC. 

Applications

This method can be applied to all low-voltage equipment such as lighting fixtures, panel boards, switches, command, control and signaling units, transformers, low and high-voltage motors and, generally, all equipment which can cause sparks or over temperature during normal operation. The electric circuits have to be enclosed within an enclosure, the heart of the system. These cases do not prevent the entry of any potentially explosive atmosphere, but in case of explosion, do not allow the flame to escape from the enclosures. The combusted gases will escape through the fittings between the parts of the enclosure at a temperature unable to ignite the outside explosive atmosphere, thus avoiding catastrophic explosions.

Main features

The main feature is a strong construction, which guarantees reliability in the long terms. The materials used are usually aluminium alloys and stainless steel. Surfaces joints that come into contact (e.g.: Bodies and lids) are machined to ensure areas which laminate and cool the flame in a linear way or in a more complex way, such as in labyrinthine joints or, more simply, threaded joints.

Reference standards

EN 60079-1 standard that governs this type of protection has been used over the years, both for the explosive atmosphere for the presence of gas and for those with the presence of dusts. In recent months, this standard is under reviewing by the European Technical Committee, as it is following the principle of separation of the modes of protection for gases and dusts. The standard, therefore, will be issued, probably within the current year and it will be dedicated only to the modes of protection for gases, vapors and mists.

Fecha de publicación: 01-03-2010

Tema: Additional information