In the world of devices suitable for use in potentially explosive atmospheres, we tend to define equipment based on their mode of protection: explosion-proof enclosures, increased safety motors, limited-breathing lighting equipment or intrinsically safe sensors.

Yet, before responding to the dictates of the specific mode of protection, there are a series of general requirements that all equipment must comply with. The regulatory reference for these requirements is the IEC/EN 60079-0 standard.

We don’t want to examine the general requirements in detail, we focus on some particularly representative characteristics of the materials used in devices suitable for use in potentially explosive atmospheres.

The IEC/EN 60079-0 standard makes an initial distinction between metallic and non-metallic materials.

The main materials of non-metallic origin in electrical equipment are plastic and elastomeric materials. Although we find glass and ceramic materials in many applications, from the lenses of lighting equipment to insulators for high voltages, most of electrically insulating parts are made of plastic. Examples are polycarbonate lenses, glass fiber reinforced polyester cases, and the countless parts that act as insulators inside sockets, plugs, connectors or terminals.

The main enemy of plastics is aging, a phenomenon of slow oxidation of polymers with which we are all familiar. The objects that surround us over the years lose mechanical resistance and water resistance, especially those exposed to bad weather and ultraviolet rays in an external environment.

For this reason, the regulatory requirements are particularly stringent: plastic materials must have the "TI" temperature index or the "RTI-mechanical" relative temperature index at least 20 degrees centigrade higher than the temperature reached in service. The same goes to elastomeric materials, with the difference that in this case the COT (continuous operating temperature) is taken as reference.

Resistance to ultraviolet rays can be guaranteed via grade (f1) according to the UL 746C standard or, alternatively, with a specific test that simulates solar exposure. [1]

The third critical aspect to take into consideration is the possible formation of electrostatic charges on the surface, which is particularly insidious when the equipment is within reach of the operator who can therefore discharge this charge to the ground through his body and generate a spark.

Among the various options to manage this eventuality there is the reduction of the surface resistivity to a value lower than 10^9 Ohm. [2] This is often achieved using polymeric compounds containing graphite or carbon fibers which increase the conductivity of the compound and give the usual black color to "antistatic" plastic materials.

Very significant for non-metallic materials is the impact test that follows the temperature resistance cycle (thermal endurance) in a climatic cell. Materials stressed by heat and weakened by cold are subjected to the impact test using a mass dropped from different heights. The greater the height of the fall, the greater the energy of the resulting impact, ranging between 1 and 20 Joules.

This sequence of tests highlights the resistance to ageing, low temperatures and UV rays (where required) of the non-metallic material.

Often, to increase its mechanical resistance, glass fiber is added to the polymeric mix, as the very widespread polyester loaded with glass fibers (GRP).

The protection methods are explained and detailed in specific standards: explosion-proof in EN/IEC 60079-1, increased safety in EN/IEC 60079-7 and so on.EN/IEC 60079-0, the general requirements standard, is to be considered valid to the extent that it does not conflict with the standard of the protection method, in which case the latter prevails. [3]

[1] IEC/EN 60079-0 26.10

[2] 10^9 Ohm is the resistivity value measured at (50 ± 5) % relative humidity.

When the risk arises from a paint, with a surface resistance >10^9 Ohm, in the case of group I and II equipment, action can be taken by limiting the thickness of the coating. In fact, it is believed that a thickness of less than 200 mm cannot accumulate a static charge with effective ignition levels (see note 6 Table 8 EN 60079-0).

Acting on the thickness is not considered valid for device group III (powders).

[3] IEC/EN 60079-1 PAR 1