In the realm of engineering structural materials, particularly with engineering plastics, it’s crucial to assess not just their mechanical and electrical characteristics, but also their thermal stability and ability to resist static electricity.
Thermal Stability
For explosion-proof electrical equipment, the plastic materials used in casings are required to demonstrate superior thermal stability. In specified testing conditions, the maximum temperature decrease should be 20K compared to the Temperature Index (TI) at 20000 hours on the heat resistance curve.
Anti-static Capabilities
Plastic materials must possess effective anti-static properties, which include measures to avert the generation and accumulation of static electricity. This can be achieved by adding appropriate conductive additives to lower the material’s volume and surface resistivity. When tested under specified conditions (10mm electrode distance), if the surface insulation resistance of modified plastic components does not exceed 10Ω, the material is deemed effective in preventing static build-up.
Beyond modifying the plastic material, static fire hazards can also be mitigated by limiting the exposed surface area of plastic casings (or parts) in explosion-proof electrical devices. Table 1 details the limits on the maximum surface area of plastic casings (or parts), while Table 2 specifies the diameter or width of elongated plastic parts, and the thickness of plastic coatings on metal surfaces.
Maximum Surface Area for Plastic Casings (or Parts)
| Catégorie et qualité de l'équipement | Catégorie et qualité de l'équipement | Surface maximale S/m² | Surface maximale S/m² | Surface maximale S/m² |
|---|---|---|---|---|
| I | I | 10000 | 10000 | 10000 |
| II | Zones dangereuses | Zone 0 | Zone 1 | Zone 2 |
| II | Niveau IIA | 5000 | 10000 | 10000 |
| II | Niveau IIB | 2500 | 10000 | 10000 |
| II | Niveau SII | 400 | 2000 | 2000 |
Maximum Restricted Dimensions for Special Plastic Parts
| Catégorie et qualité de l'équipement | Catégorie et qualité de l'équipement | Diamètre ou largeur de la bande / mm | Diamètre ou largeur de la bande / mm | Diamètre ou largeur de la bande / mm | Épaisseur du revêtement plastique sur la surface métallique / mm | Épaisseur du revêtement plastique sur la surface métallique / mm | Épaisseur du revêtement plastique sur la surface métallique / mm |
|---|---|---|---|---|---|---|---|
| I | I | 20 | 20 | 20 | 2 | 2 | 2 |
| II | Zones dangereuses | Zone 0 | Zone 1 | Zone 2 | Zone 0 | Zone 1 | Zone 2 |
| II | Niveau IIA | 3 | 30 | 30 | 2 | 2 | 2 |
| II | Niveau IIB | 3 | 30 | 30 | 2 | 2 | 2 |
| II | Niveau SII | 1 | 20 | 20 | 0.2 | 0.2 | 0.2 |
Furthermore, plastics used for making casings (or components) of explosion-proof electrical devices should also exhibit excellent flame resistance and pass various tests like heat and cold resistance, and photoaging.
