Limit requirements for surface resistance of non-metallic shells of explosion-proof equipment
In hazardous areas with explosive dust or gases, vapors, such as coal powder, flour, aluminum powder, hydrogen gas, etc., static electricity may cause fires or explosions, posing a high level of danger. Therefore, in explosive hazardous areas, the use of insulation materials should be minimized as much as possible. When choosing electrical insulation materials, it is advisable to consider maintaining a small insulation resistance to prevent discharge when non-metallic components come into contact with live parts. For non-metallic insulation materials, the anti-static ability of the equipment can be evaluated by measuring the surface resistance of the equipment casing.
Surface resistance, measured in ohms (Ω), refers to the resistance between two electrical elements in contact with a measured surface.

According to the requirements of IEC60079 series standards:

1. In Class II explosive gas environments, when the surface area of non-metallic enclosures of electrical equipment exceeds a certain limit value (as shown in Table 1), the surface resistance of the non-metallic material needs to be tested and verified to ensure that, by selecting materials, the surface resistance meets at least one of the following requirements when measured according to IEC 60079-0 26.13:
⑵ At ≤ 1 G Ω and (50 ± 5)% relative humidity;
⑵ At ≤ 100 G Ω and (30 ± 5)% relative humidity.

2. In Class III explosive dust environments, if the non-metallic shell surface area of electrical equipment does not exceed 500 square millimeters, the risk of static electricity may not be considered; If the limit is exceeded, surface resistance testing verification is required. The determination requirements are the same as the limit requirements for surface resistance testing in Class II gas environments mentioned above. For fixed installation equipment that cannot meet the surface resistance limit, the method of adding "X" after the explosion-proof mark and adding corresponding warning signs on the equipment can be adopted, and the user should be informed in the instruction manual.