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Limit requirements for surface resistance of non-metallic casings of explosion-proof equipment
In hazardous areas with explosive dust, gases, and vapors, such as coal powder, flour, aluminum powder, hydrogen gas, etc., static electricity may cause fires or explosions, posing a high risk. Therefore, explosive hazardous areas should minimize the use of insulation materials. When selecting electrical insulation materials, it is advisable to maintain 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 electrons in contact with the measured surface.

According to the requirements of the IEC60079 series of 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:
At a relative humidity of ≤ 1 G Ω and (50 ± 5)%;
At a relative humidity of ≤ 100 G Ω and (30 ± 5)%.

2. In Class III explosive dust environments, if the surface area of the non-metallic casing 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 values for surface resistance testing in Class II gas environments mentioned above. For fixed installation equipment that cannot meet the surface resistance limit, an "X" can be added after the explosion-proof sign, and a corresponding warning sign can be added to the equipment, and the user can be informed in the user manual.