Can the explosion-proof grade Ex db IIC T6Gb of the on-site instrument SPD replace Exd IIC T4 Gb?

Conclusion: The Ex db IIC T6 Gb rating fully meets and exceeds the requirements of Exd IIC T4 Gb and can be safely used as a replacement. This conclusion is based on the core principle of "higher ratings are compatible with lower ratings" in the selection of explosion-proof equipment, and the specific analysis is as follows:

Explosion-proof types "db" and "d": The "db" in the marking, according to the latest mandatory Chinese national standards GB/T 3836.1-2021 and GB/T 3836.2-2021, represents "flameproof enclosure protection" with a "high protection level." It fully covers the "explosion-proof" requirements represented by "Exd" in the old standard and is even more stringent in terms of protection level. Therefore, "db" is an upgrade and encompasses "d."

Equipment protection level "Gb": This indicates that the equipment is suitable for explosive gas environments in Zone 1 and Zone 2.

Gas group "IIC": This indicates that the equipment can withstand internal explosions of IIC class explosive gases (such as hydrogen and acetylene) without damage and can prevent internal flames from spreading to the external environment. The coverage relationship of gas groups is IIC > IIB > IIA, meaning that IIC-rated equipment can be safely used in IIB and IIA environments.

Temperature classes "T6" and "T4": Temperature classes (T-Code) specify the maximum temperature that the outer surface of the equipment may reach under normal operation or approved fault conditions. This temperature must be lower than the minimum ignition temperature of the flammable gases in the installation environment.

The T4 group allows a maximum surface temperature of 135°C.

The T6 group allows a maximum surface temperature of 85℃, which is a more stringent requirement.

When selecting equipment, it is essential to choose a device with a temperature class no higher than (i.e., a T-value no greater than) the ignition temperature class of the surrounding gas. T6 equipment, with its lower surface temperature limit (85℃ < 135℃), is suitable for all locations requiring T4, T5, or T6, providing a greater safety margin. Therefore, T6 can cover the requirements of T4.

In summary, from the explosion-proof type and equipment protection level to the gas group and temperature class, Ex db IIC T6 Gb is technically equivalent to or superior to Exd IIC T4 Gb, and therefore can be used as a substitute.

The detailed temperature parameter requirements for groups T1, T2, T3, T4, T5, and T6 are as follows: