Explosion-proof mark

In NEC’s table for classifying explosive gases by temperature, the NEC groups are: T1； T2，T2A，T2B，T2C ； T3，T3A，T3B，T3C ； T4，T4A ； T5 ； T6 ； The T4x mentioned by the poster likely refers to a general term for T4 and T4A; 4x denotes 4 or 4A, and the surface temperature indicated by the electrical meter for T4 is 135°C ； The surface temperature of the electrical instrument corresponding to T4A is 120°C ； Without a transmission line, it is not possible to upload the photo of the \"NEC Table for Classifying Explosive Gases by Temperature\" that was taken. The original poster can refer to page P1090 of the \"Petroleum and Chemical Industry Automatic Control Design Manual\", where \"Table 5-10-3 NEC Table for Classifying Explosive Gases by Temperature\" is listed. This post was last edited by Meihua Piaoxiang on 2009-4-2 at 11:42.]

I’ve learned it. Since I am now a moderator for another section, I’m not able to give a score, but thank you anyway

HOLD means pending, as I understand it. Anything that is uncertain or lacks sufficient conditions should be marked as HOLD – pending decision

Temperature (hot surfaces) is an important ignition source for explosive gases to explode. Every explosive gas has a specific temperature. At this temperature, it will ignite even in the absence of any external ignition source. Generally, this temperature is referred to as the ignition temperature (AIT) of the gas. It is another important characteristic parameter that reflects the ignition characteristics of explosive gases. 　　In accordance with the recommendations of IEC standards, China classifies explosive gases into six groups, from T1 to T6, based on their ignition temperature. North America’s classification of temperature groups is basically consistent with IEC’s, except that some of the temperature groups are divided into more detailed subcategories. Table 1-2 shows the correspondence between the two grouping systems. 　　As can be seen from the table below, the ignition temperatures of different explosive gases vary greatly and are not identical. The gas in temperature group T1 has the highest ignition temperature, while the gas in temperature group T6 is the easiest to ignite. In practice, we should strictly control the maximum surface temperature of electrical equipment to prevent it from igniting the most flammable explosive gas mixtures present in the environment in which the equipment is used; in other words, the maximum surface temperature of the equipment must not exceed the ignition temperature of the gases it may come into contact with. Therefore, in terms of the maximum surface temperature of electrical equipment, any electrical equipment designed for use in gas environments of temperature group T6 will also necessarily meet the requirements for use in gas environments of groups T1 to T5. 　　Appendix 108 lists the grades and groups for the vast majority of flammable gases and vapors. With the help of this appendix, we can look up the respective groups and subgroups for specific flammable gases and vapors. Relationship between temperature categories and ignition temperature: Chinese standards, IEC European standards, North American standards – Ignition temperature in °C, Ignition characteristics T1, T1

The above information is taken from http://www.fmdwq.com/fmdwq_Article_8498.html – knowledge on explosion protection for electrical equipment. There is also other related information on instruments; you can take a look if you are interested. There are too many regulatory standards related to explosion protection, which is quite troublesome.