The explosion-proof principle of intrinsically safe electrical equipment is: By limiting the various parameters of the electrical equipment circuit, or taking protective measures to limit the spark discharge energy and heat energy of the circuit, so that the electrical spark and thermal effect generated under the normal operation and the specified fault state can not ignite the explosive mixture of the surrounding environment, so as to achieve electrical explosion-proof, the electrical equipment circuit itself has explosion-proof performance. That is, it is safe from the "essence", so it is called intrinsically safe (hereinafter referred to as the safety type). Electrical equipment using intrinsic safety circuit is called intrinsically safe electrical equipment.

Because the circuit of the intrinsically safe electrical equipment is itself safe, the sparks, arcs and heat generated can not ignite the explosive mixture of the surrounding environment, so the intrinsically safe electrical equipment does not need a special explosion-proof shell, so that the volume and weight of the equipment can be reduced, and the structure of the equipment can be simplified. At the same time, the transmission line of intrinsically safe electrical equipment can be made of gum wire and bare wire, which can save a lot of cables. Therefore, the intrinsic safety type electrical equipment has the advantages of safe and reliable, simple structure, small size, light weight, low cost, convenient manufacturing and maintenance, and is a relatively ideal explosion-proof electrical equipment. However, because the maximum output power of the intrinsically safe electrical equipment is about 25W, the range of use is limited. At present, intrinsically safe electrical equipment is mainly used in communication, signal and control systems, as well as instruments, meters and so on.

Intrinsically safe electrical equipment is divided into two types: single type and compound type. Single-type intrinsically safe electrical equipment means that all circuits of electrical equipment are composed of intrinsically safe circuits, such as portable instruments are mostly single-type. Compound intrinsically safe electrical equipment means that part of the circuit of the electrical equipment is intrinsically safe circuit, and the other part is non-intrinsically safe circuit, such as the dispatch telephone system.

Intrinsically safe electrical equipment, like other types of explosion-proof electrical equipment, is divided into two types of equipment, class I and Class II, due to different use environments. Class II equipment is divided into three levels: A, B and C according to the minimum point current ratio. It is divided into six groups according to the different maximum surface temperatures, as specified in Table 1-3. Intrinsically safe electrical equipment is divided into two grades, ia and ib, according to the different degrees of safety. Class ia refers to the electrical equipment where the circuit cannot point the explosive mixture in normal operation or when one or two faults occur. When working normally, the safety factor is 2; When a fault occurs, the safety factor is 1.5; For two failures, the safety factor is 1. Class ib refers to electrical equipment that works properly and fails to ignite explosive gas mixtures when a fault occurs. When working normally, the safety factor is 2; For one failure, the safety factor is 1.5.

As can be seen from the above safety grade classification standards, the safety degree of intrinsically safe electrical equipment of grade ia is higher than that of grade ib. From the perspective of technical requirements, the intrinsically safe electrical equipment of grade ia is higher and stricter than the intrinsically safe electrical equipment of grade ib. Intrinsically safe electrical equipment is marked "i".

Intrinsically safe circuit is the key to design and manufacture intrinsically safe electrical equipment. The so-called intrinsically safe circuit refers to the circuit design through reasonable selection of electrical parameters, so that the circuit under the specified test conditions, whether normal operation or under the specified fault state generated spark and thermal effect can not ignite the specified explosive mixture of the circuit.

The above "specified test conditions" refers to the test conditions that take into account the most unfavorable factors (including a certain safety factor, the concentration of the test medium, etc.); "Electrical spark" refers to the contact action spark in the circuit (including buttons, switches, contactor contacts, various control contacts, etc.), and the electrical spark generated when the circuit is short circuited, broken or grounded, including static electricity and friction sparks; "Thermal effect" refers to the surface temperature and heat energy formed by overheating of electrical components and wires and the surface temperature and heat energy of electric heating bodies; "Normal operation" means that intrinsically safe electrical equipment works under the conditions specified in the design; "Specified fault state" means that, except for "reliable components or components", all electrical component damage or circuit connection failures related to essential safety performance, such as electrical component shorting, transistor or capacitor breakdown, coil turn short circuit, etc., are specified fault states. "Reliable component or assembly" means a component or assembly that does not experience faults during use, storage and transportation that would affect the safety performance of an intrinsically safe circuit.