The first half of the twentieth century was crucial in the development of electrical safety in environments at risk of explosive atmospheres. 

In those years there were very costly accidents in terms of damage to plants and loss of human lives, the echo aroused by these tragedies prompted the development and demand for safe equipment for use in mines. 

In the United States, mining represented an important sector of the economy of that time, the extraction of coal and iron ore had pushed throughout the nineteenth century the great revolution linked to the railway and the economy it generated. 

As happened in Europe, also on the other side of the Atlantic the evolution of technology and legislation, developed to improve safety at work in mines, found application in different contexts later, in the field of electrical safety in areas at risk of explosive atmospheres.

In 1910 the United States Congress instituted the creation of the "Bureau of Mines". Equipment for use in firedamp mines had already been developed by the early 1900s, but this office aimed to investigate the market and set Standards to ensure safe use in mines.

In 1911 Standards were drawn up for the construction of explosion-proof electric motors and tests to evaluate their effectiveness. However, this first effort did not have the desired effect, the standard being too vague and sparse. At the time, the most popular type of protection was made up of casings equipped with vents which were supposed to reduce the pressure during the explosion. The industry pressed for this solution as the cases, designed according to this criterion, were decidedly thinner and lighter.

With the acquisition of experience, the Standard was extended and integrated several times, the devices were classified into three types: the first referred to devices that produced sparks in normal operation (motors, fuses, switches), the second to devices that produced sparks in case of failure (batteries, terminals), the third included devices that could be operated even outside the explosive atmosphere (plug of a machine).

Devices that produced sparks in normal operation had to be placed in explosion-proof enclosures. Some images extracted from the United States Bureau of Mines Schedule 2F 1951 are particularly significant.

In figure 1, we see some types of joints and fixings used in electrical constructions approved by the United States Bureau of Mines for use in firedamp mines. The relationship with the explosion-proof joints in current use is evident.

In Figure 2, we see some examples of cable entries used in electrical construction in mining equipment approved by the United States Bureau of Mines, bushings and sealed entries are still the mainstay of cable entries in explosion proof devices today.  

Figure 1: Some types of joints and fixings according to the United States Bureau of Mines Schedule 2F 1951

Figure 2: Cable entries authorized under United States Bureau of Mines Schedule 2F 1951

Outside of mining, Extra Hazardous Locations were introduced in the National Electrical Code (NEC) in 1923. 

In paragraph 35A, entitled devices in Extra Hazardous Locations, reference was made to rooms or premises used for the manufacture, use or storage of gases, liquids, mixtures, or other highly flammable substances. The use of equipment which could create arcs or sparks was prohibited, unless protected by enclosures approved for the purpose. 

All wiring was to be in approved protective metal conduit or armored cable, lighting fitting to be fitted with vapor proof globes and fitted with protective cages. Switches and motors suitable for use were forbidden to be mounted near hoods or vent pipes.

In the United States, the path that led to the development of equipment suitable for use in areas at risk of explosion has followed an almost parallel path to Europe, addressing the issue first in the mining industry and then extending it to electrical devices in the NEC. 

The use of still current techniques demonstrates how much the more traditional methods of protection can boast a history of reliability and safety spanning more than a century.