In sites where there is a risk of the formation of an explosive atmosphere, we often find the electrical conduit in a metal protective tube (also called conduit conduit). Although it has recently been joined by more versatile armored and non-armored cable systems, with cable gland entry, its use is recommended in plants where production or use and maintenance activities can present dangerous situations for the integrity of the cables.
by Andrea Battauz, R&D Project Engineer of Cortem Group
In sites where there is a risk of the formation of an explosive atmosphere, we often find the electrical conductors  in a metal protective tube (also called conduit).
This constructive choice recalls the approach used in the American standard for electrical constructions in hazardous locations, based on the indications of the National Electrical Code in article 500, indicated by the abbreviation NEC 500.
In Europe, the reference plant legislation in the field of environments at risk of explosive atmospheres is the EN 60079-14. Before its implementation there were national standards, a trace of this remains in the paragraph of the same EN 60079-14 which reads: "the laying of the cables within the protective pipes, and the accessories required for the adequate protection technique and for the place in which they must be installed, may be subject to national approval". 
According to the EN 60079-14, the use of the protective tube in places classified as at risk of explosion is mandatory only for single core cables without sheath and for cables with “easy to strip” sheath indicated in the standard as “easy tear”. [ 3]
Its use, however, is recommended in plants where the production process and the use and maintenance activities can present situations that are dangerous for the integrity of the cables (mechanical risks, exposure to heat or corrosive substances).
Figure 1: conduit with single-core cables inside
The expression “metal pipes” refers to the pipes where the electrical conductors are inserted in protective galvanized steel pipes, consisting of a closed casing with a circular section, without interruptions.
In these conduits the cables are laid and/or replaced by pulling.
We will examine the context in which the conduit is used in conjunction with explosion-proof enclosures and devices in accordance with 'Ex d' method of protection, thus excluding the case in which the pipe is used only as a support or mechanical protection (in which case it is normally indicated as protective tube 'Open').
These pipes, together with the sealing fittings, the explosion-proof enclosures and the dedicated accessories make up the pipe systems and give them the typical appearance like a plumbing system.
Figure 2: pre-assembled skid with connections between the enclosures made of conduit
Conduit that complies with the UNI 7683  standard is considered suitable for the construction of these pipes, which concerns: "seamless and welded galvanised pipes of unalloyed quality steel for conical gas threads, cable holders, for explosion-proof electrical systems", Example of designation: AD PE 1 ½ UNI 7683.
Being a standard issued in 1977, it contains some references to standards that are outdated today. The material indicated is Fe 360, corresponding in the European nomenclature (EN 10025-2: 2004) to the current S235JR steel .
As can be seen in Table 1, this type of steel tube has an external diameter greater than the measurement in inches that designates it (1 inch = 25.4 mm). This is because the inch designation is conventional and originally referred to the actual inside diameter.
These conduits, such as BC series conduits of Cortem Group, are supplied with conical thread at the end, UNI 7683 standard reported the threads indicated as Gk according to UNI 6125, a standard now withdrawn. International standards require that the tapered thread must be made according to the American NPT; however, in Italy the variant with ISO 7/1 thread is widespread and still marketed.
The conduits are supplied after having undergone hydraulic test at 50 bar for at least 10s, tensile test and bending test.
The tube is hot dip galvanised and marked on the length at intervals of less than 1.5m.
In any case, when choosing a pipe and the necessary fittings, care must be taken to ensure that no metal combinations are created causing galvanic corrosion.
Table 1: external metric dimensions of the conduit according to UNI 7683
The system with electrical conduits in pipes has recently been joined by more versatile systems in armored and non-armored cable, with entry into the enclosures by means of a cable gland.
Despite this, this type of plant is still the most widespread system in petrochemical sites at risk of an explosive atmosphere where corrosive environments, extreme environmental temperatures and mechanical risks require high mechanical and chemical protection.
It is not uncommon to find mixed systems that integrate skids with metal pipe ducts alongside connections to cable systems.
Notes, reference standards and bibliography
 CEI 31-108 9.1.2Electrical conduit: set consisting of one or more electrical conductors and the elements that ensure their insulation, their support, their fixing and their possible mechanical protection.
 CEI EN 60079-14: 2015 par 9.5
 CEI 31-108: 9.4.1
 CEI 31-108:9.4.2