Flameproof Junction Box Design & Selection

Have you noticed some Junction Boxes have Square Doors while some have Circular Doors, Why?

Various types of junction boxes are available to protect signal and power distribution networks from harsh environmental conditions and avoid explosion hazards. This protection of electrical accessories from harsh environments & isolation between hazardous atmospheres and electrical components is achieved mainly using two methods of protection, Increased Safety And Flameproof Safety. The (Ex-d) Flameproof Safety Protection refers to IEC 6079-1 & Increased Safety Protection Class refers to IEC 60079-7 standard. For ease of understanding and text limit issues, we will speak mainly about Flameproof Junction Boxes only. Both of these standards are readily available on the internet, yet very lengthy, time-consuming & complex to understand. Thus I have tried to write the key principles and summary of IEC 60079-1: Flameproof Protection Standard.

There are various Hazard Protection Standards/Bodies such as ATEX, IECEx, NFPA, PESO, FISCO and several others. ATEX & IECEx are the pioneers and widely accepted benchmark standards, while others are derivatives of these two only, all of them follow the basic classification of hazardous environment.

The Hazardous Areas Classification is done into three groups; Group 1 - Mining Firedamp (Methane), Group 2 - Explosive Gases, and Group 3 - Explosive dust atmospheres. Group 2 is further classified into three parts.

1. IIA - Hydrocarbons (such as Propane & Ethane), Halogen, Sulfur, and Nitrogen (less explosive concentrations and types).
2. IIB - Hydrocarbons (Ethelyne & its compounds), Oxygen, Halogen, Sulfur, and Nitrogen (more explosive concentrations and types).
3. IIC - Acetylene, hydrogen, carbon disulfide. Electrical equipment in the batteries room shall be apparatus group IIC.

Now the real question arrives, how are these classifications done?

The answer is Maximum Experimental Safe Gap (MESG) and Minimum Ignition Current Ratio. We know that the Working Principle of Operation in the Flameproof protection concept is parts that can ignite in an explosive atmosphere like electrical equipment are placed within an enclosure that can withstand the force created and pressure developed during an internal explosion. Therefore if an explosion?occurs inside of the enclosure, it will either be contained within or any escaping flames would be cooled by the flame path. This reduces the risk of igniting an external explosive atmosphere. To find out these two parameters for particular explosive gas we use the below test setup.

• a - interior spherical chamber
• b - exterior cylindrical enclosure
• c - adjustable part
• d - outlet of mixture
• e - an Inlet of mixture
• f - observation windows
• g - spark electrode
• h - lower gap plate, fixed
• i - upper gap plate, adjustable
• X - Experimental Gap

1. In an enclosed cylindrical chamber another smaller spherical chamber is placed. the spherical chamber has two parts one is stationary which is attached to the body of the external chamber & another is an adjustable part with a piston.
2. Both the chambers are filled with explosive gas with known volume, later the adjustable part is moved so that a precisely known distance is kept between the faces of stationary and adjustable plates of the internal spherical chamber.
3. Then the spark electrode is used to create a spark so that the ignition happens in an internal spherical chamber. If the explosion inside the internal chamber ignites gas present in the external chamber or explosion does not occur inside the internal chamber, the test is considered to fail. however, if the internal chamber witnessed the explosion and the explosion does not occur in the external chamber then the 'X' gap & ignition current ratio is recorded for the respective volume/mixture of gas.
4. In this same manner, for various combinations of volumes and mixtures of the particular explosive gas test is conducted and the most efficient Maximum Experimental Safe Gas & Minimum Ignition Current Ratio is recorded.

Now that we have understood how this bifurcation of Group 2 gases is done, we can cater to our main question which was why these two styles of doors/covers for Junction Boxes? From the above table, we get the idea that the explosive gases in the IIC category have the min MESG, thus we have to provide the maximum Flame Path for the IIC category.

Flame Path - the place where the corresponding surfaces of two parts of an enclosure, or the conjunction of enclosures, come together and which prevents the transmission of an internal explosion to the explosive gas atmosphere surrounding the enclosure as shown in the below drawing, dimension d+c (mm).

For IIA & IIB catagory the Flanged door Square Junction Boxes can be used as this type of JBs provide the necessary flamepath to restrict the flames or cool down the hot fue gas resulting from the explosion inside the junction box. However for IIC Catagory Square edge JBs does not provide the required flame path hence we select the Round Door or circular Threaded Cover JBs for IIC catagory Applications.

The Dimesion of Flame paath, Stud size regulation, Numbers of Cable Entries per face of JB & Selection of Cable Glands for the Sqaure Edge Junction Box and Thread size and characteristics vary for Junction Boxes as the Volume and Size changes, however the minimum requirement is given in IEC 60079-1 Standard.

For those who want to read this standard its older verision is available on internet completely free or you can visit to this youtube video to quikly understand the deep concepts related to Ex-d (Flameproof) Protection Method.

Apart from the Flameproof Protection Method there are various methods of protection for Hazard Safety which we shall discuss in future. Thanks for reading this lengthy article, I hope it has added the value in your life. If any discrepencies found please let me know I will be delighted to expand my knowlege furthermore!!