Dynamic elaboration on the Art & Science of Hazardous Area Protection
Maximum dissipated power of Terminal Box Enclosure
Let us first define an Electrical Terminal / Junction Box
An electrical junction / Terminal box is a container for electrical connections, usually intended to conceal them from sight for safety & security . ( Wikipedia )
Its a simple device to connect (form a junction) of 2 or more electrical wires by providing a proper physical connection . The basic components are :
1. Enclosure or housing (Box)
2. Terminals or connectors to join electrical conductors
3. Electrical wire / conductors .These wires are brought inside the enclosure and joined together as per requirement
4. Cable Glands
(simple anatomy of a junction box)
Enclosure material can be non-metallic , Stainless-steel, sheet-steel , Aluminum or any alloy mixed specially .
Before doing this we will have to check the following points (parameters).
Let us study this table…
Most of the Ex e Junction Boxes have temperature rating of T6 or T5 (With the cover of the lid of the enclosure securely screwed or bolted)
The heat generating components inside the enclosure are the live terminals & the live conductors . The over-all generated heat is the summation of I²R loss (R= The resistance of the Terminals +Resistance of the connecting conductor in Ω.
I = The current flowing through each conductor & terminal
The manufacturers (the reputed ones only) of Ex e Junction / terminal Boxes have , as part of the certification process, carried out heat loss studies of all of the Junction Box enclosures carrying their respective LOGOS. The results are included in the data sheets.
The information is :-
1. The size of the Junction Box Enclosure
2. The maximum number of terminals of a particular size which can be installed inside that enclosure.
3. The maximum number of wires which can be accommodated in one terminal.
3. A vague guideline on How to mix terminals of different sizes and still keep the overall heat generated inside the enclosure within the T6 or T5 limits.
4. Number & size of the Cable Glands which can be accommodated on each side of the enclosure
For the Junction Box enclosure of size 340.5 X 340.5 X 150 mm
The details of the allowed size & allowed numbers of terminals & cable glands are as follows:
It is understood that the manufacturer have already done his part of the homework and all junction boxes thus manufactured & supplied will not form a source of ignition by exceeding the temperature rating. (Provided the installer is taking care about the ambient limit)
Now any changes, performed in the field , in the following parameters :-
1. Number & or size of the terminals
2. Number & or size of the Cable Glands
3. Number & or size of the conductors
can jeopardize the FINE BALANCE between the above mentioned parameters. The end result may be “NO SAFETY”.