This article tells details about what are requirement for Juncion boxes for instrumentation and electrical works. Instrument junction boxes shall be certified for use in the hazardous area in which they are installed. For intrinsically safe circuits, boxes certified Increased Safety Ex e shall be used. For non-intrinsically safe circuits, boxes certified Explosion Proof / Flame proof Ex d shall be used. Boxes shall provide IP 66 ingress protection.
The construction of junction boxes shall be as under:
a) Junction boxes with Increased Safety
Junction box construction shall be 1.5 mm 316L SS. Boxes shall be bottom entry with a removable 316 SS gland plate. The gland plate shall be drilled and tapped, 6 mm thick for tapered threads and 3 mm thick for parallel threads.
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The plate shall be bolted to the box with 316 SS bolts, sealed with a neoprene gasket and fitted with an 8 mm earthling stud. All incoming cables shall enter from the sides of the junction box. Outgoing cable shall be from the bottom of the junction box. Boxes shall have four external mounting lugs, two top and bottom, drilled with 10 mm fixing holes. A full size removable stainless steel gear plate shall be fitted inside each box.
Terminals shall be Ex e rail-mounted tunnel-type made of melamine. Wiring ducts shall be coloured blue. Boxes shall have doors hinged minimum at two points with a neoprene seal fitted all round. Hinges shall be the lift-off pintle type, constructed of stainless steel; piano hinges are not acceptable. A restraining mechanism shall prevent
opening more that 160o.
Screwdriver-operated door latches shall be fitted at the top and bottom. In addition to the manufacturer‘s certification label, each junction box shall have identification label attached to the box with 316 SS screws in a manner which will
not invalidate the hazardous area certification. The label shall be constructed of whiteblack- white laminated plastic. The label shall include an engraved Tag No. An additional tag stating ―Intrinsically Safe Circuits‖ shall be installed. This shall be blue-white-blue laminated plastic.
b) Junction boxes Explosion Proof
Explosion proof junction boxes shall be constructed from 316 SS. Boxes shall have at least four external mounting lugs, drilled with 10 mm fixing holes, and an 8 mm external earth lug. A full size removable stainless steel gear plate shall be fitted inside each box.
Cables shall enter from the bottom, and 20% spare entries shall be provided. Cable entries shall be drilled and tapped to suit the required glands, and fitted with certified 316 SS plugs. Junction box lids shall have cast hinges integral to the box with stainless steel pins. Each junction box shall have an identification label, in addition the manufacturer‘s certification label, attached with 316 SS screws in a manner, which will not invalidate the hazardous area certification. The label shall be constructed of whiteblack- white laminated plastic. Terminals shall be rail-mounted tunnel-type, made of
melamine.
Cable Junction Boxes:
Separate junction boxes shall be used for intrinsically safe wiring. Junction boxes in any cable path from field to control room shall be limited to one only. Instrument junction boxes shall not be used for electrical power cables. Field junction boxes shall be certified for use in hazardous area in which they are installed.
For intrinsically safe circuits, boxes certified Increased Safety Ex e shall be used. For non-intrinsically safe circuits, boxes certified Explosion-proof / flame-proof Ex d shall be used. All enclosure cable entries shall have NPT threads. Where this is not possible a certified 316 SS adaptor shall be fitted.
All Ex e and Ex d enclosures shall haveall spare entries sealed with suitably certified 316 SS plugs. Cables shall not enter through the top or sides of outdoor enclosures. For indoor enclosures, bottom entry is preferred. Entries shall be suitably spaced to enable ease of installation of cable glands. Tapped cable entries shall be of good quality, without stripped or damaged threads.
The thread shall be continuous and shall permit the cable gland to fit squarely against the gland plate (or enclosure). Where the gland plate is not thick enough for tapping, cable glands shall be fitted with double locknuts. The threads of all cable glands and adaptors shall be greased on assembly. Cable glands fitted to all Ex e enclosures, and industrial enclosures, shall have a nylon or lead sealing washer between the Cable Gland and Gland Plate (or enclosure) to ensure IP 66 protection. Sealing washers shall not be fitted to glands entering Ex d enclosures.
All junction boxes shall be effectively sealed, once uncrated or fitted into position. All lids shall be replaced during any break in work. Where this is impractical, e.g. large multi-cable junction boxes, a heavy PVC sheet shall be used to cover the junction box to exclude rain and dust.
All terminals (including unused terminals) shall be tightened down. For communing of terminals, feed through cross connection bar shall be used. Comb connectors shall not be used.
Requirements of Ex d enclosures:
• Minimum degree of protection shall be IP 66
• Ex d enclosures shall not be drilled or machined under any circumstances
• Damaged enclosures shall be completely replaced Flange surfaces shall be clean and undamaged
• All fixing bolts shall be present and tightened to the correct torque
• All glands, adaptors, reducers and plugs shall be certified
• All metal flanges and box lids shall be sealed after assembly with a greaseimpregnated cloth.
Requirements of Ex e enclosures:
• Minimum degree of protection shall be IP 66
• Damaged enclosures shall be completely replaced
• The contents of an Ex e enclosure shall not be altered without the prior
approval of the Company‘s Electrical or Instrument Engineer.
• Damaged terminals shall be replaced with approved Ex e terminal
• All glands, adaptors, reducers and plugs shall be certified