Page 383 - Handbook of Electrical Engineering

EARTHING AND SCREENING 371

13.4 CONSTRUCTION DETAILS RELATING TO EARTHING

This section describes some of the practical construction and installation details that are required
with metallic frames, casings, cubicles, terminal boxes and cables.

13.4.1 Frames, Casings and Cubicle Steelwork

Frames and casings are the steel or cast iron enclosures of motors and generators in particular.
Frames can also include the base plate of machines and transformers, and these are often channel
iron-welded fabrications. Cubicles are usually fabricated sheet steel box type enclosures used for
switchgear, distribution boards, control panels, fuse boards and terminal chambers.

In all locations these constructions must be connected to the earthing system. Most frames and
casings are bolted to the supporting structure, e.g. steel deck beams, concrete foundation block or
plinth. They are usually fitted with at least two large earthing bosses, which are welded or cast into
the fabrication, separated as far apart from each other as possible, and having threaded entries for
bolts of typically 10 to 15 mm diameter. The size of the bolts is related to the maximum fault current
that can flow in the fabrication. Copper bonding straps are connected to these bosses and similar
ones welded nearby on the deck beams, or onto nearby earth rods or busbars. In some situations the
frames are welded to the deck beams and so the use of bosses may not be necessary. Casings of
machines are seldom welded because the machine will need to be removed for major maintenance,
repair or re-alignment.
Cubicle steelwork is invariably bolted to the floor, a floor frame or to a wall structure, and
therefore bolted bonding straps are used in a similar manner as described above. Most cubicles are
fitted with an internal copper busbar which is bonded internally to the steel. The busbar is used to
receive the bonding connections from internal components, partitions, screening panels, cable glands,
cable armouring, cable screens and gland plates.

13.4.2 Screwed and Clearance Hole Entries

A major part of any power system installation is the termination of all the cables. Cables are terminated
at equipment by the use of cable glands. There are many different types of cable glands, and they
must be carefully chosen to suit their function and environment. Factors influencing their choice are:-
• Indoor or outdoor installation.
• Power or instrumentation cables.
• Weather and particle ingress proofing.
• Mechanical vibration or movement of the equipment.
• Non-hazardous or hazardous environment.
• Corrosive atmosphere.
• Internal construction of the cable, e.g. type of armour, use of core screens.
• Equipment submerged in liquids, e.g. sump pumps, down-hole pumps.
• Gland material, e.g. brass, stainless steel, plastic.
• Total length and type of the entry thread.