Page 319 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

The insulators are mounted in required shapes they are

Pin insulator
These are screwed on the cross arm at a pin. The single shed type insulators are used for low voltage lines,
double shed types are used for 33 KV line.

Shackle insulator
These are porcelain insulator, which are used in the overhead lines of factories and workshops. They are preferred
at the starting and terminating ends and at the bends of a low or medium voltage line.
Suspension insulator

These insulators are made of glass or porcelain in the form of disc which can be fastened together in any number
as per the requirement a wire is tied at the bottom end. They are used in EHT voltage line normally one disc on
11 KV, 3 discs on 33KV and 12 discs on 132 KV are used.
Egg or stay wire insulator
These insulators are used in stay wires to avoid any leakage of current in them from the line wires.
6 Stay wire or stay rods

Stay wires are necessary for the poles installed at bends. An egg insulator is mounted at a height of 3m in the stay
wire. A stay wire is always tied in an opposite direction to that of the tension of the line.
7 Guard
All overhead lines are provided with proper guards at crossings.

The guarding methods are
i Cradle guarding
ii Cage guarding
iii Bird guarding

iv Bead guarding
v Trolly guarding
Sag in overhead lines (Fig 5)
• While erecting an overhead line, it is very important that conductors are under safe tension.
• If the conductors are too much stretched between supports to save conductor material, the stress in the
conductor may reach unsafe value and in certain cases the conductor may break due to excessive tension.
• In order to permit safe tension in the conductors, they are not fully stretched but are allowed to have a dip or
sag.
• The difference in level between points of supports and the lowest point on the conductor is called sag.
• The sag should be so adjusted that tension in the conductors is within safe limits.
• The tension is governed by conductor weight, effects of wind, ice loading and temperature variations.

• It is a standard practice to keep conductor tension less than 50% of its ultimate tensile strength.
Sag al Sag a1/T
2

Fig 5

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CITS : Power - Electrician & Wireman - Lesson 106-116