The following points highlight the six main types of insulators generally used in low or medium voltage. The types are: 1. Pin Insulators 2. Shackle Insulators 3. Stay-Insulators or Guy-Insulators 4. Reel Insulators 5. Egg-Type Insulators 6. Aerial Fuse.
Type # 1. Pin Insulators:
An insulator set on a pin or a bolt is called a ‘pin insulator’. Both ends of the pin are threaded. The thread at one end of the pin remains tightly fixed with the thread inside the insulator, while the thread at other end of the pin is fixed with a bracket or a cross-arm by means of nuts.
There are grooves on the top of the insulator as well as at its sides. This enables a person to bind the line conductor strongly with the insulator. Pin insulators are made of different designs according to supply voltage of the line. A sketch of a pin insulator used in low or medium voltage lines is given in fig. 267.
In an overhead line when several conductors are drawn side by side in horizontal configuration, wire is drawn over pin insulators fixed to the cross-arms which are attached to intermediate poles in between starting pole and end-pole. If the line is quite straight, a conductor is fixed to the top groove of the insulator.
But if the line is deviated from the straight route at an angle not exceeding 15° to 20°, the conductor is fixed to the groove at the neck of the insulators. In addition to keeping the conductor fixed to itself a pin insulator is also used for installing horn-gap lightning arrestor etc. on overhead lines.
Type # 2. Shackle Insulators:
For low and medium voltage overhead lines both pin and shackle insulators are used. But at present in our country use of shackle insulators has increased many folds in comparison to that of pin insulators. The main reason behind this is that now-a-days in medium voltage overhead lines conductors are not arranged in horizontal configuration.
Almost everywhere at present the conductors are arranged in vertical configuration on one side of the pole. This causes inconvenience in drawing line conductors on pin insulators fixed to cross-arms. In this arrangement generally shackle insulators remain fixed to intermediate poles with the help of D-iron clamps.
At the terminal poles, i.e. at poles where from the line starts or where the line comes to an end, and at poles where the line deviates from the straight route farming a large angle, the shackle insulators remain fixed with cross-arms with the help of shackle straps. Sketches of shackle insulator and fixing clamps are shown in fig. 268 and in fig. 269.
At the middle of a shackle insulator there is a groove into which a line conductor remains fixed. Three different sizes of shackle insulators are available in the market. These sizes are 50 mm x 65 mm, 75 mm x 90 mm and 100 mm x 115 mm. usually the second and third sizes are used in main lines, while the first size is used for low voltage service connection to a house.
Type # 3. Stay-Insulators or Guy-Insulators:
Stay-wires are fixed to certain Roles of an overhead line system. This wire has two parts. The upper part remains fixed to a pole with the help of a clamp and the lower part to a stay-rod which is grouted in an earth pit. In order to keep the upper part insulated from the lower part, stay-insulator or guy-insulator is inserted at the junction of two parts. This junction should remain at a height of at least three metres from the ground level. A sketch of the stay-insulator is shown in fig. 270.
Stay-insulator is made of porcelain in a specially designed shape. There are two holes in it at right angles to each other. The lower part of the stay-wire passes through one hole. The other end of this wire is tied to the ground with the help of a stay-rod. The upper part of the stay-wire is passed through the other hole and is tied to the pole.
Stay-insulators available in the market are usually of three different colours—green, brown, and white. This type of insulator is very strong and durable, and its special feature is that even when it wears out, the stay-wires are not detached.
Type # 4. Reel Insulators:
Where overhead lines are drawn in vertical configuration, reel insulators are used for tying up phase-type ‘safety device’. A sketch of a reel insulator is given in fig. 271. There is a groove at the middle of this insulator and a whole right at its centre. In vertical configuration earth wire remains at the top and neutral wire at the bottom.
The wire for the safety device is passed through the hole of the reel insulator below, and its two ends are directly tied up with the earth wire above. At the middle of the reel insulator the neutral wire remains tied up on the upper part of the groove. This arrangement eliminates the possibility of short-circuit between earth wire and the neutral wire.
Besides safety device, reel insulators are also used in catenary system of house service connection. But comparatively large size of insulator is used in that case.
Type # 5. Egg-Type Insulators:
For binding cross lacings (small pieces of wire) of cradle guard if neutral wire is used at one side and earth wire at the other side, arrangement must be provided to keep the neutral wire insulated from the earth wire. For this purpose generally egg type insulators are used with the lacings on that side where neutral wire is drawn.
The insulator should be as near as possible to the neutral wire. The reason behind it is that, on one side-the live lines will come in direct touch with the earth wire if snapped, and on the other side the egg insulator will keep the neutral wire insulated from the earth wire.
Type # 6. Aerial Fuse:
For fixing a fuse-wire in an overhead line, a special kind of fuse-holder made of porcelain is used. At each of the two sides of the fuse-holder there are two terminal screws. With the screws at one side, the line wire remains connected by means of nuts, while with the screws at other side the fuse-wire is fixed up by means of nuts with specially designed head. A sketch of an aerial fuse is shown in fig. 272. At the time of drawing a house-service connection from an overhead line at low or medium voltage, service connection is drawn from live line or phase wire through this fuse.