Fatal accident may happen if an overhead line snaps and, while falling on the ground, comes in contact with a man or an animal. In order to assure safety against chances of such accidents, safety devices are used in medium and low voltage overhead lines.
A safety device is usually made of galvanised iron wire. One end of this wire is directly tied up with the earth wire of the line, while its other end is tied up with the neutral wire through a reel insulator or an egg-type insulator. As soon as the live conductor of an overhead line snaps, it comes into contact with the safety device before falling on the ground. This contact causes short-circuit between live wire and earth.
As a result either the line fuse burns out or the circuit breaker trips, and the flow of current through the live line is at once stopped, i.e. the live line is disconnected from the source. Hence, even if the wire falls on the ground, there is no risk of any fatal accident.
Safety devices of different varieties are found to be in use in distribution lines.
Some discussions on these safety devices are given below:
(i) Phase-Type or Box-Type Safety Device:
A phase type or box type safety device is used in cases where overhead lines are drawn in vertical configuration, i.e. one line is drawn vertically above or below the other. The connection of such a safety device is shown in fig. 278. The top most wire of the line is the earth wire and the neutral wire is drawn at the bottom of the configuration.
The phase wires or live wires are drawn in between these two wires. Safety devices are tied up with the lines about 60 cm to 75 cm away from the poles. Two devices are used on both sides of an intermediate pole, but one device is enough for a terminal pole, as overhead line is drawn on one side only of this pole.
At the lower end the wire of a safety device is passed through the hole in the reel insulator and the two ends of this wire are drawn up to top most position and tied up with the earth wire. The neutral wire is placed in the groove of the reel insulator and tightly bound with it with the help of binding wire.
In some service connections supply is given through two live wires (two phase wires in case of a.c. supply and positive and negative outers in case of d.c. supply). As neutral wire is not used in such cases, safety device is tied up with only earth wire at one end. This is known as box type safety device. It remains suspended from the earth wire. Fig. 279 shows the connections of such a safety device.
(ii) Ring-Type Safety Device:
A ring type safety device can be conveniently used irrespective of whether a line has been drawn in vertical or in horizontal configuration. But it is not usually found to be used in overhead lines drawn in vertical configuration.
A galvanised iron ring with bracket remains fixed to the pole. Each ring and its bracket must have good electrical connection with earth wire. Every live conductor of the line is drawn through a ring so that in the event of snapping, it comes in contact with the ring and hence becomes electrically connected with the earth wire. This causes the line fuse to be burnt out immediately as a result of which flow of current through the live wire is stopped. The distance maintained between a ring and the corresponding pole is about 60 cm to 90 cm (two to three feet).
(iii) Tray-Type or Bracket-Type Safety Device:
This type of safety device is specially suitable for an overhead line drawn in horizontal configuration. All live wires drawn parallel to one another horizontally on a cross-arm are covered by a single tray type safety device. This is shown in fig. 282.
A tray made of galvanised iron flat with bracket remains fixed to a pole. Tray and bracket must have good electrical connection with earth wire. As the phase wires or live conductors of an overhead line are drawn over the tray in horizontal configuration, as soon as a conductor snaps, it falls into the tray and thus becomes electrically connected with the earth wire. A tray is kept at a distance of 60 cm to 75 cm (2 to 2.5 feet) away from the respective pole.