At most air-insulated substations, the arrester mounted at the transformer is close enough to other equipment to provide effective protection from flashover.

Line Entrance Arresters

However this applies only if the breakers are closed. If a breaker is open, the insulators (such as pin insulators) on its line side – including its line side bushing – are not protected. In this regard, installing line entrance arresters will protect all points within the station when critical breakers are open – and this applies to all system voltages, including distribution. In fact, because insulation withstand levels are lower at distribution voltages risk becomes even higher.

Under hung configuration Importance of Line Entrance Arresters
Under-hung configuration line entrance arresters:
It’s probably worthwhile defining a line entrance arrester since it’s possible other names are also being used to describe it. As discussed above, this terminology applies to an arrester mounted on the first structure on an incoming or outgoing line at a substation. I estimate that about 30% of all substations with breakers are currently equipped with line entrance arresters according to this definition. Possibly, some readers may ask: “why waste money on an arrester at that location?” Other might wonder: “I thought everyone used them?”

Line entrance arresters can used with some electrical insulators to protect the lines, such as porcelain pin insulator and post insulatos.

Purpose of Line Entrance Arrester
The basic role of a line entrance arrester is to protect a circuit breaker that is ‘open’ from any lightning surge that enters the station along a connected line. Since it’s generally accepted that breakers on incoming lines are usually not left open for long, one might conclude there is little cause for concern. True, breakers normally remain closed. But if there’s a lightning induced fault on the system, the breaker is called into action to interrupt it. That means it can be open for some 100 ms or longer whenever a line fault occurs. Electrical insulator can protect the circuit. The arresters can be protect the flashover. This may seem a short duration yet, when it comes to lightning, it’s quite long. Since most lighting events include multiple strokes, the resulting surge can present itself to the power system with anywhere from 2 to 14 independent surges of different magnitudes, each separated by some 50 to 150 ms.