In operation, an overhead line for transporting electricity is continuously subjected to polyphase alternating high voltage on which transient surge voltages may be superposed, as can happen particularly during the operations of opening or closing the line, and as can also happen, exceptionally but with much greater magnitude, in the event of a lightning strike. It is therefore the practice to associate spark-gaps with an overhead line to limit the effects of such voltage surges, and more recently to associate lightning arresters therewith to provide additional protection.
Such spark-gaps and arresters are commonly associated with the line wire support assemblies of the pylons that support an overhead line for transporting electricity.
Document EP-A-0 431 528 includes a FIG. 1 showing a conventional line wire support assembly in which an insulator element made up of a string of insulators is suspended via one end from the underside of a pylon arm via a mechanical suspension structure, with said end being connected to ground. The wire is secured to a load-carrying mechanical structure which is fixed beneath the other end of the insulator element. The assembly also includes a circuit branch having a horned spark-gap connected in series with a lightning arrester between a line wire and ground.
The spark-gap has two horns, one fixed to a first end of the insulator element which is connected to the line wire, and the other fixed to the lightning arrester via which it is connected to ground and to a second end of the insulator element.
The distance between the free ends of the horns of that spark-gap is selected to ensure that an arc is always struck between said ends and thus via the lightning arrester whenever a dangerous voltage surge appears, and in particular in the event of lightning.
In the event of a lightning strike, and during a short time interval, the current which appears when an electric arc is struck between the horns of the spark-gap can be very large, for example of the order of 5,000 amps to 200,000 amps. There is therefore a major risk of the lightning arrester that is connected in series with the spark-gap being destroyed, if the lightning energy is greater than the energy capacity of the arrester, and under such circumstances there is thus a risk of the arc being maintained after the voltage surge has disappeared and until a line-protection circuit breaker comes into action.
It is therefore known to associate a device for flagging a malfunction or a failure to operate in order to indicate that a lightning arrester has failed, with such devices being described in particular in above-mentioned document EP-A-0 431 528. In a conventional assembly as shown in FIG. 2 of that document, the malfunction flag is provided between ground and the lightning arrester with which it is associated so as to carry the current which passes through the lightning arrester in the event of a voltage surge giving rise to an arc between the horns of the spark-gap in series with the arrester.
That arrester must therefore be connected to ground solely via the malfunction flag, and it is held in position in the assembly by means of a support block of insulating material fixed to the end of the insulator element which is connected to ground. Such an assembly presents significant drawbacks insofar as it implies that the lightning arrester is insulated from the grounded metal structure of the pylon. The insulating material support on which the arrester is mounted gives rise to additional risks of mechanical failure and, if the support is to be found in an environment that is hostile, it can sometimes lose its mechanical and electrical characteristics more quickly than expected.
In addition, the malfunction flag which must be visible to an observer, generally on the ground, can itself be hidden by other elements of the assembly because of its position at the top of the insulator element close to the top end of said element.