Starting in 1975, Chubu Electric replaced almost all of the pin-type insulators on its main lines with solid core pin post insulators and increased the main line BIL to 100 kV. This was done since one of the countermeasures the utility employs against lightning is differential insulation designed to induce the surge from lightning to the transformer rather than to the main line. Insulation co-ordination to accomplish this requires a higher than normal main line BIL provided by the pin post insulators, while pole-mounted transformers are equipped with a primary bushing having an outside BIL of 60 kV. Says Iizuka, “this is part of our philosophy that it is more efficient to protect the main line than the transformer so as to shorten any interruption time. If there is an interruption, it will tend to be limited to the area immediately around the transformer while any interruption to the main line would take much longer to repair.”

Increasing the BIL of the main line to 100 kV resulted in a lower number of conductor drops due to burn down. “However,” says Assistant Manager of Engineering, Taikichi Kondoh, “that was still not acceptable and we asked ourselves what we could do to reduce these even further.”

Starting in 1982, Chubu Electric adopted widespread use of a current limiting arcing horn device (CLAH) and began a six-year program of installing these throughout its distribution network. The impact of this program was immediate as the incidence of conductor burn down decreased directly with the increasing number of CLAH units installed on the network. Today, with virtually complete installation, the number of instances of conductor burn down has been reduced to only some 10 per year.

Basically, each CLAH unit incorporates integrated EPDM-housed zinc oxide elements designed to diminish the power-follow current. The CLAH could be installed either as a complete unit attached to an insulator or, more typically, retrofitted live line to existing solid core insulators using simple attachment hardware. The CLAH unit utilized by Chubu has a BIL of 75 kV, implying a 50 per cent flashover voltage of 90 kV.

During the period between 1983 and 1996, Chubu Electric installed a total of some six million of these devices. Kondoh explains that while they are now used universally on the network, CLAH units have not generally been applied to dead end structures since, as he points out, “if there is a flashover in this situation, the arc will tend to run to the load side and no conductor burn down will occur on the dead end pole.”