Disc Insulator are the version of suspension insulation devices and are intended for fastening the strand wire of overhead transmission lines to the supporting structures. Glass Disc Insulator has the porcelain or glass disk (plate) with a spherical head. The bottom surface of a disk is manufactured as ribbed surface in order to increase the creepage distance, the top surface of the disk - smooth with a small bias for the rain running off. The form of a dropper is given to the edge of a disk to provide running off of water without wetting the bottom surface. There is the steel pin inside of a head assembled by means of cement. The top head of glass disc is assembled with a cap made from malleable cast iron with a socket for connection with a pin of the other insulator or for fastening a string to the supporting structure.
All connections are executed as socket ball type. Internal and external surfaces of the head are made in form that in case of tension of the conductors provide only compression load for the material (it is known that strength of porcelain and glass at compression is considerably higher than at tension). That is the way to provide the high mechanical strength of glass string cap & pin insulators. They are capable to with stand the tension load of the conductors in thousand kilos. Cap & pin insulators are designed in a way so that in dry condition the puncture voltage exceeds the flash over voltage approximately in 1.6 times that provides the absence of puncture at over voltages.
To increase the reliability of the insulation and increase the flash over voltages the glass string cap & pin insulators are connected in strings. Such strings are mainly used for insulating of the conductors in transmission and traction lines. As long-term experience of use of insulation devices in traction lines has shown that the cap & pin porcelain insulators are much less reliable in comparison with the glass insulators in a reason of the cracks appeared in porcelain not found out at survey and lead to full loss of the insulation properties. The cracks in glass insulators lead to breakage of the glass disc and the defect becomes obvious.