Insulator rod is the heart of insulators. Insulator rod can be divided into two types, Insulator Rod for Tension Applications and Insulator Rod for Bending Applications.

Insulator Rod for Tension Applications
The glass grade influences the susceptibility to failure by electrolytic stress corrosion (brittle Fracture – BF). This phenomenon is characterised by a destructive acid attack on the glass fibres, followed by a mechanical insulator failure, when the remaining fibres can no longer bear the service tensile load. Recent investigations in CIGRE and IEEE as well as the service experiences have shown that with specially formulated glass fibres (low or Boron-free content, so-called E-CR glass) the likelihood of a brittle fracture can be significantly reduced.

Insulator Rod for Bending Applications
SEFAG uses such BF resistant rods. Currently, Working Group 12 in IEC TC 36 is responsible for defining an appropriate standard for the qualification of rods, especially for the property of stress corrosion.
Rod diameters are chosen on consideration of both operational and ultimate cantilever load and permissible deflexion at operational load.
As an advantage of composite post insulators, the so-called safe failure mode has been found. This means that a composite post insulator developed with appropriate design would not fail by part separation after overload, but only by inter- laminar shearing in the neutral zone.

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