Fault arcs on busbar sets and switchboards

Evolutive causes

These result from a gradual decrease in the inter-phase or inter-phase to ground insulation resistance. This decrease can be the result of deposits which, subsequent to exceptional humidity conditions, can result in the formation of a superficial resistance bridge such that a conductive path on the surface ot insulators can be created.

Depending on the type of insulators, this initial fault may be self-eliminating, or become worse, resulting in the creation of a fault arc. This phenomenon is sometimes observed when an installation is switched on again alter being shut down for a period of several days. during which condensation may have occurred due to the fact that the installation temperature is no longer higher than ambient temperature. ln particular, this applies to glass-making installations, in which the atmosphere carries sodium carbonate dust. and in which the risks of hygroscopic variations are high.

The finai resllt will be the same it the superficial pollution of the insulators results trom splashes of fluids containing electrolytes. The accident may pass unperceived, and occur during transport and handling of the installation components. The detect will be revealed later during operation under exceptional hygroscopic conditions. This also applies to condensation on equipment stored in coastal areas, and which is incorrectly protected. The gradual degradation of the insulation may also result from accidental local heating due,  for example, to incorrect tightening or gradual loosening of a connection. The increase In temperature immediately round the defective point can entail decomposition and progressive carbonization of the adjacent insulators, resuitlng in a fault arc, first of all between phases, or between phases and ground, and degenerating into a three-phase fault.

AUTHOR: Schneider Electric experts | G. BOUVIER, A. DUCLUZAUX