High Voltage Equipment Maintenance Information

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There are numerous inspection and testing procedures for in-service equipment, however they can be broken down as per the following list.


Maintenance routines for both high and low voltage switchgear are similar.

• Circuit breaker testing includes the following:

  • Insulation resistance testing
  • Main contact resistance testing
  • Mechanism inspection, lubrication and physical operation
  • Auxiliary contact inspection and operation/test
  • Inspection, resistance and operational checks of fitted accessories such as trip & close coils, spring charge motors, mechanical fuse trip linkages etc

   (Oil insulated / filled unit specific)

  • Oil inspection and testing
  • Carbon and sludge inspection
  • Contact condition inspection
  • Internal mechanism inspection
  • Dashpot inspection

   (Gas insulated specific)

  • Gas pressure verification

• Bus bar & bus chamber:

  • Visual inspection
  • Contact resistance testing
  • Insulation resistance testing

• Protection relay servicing:

  • Documentation of settings
  • Downloading of relay file (microprocessor based)
  • Cleaning of mechanism, induction disk, magnets and contact sets (electro-mechanical)
  • Injection testing to prove various relay elements
  • Trip and operational tests


• Insulation resistance testing of high and low voltage windings
• Insulating oil analysis

  • DGA – (Dissolved Gas Analysis): This test determines quantities of gases that may be present. As the name suggests, various gases dissolve in the oil. This is considered the principle oil test for determining the overall condition of the oil.
  • Acidity – The acidity (neutralization value) of oil is a measure of the acidic constituents or contaminants in the oil. Determines acid values which can damage the transformer core winding insulation properties.
  • Water content – Determines the amount of free water that may be present; important in outdoor units. Degradation of cellulose (transformer winding insulation materials), and/or the oil itself are the two sources of water increase.
  • Resistivity – This parameter is sensitive to the presence of soluble polar contaminates, ageing products or colloids in the oil.
  • Dielectric strength – This is a measure of the ability of the oil to withstand electric stress, its primary function.
  • Furan – The main goal of furan testing is to determine whether the insulating paper in a given transformer has been or is being damaged by heat. Degradation of the paper causes it to lose its tensile strength and results in the release of furans

• Cable box inspection
• High and low voltage cable termination inspection
• Inspection and testing of protection devices, such as temperature indicators and bucholtz relays
• Visual inspection of unit for general condition, oil leaks, valve condition, breathers etc
• Secondary control checks i.e. cooling fans, tap changer units

High Voltage Cables

• Insulation resistance testing
• Termination inspection and cleaning

Earth System

• Visual inspection of bars and associated conductor connections
• System resistance measurement to general mass of earth

DC Systems

• Visual inspection of batteries
• Voltage measurements
• Load testing