Routine monitoring requires monthly recording of secondary circuit impedance (should be Ω), and checking connection point temperature with an infrared thermometer (should be °C). A maintenance team detected three core insulation aging faults early by regularly measuring the secondary current phase angle.
Annual preventative testing includes measuring insulation resistance with a 500V megohmmeter (standard value >100MΩ), power frequency withstand voltage testing (2kV/1min without flashover), and verification of the volt-ampere characteristic curve (inflection point voltage not lower than 90% of the nominal value).
During fault diagnosis, if metering inaccuracies are found, first check if the secondary load exceeds limits. In one case, a shopping mall experienced excessive secondary impedance due to the installation of a monitoring module; replacing it with a low-power device restored normal operation. Abnormal heating may indicate a loose core; one case showed a 15°C temperature drop after tightening the bolts. Phase angle monitoring can predict insulation aging.