Faults in transformers are caused in many ways and the development time may vary from microseconds to tens of years. Very fast mechanisms, often involving mechanical failures, cannot normally be prevented by condition monitoring, although the extent of damage may be restricted by fast tripping mechanisms. Occasionally, detection of precursors to such a fault, such as overheating at a loosened bolt may allow preventative maintenance to arrest its progress. Specialist on-line monitors, such as those for on-load tap-changers are now available.
Transformer oil carries information about the condition of the transformer. Analysing the oil in service can therefore give early warnings about paper degradation, hot spots, electrical faults and problems with moving parts such as tap-changers and pumps. It allows optimisation of inspection or repair times and resources and assists in spares management.