As part of the COMA agreement an annual substation clean and inspection will take place during which an oil sample will be taken. The sample is sent to an independent laboratory for testing.
Once testing is carried out a report will be produced that will flag any potential issues with the transformer, then action can be taken to prevent a fault or unnecessary down time.
Regular testing will also prove an invaluable tool as a log of past results can be complied and compared to check for a trend in degradation if one is present.
On the initial oil sample a test is carried out for PCBs (Polychlorinated Biphenyls), this is a toxic substance and if found in quantities greater than 50ppm will require the transformer or switchgear to be registered with the environment agency, and ideally be decontaminated / changed.