Robot transformers are very robust. They can last for years, if water cooled properly and operated under their rated duty cycle. Overheating a transformer damages the internal insulation and will lead to a short between the primary and secondary coils. Exceeding or running at the rated duty cycle will shorten the life of the diodes in the system.
With that said if an AC transformer fails one checks it to see if it is shorted out between the secondary and primary.
In the case of the MFDC and MFAC equipment, it is not that simple. The transformer manufacturers have data on their website explaining how to evaluate a potentially failed transformer. The first three lines state: Don’t proceed or perform the test unless you are a knowledgeable trained personnel.
If you have a failed MFDC transgun swap it out and call the transformer manufacturer for assistance.
Reference: RWMA – Resistance Welding Manual 4th Edition