Failure of Transformer Incomer Without a Fault Indication

Did you send the faulty breaker back to the manufacturer for failure analysis?

Evil pixies.

Suddenly a high voltage surge might have attacked and the circuit would have tripped to avoid damage

Initially I would suspect the breaker or a loose connection....I would check the operating temperature of the breaker with an infrared thermometer....and also all the wiring connections....

The answer must be Yes, because when they turned it back on, it is running normally now.

old age!

Make an analysis for the following:

How long this breaker was in service?

How many such incidents the breaker had before?

If the breaker has a UV relay in the protective circuit, check the set values for voltage minimum/maximum and the withstand time;

May be "the hand of god"?

Did it correspond to the arrival of the latest CME (Thurs.)?

Did the breaker fail? Or did it perform correctly and trip/open?

Don't know about your location but we suffer inadvertent trips all too often caused by human error when they are troubleshooting or calibrating protective equipment and get into the wrong circuit or forget to disable the trip circuit.

Once this happens it is difficult and in most cases impossible, to find out what actually happened due to the fear of reprimand by management.

The Breaker might have been tripped due to sudden under-voltage in incoming power supply system. Please check setting and operation of under-voltage relay.

Regards,

Manindra.