External faults are those faults or hazards that occur outside the transformer. These hazards present stresses on the transformer that may be of concern and may shorten the transformer life. These faults include the following. • OVER LOADS Overloads cause the transformer to overheat and have the potential to cause permanent damage or loss of life to the unit. The time constant for overheating is long, however, and it may take many hours of exposure for the condition to become serious. In most cases, no protection is provided for overload, but an alarm will often be used to warn operating personnel of the condition. One cause of overload may be due to unequal load sharing of parallel transformers or unbalanced loading of three-phase banks. • OVER VOLTAGE Over-voltage can be either due to short-term transient conditions or long term power-frequency conditions. Transient over-voltages cause end-tum stresses and possible breakdown. These transients are protected against by surge...