Delta T Criteria for Electrical Systems

The infrared thermographer may use the following Delta T (temperature difference) criteria to evaluate the temperature severity of an exception. These Delta T criteria are reported as the temperature rise of the exception above the temperature of a defined reference, which is typically the ambient air temperature, a similar component under the same conditions or the maximum allowable temperature of the component:

NETA Maintenance Testing Specifications, for electrical equipment

Priority	Delta T between similar components under similar load	Delta T over ambient air temperature	Recommended Action
4	1 to 3C°	1C° to 10C°	Possible deficiency; warrants investigation
3	4 to 15C°	11C° to 20C°	Indicates probable deficiency; repair as time permits
2	-	21C° to 40C°	Monitor until corrective measures can be accomplished
1	>15C°	>40C°	Major discrepancy; repair immediately

Military Standard, for electrical equipment

Priority	Delta T	Recommended Action
4	10 to 25C°	Component failure unlikely but corrective measure required at next scheduled routine maintenance period or as scheduling permits
3	25 to 40C°	Component failure probable unless corrected
2	40 to 70C°	Component failure almost certain unless corrected
1	70C° and above	Component failure imminent. Stop survey. Inform cognizant officers

Experience-Based, for electrical and/or mechanical equipment. Any Delta T classification system based on experience, such as the following

Priority	Delta T	Recommended Action
4	1 to 10C°	Corrective measures should be taken at the next maintenance period
3	>10 to 20C°	Corrective measures required as scheduling permits
2	>20 to 40C°	Corrective measures required ASAP
1	>40C°	Corrective measures required immediately

Motor Cores, (on test bench, not in service)

Priority	Delta T	Recommended Action
3	1 to 10C°	No exception likely
2	>10 to 20C°	Possible exception, consult motor core test data
1	>20C°	Exception likely