We had an incident occur at a customers location this past week. Parrllel 500 kcmil in seperate 2 1/2" EMT install 50' between panelboard and customer equipment, 480v 800A three phase. The conductors were all tested using our Ideal 61-791 insulation tester, which showed no problems. the 800A breaker was closed and the circuit energized. 2-3 hours after energizing, the wire short circuts inside the conduit and blows a dime size hole and an arc flash. Fortunately no one was in the vacinity of the explosion and the breaker operated correctly and tripped as intended.
Here is the question- has anyone had a problem like this where the conductors insulation failed even after a megger test? We are planning to investigate the test procedure that the tech used and the method of wire installation. We are also having the 61-971 inspected and recalibrated. ANy other suggetions would be appreciated.