Tan Delta Test

1. UST mode ("Ungrounded Specimen Test") is used for testing between 2 points that normally are both insulated from ground, such as across the open contacts of a circuit breaker pole. The GST mode ("Grounded Specimen Test") is for testing of a portion of apparatus that normally is operating phase to ground, such as a circuit breaker or transformer bushing from the conductor to the case or tank (which is grounded in service). GST tests can be done in the UST mode of the test set. However, UST tests cannot be made in GST mode, because the contribution of the insulation to ground element cannot be eliminated. Having both modes allows speedy testing of the various elements of the insulation in a transformer or circuit breaker without having to move the test leads around many times, just by changing the mode switch.

2. What does the project's test specification or the current transformer instruction manual say? I would normally expect "Yes," but that might not always be the case.