14 CFR § 25.1363

When laboratory tests of the electrical system are conducted—

The tests must be performed on a mock-up using the same generating equipment used in the airplane;

The equipment must simulate the electrical characteristics of the distribution wiring and connected loads to the extent necessary for valid test results; and

Laboratory generator drives must simulate the actual prime movers on the airplane with respect to their reaction to generator loading, including loading due to faults.

For each flight condition that cannot be simulated adequately in the laboratory or by ground tests on the airplane, flight tests must be made.