Instruction for the testing of distribution transformers as sources of voice-frequency noise for communication circuits is provided. The tests measure the degree to which a transformer may contribute to electrical noise in communication circuits that are physically paralleling the power-supply circuits feeding the transformer. They provide a standard method for use by the transformer manufacturer, user, and others in the industry for the purpose of better evaluating the design choices that are available and moving toward industry objectives with regard to levels of harmonic exciting current.

The practice described herein provides instruction for the testing of distribution transformers as sources of voice frequency noise. These tests measure the degree to which a transformer may contribute to electrical noise in communication circuits that are physically paralleling the power-supply circuits feeding the transformer.Transformers have a characteristic that is common to other iron-core devices of causing harmonic currents at voice frequencies to flow in supply circuits to the transformers. The magnitude of these currents, and of interference that may result, varies according to the design of the transformer as well as the excitation voltage. The tests described in this practice provide a standard method for use by the transformer manufacturer, user, and others in industry for the purpose of better evaluating the design choices that are available and moving toward industry objectives with regard to levels of harmonic exciting current expressed as I T (current x telephone influence factor [TIF]).