Electrical current or voltage can be measured by bringing it to a suitable instrument, but in many electrical power circuits the current and voltage are both so high that it is desirable for both cost and safety to bring the circuits to the primary winding terminals of a potential transformer to measure voltage or through the primary winding of a current transformer to measure the current.
Standard secondary voltages are 100V, 115V, 120V with output ratings between 10A and 300VA, standard secondary currents are 1A, 2A and 5A.
They includes current transformers and potential (voltage) transformers.
