TABLE OF CONTENTS AC voltages and currents that are too high for direct connection to meters or protective relays can be measured with the aid of voltage (potential) and current transformers. Voltage transformers are transformers with low exciting current and leakage reactance that are insulated for high voltage on the primary. The usual secondary voltage is 120 V. These transformers are made with both single-insulator primaries for measuring line-to-ground voltages and double-insulator primaries for line-to-line metering. The transformer frame should be solidly earth grounded for safety on either type. Single-insulator transformers can be fused with only one fuse on the high-voltage side, but double-insulator transformers must be fused on both primary connections to protect against a ground fault in the primary winding. Medium-voltage fuses, specially rated for voltage transformers, are available. Since the primary fuses must be large enough to withstand inrush currents, they only serve as short-circuit protection. The transformer should be thermally protected by a secondary fuse and have one side of the secondary earth grounded for safety.
Current transformers are available in both wound primary and bushing types. The wound primaries are like an ordinary transformer with a few primary turns and a large number of secondary turns so that a large primary current is transformed into a small secondary current, the secondary being short circuited by the connected ammeter or relay. This type is used mostly for low currents or high-accuracy applications.
Bushing transformers are toroidal cores with a wound secondary, the winding operation being analogous...
