Potential Transformers (PT's)

Potential Transformers Picture Potential transformers, or voltage transformers, are used to transform high distribution level voltages to a lower voltage, usually 120 volts, that can be safely measured by standard watt-hour meters. PT's are usually simple, two-winding transformers and are similar in overall design to any step-down transformer, except that the current and the kVA capacity are very low.

PT's may be required on 480-volt systems for safety reasons, even though 480-volt meters are available. However, all 2400-volt and above applications for metering and most relaying applications require PT's.

PT's have a voltage reduction ratio given on their nameplate. A PT with a voltage reduction ratio of 200: 5 would reduce the voltage by a ratio of 200 divided by 5 or 40 times. For every 200 volts going to the customer, 5 volts would go to the metering equipment. Therefore, the meter reading must be multiplied by the PT ratio of 40 to bill the customer for their actual usage since the meter is only measuring 1/40th of the electricity being used by the customer.