A potential transformer (PT) is a device that lowers the voltage going to metering equipment. It is similar to a current transformer except that it lowers voltage rather than current. PTs are required when the system or service voltage exceeds the rated voltage level of a standard meter. The industry standard single-phase revenue meter can handle up to 240 volts, and the standard three-phase meter can handle up to 600 volts. If a customer is taking primary distribution or transmission service, the voltage will be higher than the meter can accommodate. By lowering the voltage with the PT, a standard meter can be used to measure the power usage.
A PT has a voltage ratio that is used to convert the meter reading to account for the reduced voltage. Here is an example for a PT with a ratio of 200 to 5 volts. For every 200 volts flowing through the service to the customer, 5 volts are sent by the PT to the metering equipment. The metering location will have an account multiplier associated with the PT, which is 40 because only 1/40 of the actual power going to the customer is actually measured and recorded on the meter. So the kWh or kW values measured by the meter must be multiplied by 40 to determine the difference between measured or metered usage and actual usage.
In some cases, meter installations require both and PTs and current transformers (CTs) on an individual customer's installation. Some services will require both CTs and PTs because of high current and high voltage. These would typically be primary meter customers, where the customer is metered before the transformer, or any service whose voltage exceeds 480 volts. CTs reduce the current and PTs reduce the voltage, and the account multiplier is determined by multiplying the CT ratio by the PT ratio. For example, if the CT ratio is 40 and the PT ratio is also 40, the multiplier is 40 times 40, or 1600. Because there is both a CT and PT on this particular installation, the meter will measure only 1/1600th of what is delivered to the customer.