The Current Transformer (CT) is used to sense AC current in single-phase or three-phase mains circuits. The CT typically has a 1A or 5A AC secondary that connects to a current, power or energy meter. This allows the meter to be located away from the main wiring. CTs are available in various sizes and styles, with standard ratios of 50:5 to 4000:5. Split core models easily retrofit around existing wiring. Solid core models offer lower cost.
Some monitoring systems are supplied with current transformers that have a voltage output. The full scale on these devices is not standardized but usually is between 0.3-2V AC. Despite the lack of standardization, there are several advantages to using a CT with a voltage output. It eliminates the need for heavy leads or a high VA rating. The voltage output also allows a greater distance between the CT and the meter. Another consideration – an open secondary loop on a 1A or 5A CT can produce hazardous high voltage. Models with a voltage output are clamped to a safe level.
Current transformers are specified by size (VA rating), ratio and accuracy. The VA rating determines the maximum secondary impedance (lead+terminal+meter impedance) that can be driven at the stated accuracy. Metering CTs are specified for a 0.9 power factor at 60 Hz. Relaying CTs are specified at 0.5 PF.
Current Transducers also use a solid-core or split-core transformer to sense an AC current. However, they have circuitry to convert the output to a low level DC signal, either volts or mA. Models with a DC voltage or 1mA current output may be self-powered. Models with a 4-20mA DC output usually require an external power supply.
See product info for Current Transformers or AC Current Transducers.