What is a 3 wire system?
: a constant potential system of electric distribution in which lamps or other receiving devices are connected between either one of two main conductors and a third wire and motors and heavy duty appliances are usually connected across the main conductors.
Why do RTDs have 3 wires?
To compensate for lead wire resistance, 3 wire RTDs have a third wire that provides a measurement of the resistance of the lead wire and subtracts this resistance from the read value. Because 3 wire RTDs are so effective and affordable, they have become the industry standard.
What is 3 wire and 4-wire RTD?
The 3-wire RTD uses 1 additional wire and the 4-wire RTD uses 2 additional wires to compensate for the wire resistance. Generally speaking, field devices have inputs for 3-wire sensors to provide sufficient compensation of the wire resistance.