different between 2 wire and 3 wire

in a 3 wire instrument the third wire is ment for earthing the instrument - a safety feature that prevents damage to humans in case of a faulty electrical setup

Read these Wikipedia articles

Earthing system

Ground (electricity)

In the case of a resistance-temperature device [RTD], the third wire is there to compensate for the other variable, which is the resistance of the cable due to its length.

Consider a Wheatstone Bridge circuit, where one of the resistors is a 3-wire RTD with long cables, and all will become clear.

There again 2 terminal measurements are okay for non-accurate measurements, but 3 terminal measurements not only reduce the connection impedances but also compensate for internal impedances.

So a capacitor in a screened box will have capacitance to the screen on both sides of the capacitor, with three terminal measurements the screen of the box is connected separately cancelling the parasitic capacitances, so increasing accuracy...

For highest accuracy 4 or 5 terminal measurements should be made...

However, if you are referring to mains connections post #1 is right, its safer to have an earth wire

John.

A two wire sensor is powered via leakage current.

The advantage is you can use one sensor for both sinking and sourcing inputs, the disadvantage is that if the leakage current is large enough, it will turn on the input.

A three wire sensor needs a supply (two of the wires), and provides a switched output on the third.

The avantage is you may get more reliable switching, the disadvantage is you may need a differnet part for sinking and sourcing inputs.

I assume you are not referring to 2 or 3 wire temperature probe instruments, or referring to 240VAC or 110VAC powered instruments but are talking about 2 or 3 wire 24VDC powered instruments. 2 wire transmitters that transmit in (say) 4-20mA signal range must be able to consume less than 4mA when they at their zero point of range. If it consumes more than 4mA at zero range they must have a 3rd wire, one for 24VDC+ and one for 24VDC- and one exclusively for 4-20mA output signal. That way the instrument could consume 6mA and still output only 4mA on the 3rd wire.

Thanks for good answers. I will take and use all of them.

thanks again.

Everybody's already given good answers. In my case, a three-wire cable went into a two-wire instrument because it was the only available cable they had.

Two wire instruments uses only two wire and connected serially with other two!(Power supply and say controller). Advantage is to get rid of additional power supply line.Disadvantage is that can not be used with zero based signals(like 0-20mA).

3-wire connection uses 2 line for signal plus 1 line for power. One of this 3 wire is common for signal return and power return.