Measuring Voltage/current

[areofall]

Hi
I working on an assignment and woundering if I' right. A task said why do transducers normally have a output of 4-20mA or a 0-10V,
I have most of the reasoms but I this one correct, Is it easier to detect a break in a cable with a voltage signal than a current, I think it is voltage as current has to be measured in series. Is this correct? Or can someone help me expand on this?

 

[rsdoran]

Take a look at this thread:
http://www.plctalk.net/qanda/showthread.php?s=&threadid=1770

 

[Steve Bailey]

Assume that just before the wire breaks, your signal is at its minimum level. With which type of signal would you be able to tell the difference between the signal level and the broken wire?

Your observation on the method of measuring the signal is relevant to the question of why both types of signals are found in common use. Which type of signal is easier to measure with a portable multimeter? With which type of signal are you likely to measure different values depending on where in the circuit you make the measurement?

 

[Tom Jenkins]

Voltage signals are measured in parrallel, and current must be measured in series. You can several devices on either signal. Current is better for transmitting long distances, and is generally more immune to electrical noise.

Broken wire detection is a function of having a live zero. On 4-20 mA you can distinguish between a zero process value - 4 mA - and a broken wire - 0 mA. You can do the same thing with a 1-5 VDC signal, but this is not as common.

You can measure the mA signal in parallel with the readout or PLC if you know the impedence. For example, many PLCs use a 250 Ohm resistance, so you can measure the voltage across the input as 1-5 VDC.