Why we are using 4-20 ma signal instead of 0 -20ma?

if0 is used instead of4 mA,we cannot differentiate the actual0 value of the sensor or the problem in the transmission of signal. for eg. cable is cut or damage of cable.

20 mA is used as maximum because the human heart can withstand up-to30 mA of current only. so,as a safety point of view20 mA is chosen

This is a common interview question for Instrumentation Engineers and here is some of the best anwers from my research:

Why we are using4- ma signal in instrumentation instead of0 -ma?

1. All transistors needs some amount of voltage for turning it on, this is a cut in voltage4mA - up to this voltage their exists a non-linearity in its characteristics, beyond this cut in voltage the characteristics of transistor is linear. This non linear region lays between0-4mA, Beyond4mA it's linear and this's why4mA is used.

2. Can be effectively used to detect open circuits in a DC system. If the line breaks in a4-mA system the current drops to zero raising an alarm, whereas in a0-mA system this cannot be done thus open circuit cannot be differentiated from a live circuit carrying minimum current. This is also known as live zero.

3. Most two wire devices are self powered and require a minimum of4mA to operate. Also, in4-mA communication systems the sensor transmitter can be powered by the same two wires that carry the loop signal. This helps in lowering wiring cost.

4. Also, at4-mA with+V power supply, no spark will be generated, making it intrinsically safe in hazardous areas, for these reasons only up tomA is selected.

5. Historically a-mA standard was used, however, because of higher current losses while using theI2C (sychronous serial) communication protocol it was replaced with4-mA protocol.

6. Note that the signals that used in Instrumentation is always where the Span to LRV Ratio is4:1

3-Psi Span = and LRV =3. Ratio is:3 =4:1

4-mA Span = and LRV =4 Ratio is:4 =4:1

-mA Span = and LRV = Ratio is: =4:1

With0-20 mA you can not distinguish between minimum field value and connection break.  With4-20 mA, internal circuit can distinguish between connection break of minimum value. Normally when the value is minimum the transmitter will give you4 mA while in case of connection breakage it will give0 mA.

this is because0 mA current is not always possible due to inertia of electrical system there is always some current flow which makes system unreliable.

Normally all instruments we use operate at4- m A...therefore we use4- m A instead of0- m A. 

This is live zero.

We can find that signal from object  is coming or not.