Project ForgeInstrumentation & Process Control
Measurement Instruments
GATE 2011
Easy
The range of standard current signal in process instruments is 4 to 20 mA. Which ONE of the following is the reason for choosing the minimum signal as 4 mA instead of 0 mA?
- ATo minimise resistive heating in instruments
- To distinguish between signal failure and minimum signal condition
- CTo ensure a smaller difference between maximum and minimum signal
- DTo ensure compatibility with other instruments
Explanation
A live-zero of 4mA lets a broken wire or dead transmitter (reading 0mA) be immediately distinguished from a genuine minimum-value signal (4mA), which is impossible if 0mA were used as both the minimum reading and the failure indication.
Source: GATE CH 2011 (2011)
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