Instrument signal loop
1V-to-5V signal for an Instrument. At 4mA, the voltage measured by the instrument is 1V (250 ohms2.004A). At 20mA, the measured voltage is 5V. Normally, the instrument scale is calibrated so the voltage reads directly in DegrF, psi, etc.
Instrument ground
For LV (Instrument) system it should be less than 1 ohm and
For MV (electrical) system it should be in rage of 1 to 5 ohm
Ideal is 0 resistance but practically it is not possible
The major consideration is for:–
· Ground system maintains reference potential for instrument/Equipment safety
· Protect against static Charge
· Limit the system to casing voltage to enhance operator safety
· Grounding provides a discharge path for short duration current during lightening and prevent damage from surges on power lines
Credits - Amit Upadhyay