Answer / carl_ellis

Reason: ground current added to loop current through a
ground created by moisture.

The analog input on the DCS references ground at some point.
The moisture in the junction box or remote terminal panel
makes a connection to a local ground at that point. The
difference in potentials between the grounds causes a ground
loop - current added to the modulated loop current. The
DCS's analog input 'sees' this current, even though the
transmitter is turned off, because the current is driven by
the potential of the ground loop, not the potential from the
power supply.

When the moisture (and hopefully, the dirt) is removed, the
local ground connection is broken and the loop is isolated,
except for the DCS's AI reference to ground. So the DCS AI
'sees' only the loop current modulated by the pressure
transmitter and reports the correct loop pressure.

Some handheld calibration instruments can detect current
leakage in loop powered loops. An example is the Practical
Instrument Electronics (PIE) 334 Plus calibrator.

http://www.piecal.com/products/evolution/334Plus_mA.php

Carl Ellis
Measure First

Why Pressure Transmitter is NOT installed before up-stream of an ORIFICE PLATE? in a flow measurement.

4 Answers   Mecon,

---

what is the difference between signal cable and ontrol cable?

14 Answers   Essar,

---

whats diffeence between dcs & scada operation.

3 Answers   Orchid, Reliance,

---

How to Calibrate Interface Level Troll in field

0 Answers  

---

how to rectify the poor signal strength in utrasonic digtial flow meter ,model:ge digital xce868i

0 Answers   KNPC,

---

What is the difference between modbus,fieldbus & profibus

0 Answers   NSL,

---

Why do we need to install hook-up line differential pressure type level transmitter when the differential pressure type level transmitter with capillary are available?

2 Answers  

---

In field side transmitter voltage droping here no loose connection then what will happen?

2 Answers   Edac,

---

ABB DCS (AC 800 M)redundency take howmany milliseconds to switch from master controller to slave?should be less than DCS scanning time?

2 Answers   SCS,

---

when a magnet is heated?

0 Answers  

---

what is the principle of vibration

3 Answers  

---

What is voltage regulator?

0 Answers  

---