Question { ITC Infotech, 4127 }

In the field and in the control room different power
supplies if used How to handle the ground

Answer

Normally there should be different grounds in control room.
DCS or PLC vendor generally gives the drawing.

Mostly it is taken as convension that there should be 3
different grounding busbars in control room.

1. One is for the power earth. This will give the earthing
to the power distribution panel of the DCS or PLC.

2. Second is the Signal earth. All the shield cables will be
connected to this earthing busbar to minimize the
interference in the analog signal cables.

3. Thrd is Panel earth. This will be connected to the panel
body.

All these 3 busbars should be connected to 3 different
earthpits by 3 different Copper cables. Redundancy of
earthpits or copper cables may be there if possible.

Question { Siemens, 3989 }

If the average power during generation is zero, why do the
analog meters have + and - ?

Answer

The average power during generation is never zero. The load
is never 100% resistive. It has capacitive as well as
inductive components also. The power drawn by only resistive
load is active power and the power drawn by inductive load
is reactive power. The resultant is never zero.