Answer / mugil

Instrument Earth:
Instrument Earth is also called as Reference earth.Instrument Earth all individual shields(Screen) and
over all shields single or multi pair cable shall be isolated from electrical earthing due to avoiding interference and terminated different bus bar.
Instrument bus bar will be connected to grounding dispatcher by 25 sqmm green- yellow cable.Grounding dispatcher to earth pit is connected to 70 sqmm green -yellow cable.

Electrical Earth:
Electrical earthing also called as dirty earth or protective earth.It is used to protect the power system,electrical equipment and personnel from electric shock.

Answer / jigaranadkat

Dear Shashank,

There are mainly three types of earthing used in instrumentation.
1) Power earth
2) Signal / Instrument Earth
3) IS Earth

Power earth :
Provides ground reference to all power cables. Usual practice to have MAX . Earthing resistance ~ 100 Ohms (There is no harm if it is lower)

Signal / Instrument Earth :
Provides ground reference to all Signal cables (including ICS systems).
Usual practice to have MAX . Earthing resistance ~ 10 Ohms (There is no harm if it is lower)

3) IS Earth (Intrinsically Safe Earth) :
Provide ground reference for all IS signals. MUST NOT EXCEED 1 OHM.

You can have multiple earthing pits of same kind of earth and can be connected in parallel in order to reduce the over all resistance.

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