Instrument earthing system shall consist of the following earth type:
· Electrical earthing (also called dirty earthing or Protective Earthing (PE))
· Instrument earthing (also called Reference Earth (RE))
· Intrinsically safe earthing
1. Electrical earthing is used to protect the power system, electrical equipment, and personnel from electric shock.
How to do an electrical earthing?
· Armor of field instrument cable shall be terminated at cable gland.
· Armor of single and multi core cable going to junction box shall be terminated at cable gland. If the junction box made from metal, then it only needs to connect the earth stud bolt to the nearest steel structure. If the junction box made from non metal, the earth stud bolt will be located at the metal gland plates which have direct contact with the cable gland.
· Armor of single and multi core cable going inside or outside marshalling and system cabinet shall be terminated and connected to a bus bar inside the cabinet. Each bus bar inside the cabinet will be connected to a grounding dispatcher by 35 mmsq cable (usually green – yellow stripped). This grounding dispatcher will collect all connection from individual bus bar and then connect it to a general electrical earth loop (to structure steel) by a 70 mmsq cable. In general used, the earth bus bar is made from copper and has 1 ½” width and ¼” height.
2. Instrument earth
· The general principle of instrument earth is all individual shields (screen) and overall shield (screen) of single or multi pair cable shall be isolated from electrical earthing and terminated at different bus bar. This instrument earth usually also called reference earth since it serve the reference point of the instrument loop (ground of internal electric circuit inside the instrument).
· Individual shield (drain wire) of single pair cable shall be terminated at earth or ground terminal block inside the instrument enclosure.
· Individual shield from analog single pair cable going inside the junction box shall be terminated to terminal block. Individual shield from digital single pair cable going inside the junction box shall be terminated to terminal block and jump out each other then connect it to bus bar.
· Individual shield from multi pair cable going inside the junction box shall be also terminated to terminal block match with the individual shield from single pair cable.
· Overall shield from multi pair analog cable going inside the junction box shall be terminated to terminal block or bus bar (overall shield at analog cable doesn’t have a pair with the shield from single pair cable). Overall shield from multi pair digital cable going inside the junction box shall be terminated to bus bar.
· All individual and overall shields (screen) from multi pair cable shall be terminated into respective instrument earth bus bar at marshalling cabinet.
· Instrument bus bar will be connected to grounding dispatcher by 25 mmsq green-yellow stripped cable. From grounding dispatcher, it will be connected to main instrument earth loop by 70 mmsq green-yellow stripped cable.
3. Intrinsically Safe earthing
· Isolation and termination of IS field cable shields (screen) at field devices, junction boxes and marshalling cabinets shall be done in the same manner as for instrument earth explained above. However the overall shield (screen) of multi pair cable for IS signals goes to marshalling cabinet shall be terminated individually and connected to its IS bus bar. The individual shield (screen) of this cable will be terminated directly to a galvanic isolator and then connected to the respective IS bus bar.
The following maximum resistance limits shall be achieved after the instrument earthing system installed. This resistance is minimized as much as possible so the un-normal current can be safely grounded at steel structure.
· Between instrument earth bus bars and grounding dispatcher not greater than 0.5 ohm.
· Between electrical equipment frame and nearest local stud earth on structural steel not greater than 1 ohm.
· Between intrinsically safe installation and grounding dispatcher not greater than 0.5 ohm.