Wednesday, October 17, 2012
Instrumentation Earthing System
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.
EARTHING ANALOG SIGNALS
EARTHING DIGITAL SIGNALS
EARTHING MARSHALLING RACK (Above)
EARTHING CABLE TRAY (Below)
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.
- 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.
Labels: cables, earthing, epc-school, Rahul Kapoor, safety
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