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Instrumentation Earthing

Earthing is the process of creating an alternative path for the flow of fault/excessive currents safely into the ground in the presence of minimal resistance or impedance.
Instrumentation Earthing


The primary purpose of earthing is to reduce the risk of serious electric shock from current leaking into uninsulated metal parts of an appliance, power tool, or other electrical devices.

In a properly earthed system, such leaking/fault current is carried away harmlessly while tripping the fuse.

Earthing also provides protection from large electrical disturbances like lightning strikes and power surges. It also aids in the dissipation of hazardous static electrical charges.

Although most electrical systems have fuses or circuit breakers for protection against a fault current, the human body may be fatally electrocuted by a current of less than one ampere which is well below the point at which a fuse or breaker will operate. Earthing helps minimize such hazards from occurring.

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

Electrical Earthing

It 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.

Instrument Earthing

  • 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 paire 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.

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.

Typical Earthing Installations

Cable Tray Earthing

Cable Tray Earthing

Earthing of Analog Signal Junction Box

Earthing of Analog Signal Junction Box

Earthing of Digital Signal Junction Box

Earthing of Digital Signal Junction Box

Marshalling Cabinet Earthing

Marshalling Cabinet Earthing

Also Read: Difference between DCS & PLC Systems

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4 comments

Anand January 24, 2016 at 1:55 pm

Can you please explain the earthing concept with a diagram ???

Reply
Naresh Kumar Tirlingi May 5, 2017 at 6:59 am

what is the value of difference in between Electronic and Electrical earthing ?

Reply
Ren Kitchener November 6, 2019 at 10:46 pm

The Intrinsically safe earthing approach is only correct if the entire earth or equipotential circuit has a zero potential from end to end, even with an electrical fault present, as per IEC60079 Part 14. In some cases, even at PD of 1 Volt can be incendive. Isolation testing as per the routine maintenance requirements (Part 17), particularly for Zone 0, with grounded instruments is time consuming and costly. Note: Cutting back and insulating the shield in the hazardous area with insulation tape is not permitted, if this is a requirement to conform to Part 14, when using a bond or supplementary bond that could carry fault currents (which can lead to a PD). Always remembering that a shield is one long and large inductor, with very little mutual damping.

Reply
Vignesh December 1, 2019 at 1:19 am

We have issue based on earth leakage current that an alarm came du to El detector not get resettled again we check in field side , at jb side they used drain wire comes with mesh armour as a shield and connected which creates all shield having contuinity with Jb ,after we removed both ends shield then alarm clears yet when we put back alarm raises again.

What’s the reason for the alarm can we use the earth wire comes with mesh armour as a shield

Reply

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Instrumentation Earthing

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