Instrumentation earth system

What is the earthing system?

The earthing method is the protection of harmful potential differences that could result in injury, protection and dissipation of static charges, testing and elimination of disruption in instrumentation systems, as well as providing a reference point for inherently secure Zener barriers.

Basic functions of an Earthing system:

  • Provides low impedance return path for fault current for so that the fault can be detected and the source of power removed as quickly as possible.

  • To avoid potential differences that create a potential hazard

  • To minimize damage caused by lightning strikes and related surges

  • Minimizing electromagnetic noise of instrument and contact signals

  • To provide a point of reference to allow intrinsically safe devices to reduce energy in potentially explosive atmospheres.

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:

Electrical earthing is used to protect electrical equipment in a power system.

How electrical earthing is achieved?

  • Field instrument cable armour shall be prevented at the cable gland.

  • The steel of the single and multi-core cable to the junction box shall be halted at the cable gland. If the metal junction box is made, then the earth stud bolt must only be attached to the closest steel structure. If the non-metal junction box is made, the earth stud bolt will be mounted on the metal gland plates having direct contact with the cable gland.

  • Armor of single and multi-core cable inside or outside marshalling and system cabinet shall be terminated and connected inside the cabinet to a bus bar. Each bus bar inside the cabinet is connected by 35 mm. sq cable (usually green-yellow stripped) to a grounding dispatcher. This grounding dispatcher collects all connections from the individual bus bar and then connects them through a 70 mm.sq cable to a general electrical earth loop (to the steel structure). The earth bus bar is typically made of copper and has a diameter of 1 1⁄2 “and a height of 1⁄4.”

Instrument earth:

Instrument Earth is also referred to as Earth Guide. Instrument Earth all individual shields (screen) and single or multi-pair cable overall shields are separated from electrical earth due to the prevention of interference and the removal of different bus bars.

How to achieve instrument earth?

  • Individual shield (drain wire) of a single pair of cables shall be terminated inside the instrument enclosure at the earth or ground terminal block.

  • Single shield inside the junction box from analog single pair cable shall be terminated to the terminal block. Specific shields from the digital single-pair cable inside the junction box are terminated to the terminal block and then jump each other out and attach to the bus pole.

  • Individual shield from multi-pair cable inside the junction box shall also be terminated from single-pair cable to terminal block match with the single shield.

  • The overall multi-pair analogue cable shield inside the junction box shall be terminated to the terminal block or bus bar (the overall analog cable shield does not have a single-pair shield). The overall shield from the digital multi-pair cable inside the junction box to the bus bar is terminated.

  • At the marshalling cabinet, all single and complete shields (screen) from the multi-pair cable are terminated into the respective earth bus bar of the instrument.

Intrinsically Safe earthing:

Infield instruments, junction boxes and marshalling cabinets, the separation and termination of IS field cable shields (screen) shall be done in the same manner as the earth instrument described above.

Furthermore, the total multi-pair cable shield (screen) for IS signals goes to the marshalling cabinet to be separately terminated and attached to its IS bus bar. This cable’s shield (screen) will end directly with a galvanic isolator and then connect to the respective IS bus bar.

Upon implementation of the instrument earthing device, the upon total resistance levels shall be met.

  • Between instrument earth bus bars and grounding dispatcher not greater than 0.5 ohms.

  • 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 ohms.