Instrument Earthing Systems

Three main types of earthing systems are employed in plants, each with a specific role to ensure safe and efficient operation. 

These include:

Safety Earth (SE)

The Safety Earth, often called Dirty Earth, Protective Earth, Electrical Earth, or Power Earth, is the primary grounding system for safety applications. 

• It is intended to transmit fault currents while protecting personnel and equipment from hazardous electrical potentials and static electricity charges.
• This earthing system is an important feature of the design of electrical distribution networks, as it provides protection against electric shocks and fires.
• Its job is critical to the plant’s overall safety policy, as it ensures that electrical failures are safely handled.

Instrument Earth (IE)

Instrument Earth, also known as Electronic Earth, Reference Earth, Clean Earth, or Signal Earth, has a distinct role from Safety Earth. 

• Its primary purpose is to ensure that the earth potentials at various measuring and controlling equipment stay constant and consistent. This form of earthing is required for the proper operation of sensitive electronic instruments.
• Instrument Earth stabilizes electrical equipment that is susceptible to low-level interference. Ground signal leakage into a device can cause performance difficulties and operational breakdowns.

Key principles of Instrument Earth 

• Individual shields (screens) and overall shields (screens) of single or multi-pair cables must be disconnected from electrical earthing and terminated on separate bus bars.
• A single-pair cable’s individual shield (drain wire) should be terminated to the earth or ground terminal block located inside the instrument enclosure.
• For analog single-pair cables, the individual shield entering the junction box must be terminated at the terminal block. For digital single-pair cables, the shield is terminated at the terminal block and linked to the bus bar.
• The overall shield of multi-pair analog cables that enter the junction box should be terminated at the terminal block or bus bar.
• All shields from multi-pair cables must be connected to the appropriate instrument earth bus bar at the marshalling cabinet.
• The instrument bus bar connects to the grounding dispatcher via a 25 mm² green-yellow stripped wire, which is then linked to the main instrument earth loop via a 70 mm² green-yellow stripped cable.

Intrinsic Safety Earth (IS)

The Intrinsic Safety (IS) Earth system is intended for devices and circuits in hazardous locations where just a single spark or arc can ignite an explosive atmosphere. 

• This earthing method is essential when employing inherent Safety instruments to ensure that the circuits retain their inherent safety properties.
• The IS Earth system adds an extra degree of safety by preventing sparks from static electricity, which could cause an explosion or fire.
• IS field cable shields (screens) should be isolated and terminated in a similar manner to Instrument Earth. However, the overall shield of multi-pair IS cables entering the marshalling cabinet must be terminated individually and connected to a specific IS bus bar. Before connecting to the IS bus bar, connect the cable’s individual shield to a galvanic isolator.

Maximum Resistance Limits for Earthing Systems

After the installation of the earthing system, the following maximum resistance limits should be met to ensure proper grounding and safety:

• The resistance between instrument earth bus bars and the grounding dispatcher should not exceed 0.5 ohms.
• The resistance between electrical equipment frames and the nearest local stud earth on structural steel should not exceed 1 ohm.
• The resistance between intrinsically safe installations and the grounding dispatcher should not exceed 0.5 ohms.

Principles of Instrumentation Earthing

A well-designed instrumentation earthing system follows these key guiding principles:

Equal Potential

Minimizing voltage differences between systems is essential to prevent electrical shocks and equipment damage. This is accomplished by maintaining a common earth reference point, utilizing homogeneous materials, and ensuring the use of highly conductive electrodes.

Immunity

Instrumentation signals must be immune to electromagnetic interference (EMI) and radio frequency interference (RFI). Achieving this requires proper shielding, grounding, and electrical isolation of the equipment.

Integrity

The earthing system should remain free from corrosion and physical damage to maintain its effectiveness. Regular inspections and testing are necessary to detect and rectify any faults.

Separation

Proper segregation of different earthing systems is vital to avoid cross-system interference. This is achieved by using distinct earth conductors, electrodes, and maintenance bonds for each system.

Click here for Difference between Earth Fault and Ground Fault

Comparison of Safety Earth (SE), Instrument Earth (IE), and Intrinsic Safety Earth (IS)

Each system plays a distinct role in ensuring the safety and reliability of electrical and instrumentation systems.

Adherence to Standards – Instrumentation Earthing

Strict adherence to safety standards is vital for ensuring the effectiveness and safety of earthing systems. The International Electrotechnical Commission (IEC) provides key guidelines in the IEC 62755 standard for the earthing and bonding of instrumentation. Following this standard is essential for maintaining a reliable and safe plant earthing system. 

Each type of earthing system plays a crucial role in plant safety and reliability:

• Safety Earth (SE) handles significant fault currents to protect against electrical shocks and fires.
• Instrument Earth (IE) ensures stability for sensitive electronic circuits and instrumentation.
• Intrinsic Safety Earth (IS) provides additional protection in hazardous environments, preventing potential ignition from sparks or static electricity.

Ensuring that these systems are installed, maintained, and tested according to IEC standards is critical for the safe operation of any industrial facility.

Click here for What is an Earth Pit?

Earthing System for an Analog Signal Junction Box

This drawing represents the earthing system for an analog signal junction box, used to connect field instruments to control systems like Distributed Control Systems (DCS) or Programmable Logic Controllers (PLC). 

Let us explain the key components and flow of the earthing arrangement:

Key Components of the Earthing System for an Analog Signal Junction Box

Field Instrument (Top left side)

• The field instrument is connected to the junction box using a cable with individual signal wires and a drain wire (screen).
• The cable gland seals the cable entry and terminates the armor (if present), providing mechanical protection and electrical continuity for earthing.

Drain Wire (Screen)

• The drain wire or shield in the cable protects against electromagnetic interference (EMI) and radio frequency interference (RFI). 
• The drain wire is shown connecting from the field instrument and is terminated inside the junction box.

Junction Box

• Pair Individual Screen: Each pair of signal wires has its own screen, represented as a dotted line connecting to terminal points inside the junction box.
• Overall Screen: In case of a multi-pair cable, an overall screen covers all pairs. This is terminated at the bus bar inside the junction box.

Insulated Reference Earth (RE) Bus Bar

• The screens of the individual signal pairs and the overall screen (for multi-pair cables) are all connected to the Insulated Reference Earth (RE) bus bar inside the junction box. 
• This bus bar provides a stable, noise-free grounding reference for sensitive instrumentation signals.

Earthing Cable and Earthing Stud Bolt

• An earthing stud bolt on the junction box provides a connection point for an earthing cable that runs to a grounding boss or earthing system in the plant. 
• This ensures that any leakage current or fault condition in the junction box or instrument enclosure is safely grounded.

Instrument Support and Earthing Boss

• The earthing boss shown on the instrument support ensures that the instrument housing is properly grounded for safety.

Multipair Cable to Marshalling Cabinet

• A multipair cable from the junction box connects to the marshalling cabinet, where signals are further routed to the control system. The cable contains both signal wires and shields, which continue to maintain signal integrity through proper grounding.

Typical Earthing Configuration for a Marshalling Cabinet

This above drawing illustrates the typical earthing configuration for a marshalling cabinet that receives signals from field instruments, both analog and digital, including intrinsically safe (IS) signals. 

Here’s a detailed explanation of the key elements:

Key Components and Signal Flow for Earthing Configuration for a Marshalling Cabinet

Signal Types from the Field

• Analog I.S. (Intrinsically Safe) Signal Multipair from Field: This group carries intrinsically safe analog signals from field instruments. The signals have additional safety requirements to prevent sparks or ignition in hazardous environments.
• Analog Non-I.S. Signal Multipair from Field: This section carries standard analog signals that don’t have intrinsic safety requirements.
• Digital Multi Pair from Field: This represents the digital signals arriving from the field instruments, such as discrete inputs/outputs or digital communication signals.

Cable Overall Screen (TYP)

• The overall screen for each multipair cable provides shielding for the entire bundle of signal pairs, protecting them from electromagnetic interference (EMI). These screens are terminated at specific bus bars for grounding purposes.
• In the case of I.S. signals, the overall screen is connected to the Galvanic Isolator Barrier Bus Bar to ensure safe isolation and grounding.
• For Non-I.S. signals, the overall screen is terminated at the appropriate bus bar for effective grounding.

Pair Individual Screen (TYP):

• Each signal pair in the multipair cable has its own individual screen, which provides localized shielding for that specific pair. 
• These are terminated at the terminal blocks within the marshalling cabinet.

Terminal Blocks:

• The signal wires are terminated at terminal blocks inside the marshalling cabinet. These terminal blocks act as the connection points where signals from the field instruments are received before being routed to the control system (like DCS or PLC).
• Both individual pair screens and overall screens are routed and grounded through these terminal blocks and corresponding bus bars.

Earthing Bus Bars in Instrumentation Marshalling Panels

The drawing shows three distinct earthing bus bars for proper segregation and grounding of different signal types:

Insulated Protective Earth Bus Bar (P.E.):

• This bus bar is responsible for safety grounding. It handles the grounding of equipment frames and other structural parts to provide protection against electrical faults.

Insulated Reference Earth Bus Bar (R.E.):

• This bus bar provides a clean reference ground for instrumentation signals, ensuring stable grounding and preventing noise or interference in the signal loops. It is typically connected to the screens of non-I.S. signals to keep them noise-free.

Insulated Intrinsically Safe Earth Bus Bar (I.S.E.):

• This bus bar is used for intrinsically safe (I.S.) signals. It ensures that the signal grounding meets the specific safety standards required in hazardous areas. This helps prevent static discharge or any other risk of ignition in explosive atmospheres.

Earthing Cable in Instrumentation Marshalling Panels

Each of the bus bars (P.E., R.E., and I.S.E.) is connected to the plant’s grounding system via an earthing cable, which safely dissipates any electrical faults or stray currents to the ground.

Click here for Panel Door Earth Bonding Procedure: Ensuring Safety and Reliability

Typical Earthing Arrangement for a Digital Signal Junction Box

This above drawing shows the typical earthing arrangement for a digital signal junction box, which connects field instruments to a marshaling cabinet. 

Here’s a detailed explanation of the key components:

Key Components of Earthing Arrangement for a Digital Signal Junction Box

Field Instrument

• A typical field instrument (such as a sensor or transmitter) is connected via a cable gland and an armored cable. 
• The armor is terminated at the cable gland for mechanical protection and grounding purposes.

Drain Wire (Screen)

• The drain wire represents the shielding of the cable that protects the signal from electromagnetic interference (EMI). 
• The drain wire is connected to the grounding system to eliminate any induced noise.

Earthing Cable from Instrument Support:

• The instrument support is grounded via an earthing cable connected to an earthing boss. This ensures that the metallic parts of the instrument are safely grounded, preventing electrical hazards.

Pair Individual Screen:

• Each pair of signal wires in the cable has an individual screen for added protection from noise and interference. 
• The individual screen is terminated within the junction box to the insulated reference earth (RE) bus bar.

Insulated Reference Earth (RE) Bus Bar:

• The insulated reference earth bus bar collects the grounding points from all individual pair screens and the overall screen. This bus bar ensures a common grounding reference for all signals, minimizing noise and interference.

Overall Screen:

• The overall screen provides shielding for the entire multipair cable. It is also terminated to the insulated reference earth bus bar within the junction box to provide additional EMI protection.

Earthing of Junction Box

• The junction box is supported by a structure, which is grounded using an earthing boss and an earthing cable. This prevents the junction box from becoming electrically charged due to faults or interference.

Multipair Cable to Marshalling Cabinet

• The signals are routed through a multipair cable that connects the junction box to the marshaling cabinet. This multipair cable carries the digital signals from the field instruments to the control system for processing.

Earthing Stud Bolt

• The earthing stud bolt is where the earthing cable is connected, which ensures that the metallic parts of the junction box are grounded effectively.

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Typical Earthing (Grounding) Arrangement for a Cable Tray System

This above drawing illustrates the typical earthing (grounding) arrangement for a cable tray system in an industrial or commercial installation. 

It provides details on how to ensure proper grounding of cable trays to maintain electrical safety.

The breakdown of the components and details shown in the drawing:

Key Components of Earthing (grounding) Arrangement for a Cable Tray System

Cable Tray

• The drawing shows multiple layers of cable trays (analog and digital) used to support and route cables.
• They are perforated trays, which allow for easy cable management and heat dissipation.

Cable Tray Connectors

• These are used to connect sections of cable trays. The connection is essential for maintaining continuity across the entire cable tray system.

Earthing (Grounding) Boss

• The drawing specifies earthing bosses, which are welded or bolted points on the tray where grounding connections are made. These points ensure the cable tray is properly grounded to prevent electrical faults.

Earthing Cable

• The earthing cable connects the grounding boss on the cable tray to the structure or column. This is essential for providing a low-resistance path to ground in case of a fault.

Detail A

• This part of the drawing focuses on the grounding connections in detail. It shows the cable tray connectors, earthing boss, and the route of the earthing cable to a structural point or column.

Bill of Material (BoM)

The BoM lists the items required for installation, including:

• Cable Lugs: These are used to terminate earthing cables to the earthing boss.
• Earthing Boss: A designated grounding point on the cable tray.
• Earthing Cable: The cable used to connect the cable tray to the earthing network.

Notes and Annotations:

• Note1 mentions that the cable tray must be continuous and bonded to maintain electrical continuity.
• Different trays are used for different types of cables (analog and digital) to prevent electromagnetic interference (EMI).