The process of immediate release of electrical energy to the earth through a low-resistance path is called grounding.

Instrument ground

The main objectives of a factory grounding system are described below:

• It provides a low impedance path to ensure proper system operation under fault conditions

• Ensure personnel are not exposed to unsafe potential risks due to unspecified failures

• It ensures compliance with EMC requirements

• IS earth avoids sources of ignition in hazardous areas.

Items or equipment that require grounding, but not limited to the following,

• Field instruments,

• Control system cabinet,

• Analyzer shelter,

• junction box,

• enclosure,

• pipeline,

• Cable tray,

• Pillar,

• Live local panel,

• Console,

• Motor,

• Tank,

• Ships,

• pipeline,

• Steel structure, etc.

This article focuses on the grounding requirements for any industrial plant instrumentation project.

Type

There are three main types of instrument grounding systems.

1. Safety Earth (SE)/Dirty Earth/Protective Earth/Electrical Earth/Power Earth

2. Instrument ground (IE)/Electronic ground/Reference ground/Clean ground/Signal ground

3. IS The intrinsically safe (IS) ground of a circuit. This is only necessary if you have an IS instrument.

Instrument ground

The primary purpose of instrument ground/signal ground is to provide a low impedance path for noise currents caused by RFI/EMI, which may result in erroneous unwanted noise signals being induced into analog signals.

The shield of the single/multiple pair analog instrument signal cable is connected to this ground (IE).

Safety ground

Undetected ground faults pose safety risks to personnel and can also lead to safety hazards such as equipment failure, fire, and electric shock. This is why we need electrical grounding/safety grounding.

Ground resistance

Instrument grounding system design should take into account the allowable grounding resistance as recommended by the code or control system supplier.

Below is a reference table that can be updated based on project specific requirements.

There should be isolation between each of the above ground types. Without isolation, the purpose of the above grounding requirements cannot be achieved.

There are two main areas in which grounding systems are conceived.

1. Indoors (such as control rooms, switchgear rooms and other buildings where cabinets/panels/consoles, etc. are installed)

2. Outdoors (process areas where instrumentation and related installation items are installed)

The control panel in the control room is of the indoor type. Depending on the function of the cabinet, each type of ground rod is provided.

Control room equipment including system cabinets and marshalling cabinets, distribution boards, complete control system cabinets, workstations, auxiliary consoles, printers, etc. should consider grounding requirements.

For example, systems and marshalling cabinets should have - SE, IE and IS earth strips.

Network cabinets/server cabinets/power distribution boards should have - SE only.

Inside the cabinet, the instrument ground rod should be isolated from the safety ground rod by mounting the instrument ground rod on the insulated busbar.

All instrument ground bars in the cabinet shall be connected to the common instrument ground bar provided in the false floor/cable basement using insulated redundant copper conductor cable.

This common instrument ground bar should be further connected to the nearest instrument ground grid using copper cable.

The grounding principle of the cabinet ground bar and building ground bar network should be agreed with the control system supplier. For example star connection or series connection.

Grounding principles should include at least scope of supply, ground wire/ground wire size, ground stud size, wire color.

Series ground connection

Start/Peer-to-Peer Earth Connection

All safety ground bars in the cabinet shall be connected to the public safety ground bar provided in the false floor/cable basement using insulated redundant copper conductor cable.

This public safety ground bar shall be further connected to the nearest safety ground grid using copper cable. The main grounding grid conductor size can be calculated based on the required fault current during the required grounding time.

Ultimately, this network/grid connects to the pit. The soil around the pit must be kept moist. The effect of soil temperature on soil resistivity is greater near and below freezing, which will result in ground electrodes being installed at depths where frost will not penetrate.

Good practice

• The cable shield should be insulated and floating on the field instrument side (cut and taped). Cable shielding is a drainage line with an aluminum mylar or metal braided cover attached to the instrument signal cable, that is, 4-20 mA analog signals, low-voltage digital signals, etc.

• The shielding of the main cables (i.e. from the junction box and control room) should be grounded at one point, the control system end Instrument ground rod Isolated type. All connections within the cabinet to the instrument ground bar shall be of threaded, non-corrosive metal.

Likewise, for intrinsically safe cables, the individual and overall shields should be connected to the intrinsically safe ground bar within the cabinet.

Ensure continuity of all ground cables to ground rods and pits.

• Cable armoring should be grounded for lightning protection at both ends, i.e. at the junction box and control system end. Since the junction box body and the control system cabinet are connected to the safety earth bar, the cable armor is ultimately connected to the safety earth.

• The body of all instruments (24V DC or 110 V /220V AC operation) should be grounded to the nearest site safety ground rod on the steel structure.

• Ground continuity should be ensured between pallets, pallet accessories and pallet sections. This will be ensured by grounding cables/straps for every 25 meters of pallet length.

• The ground rods should be connected so that painting does not create undesirable contact between the ground rod and the structure. Continuity should be ensured.

Junction box ground (safety ground)

Transmitter ground (safety ground)

Junction Box Shield Termination on Isolated Instrument Ground Terminal

Instrument Cable Tray Grounding (Safety Grounding)

Instrument grounding in the control system cabinet (isolated from safety ground)

Safety grounding within the control system cabinet (non-isolated)