Equipment grounding is a process of connecting all non-current carrying metallic parts like panels, equipment enclosure, motor, cable trays and various structures together and eventually to the neutral ground bonding point through a low impedance path, ensuring all metal parts remains at same potential. Every electrical installation depends on it as faults in the system are inevitable, insulation of equipment can fail, cables can get damaged, energizing the equipment bodies.
Without proper equipment grounding, fault current will have no return path, increasing the risk of shock hazard, relay mis operation, equipment burnout and risks of electrical fire. Improper equipment grounding can turn a minor insulation failure into a lethal situation. In this article the equipment grounding is discussed in details maintaining the technical accuracy.
Table of Contents
Objectives
Personnel safety: Equipment grounding keeps the exposed metal parts at the ground potential. During a fault, the equipment grounding conductor diverts the leakage or fault current away from the body preventing dangerous touch voltage rise. This makes sure that any accidental contact with the equipment remains within the safe limit.
Equipment protection: The low impedance equipment grounding path ensures that any phase to metal fault, produces high fault current that quickly returns to the source. This high current is detected by the protective devices like MCB, MCCB, fuses or relays causing them to trip within milli second. Faster clearing of the fault prevents the equipment overheating, conductor damage, insulation breakdown or arcing escalation.
System stability: The equipment grounding maintains all metal parts at same ground potential, which suppresses any unwanted circulating currents between the metallic parts such as cable trays, structures, panels etc. It also reduces the electro magnetic interference which reduces the noise in the control and communication wiring, thereby improving the system stability.
Components of Equipment Grounding
Equipment grounding conductor: The equipment grounding conductor is a Galvanized iron or copper strip or cable which provides a low impedance path from equipment to the grounding bus. The typical sizes of these conductors for copper ranges from 16-70 mm2 cables for LT panels and 70-120 mm2 for MV equipment, while galvanised iron with sizes from 25mm x3mm to 50mm x 6 mm are common in substation depending upon the level of fault.
Grounding Bus or Bar: The grounding bus is the centralised metallic bar usually of copper or galvanized iron, installed inside panel, switch board or rooms for the termination of multiple equipment grounding conductors. The bus consolidates all terminations from various equipment and frames into one equipotential node before routing them to the external ground via grounding electrode conductor. It ensures uniform potential in all metallic frames and structures while simplifying the maintenance and testing.
Bonding Jumpers: Bonding jumpers are short flexible conductors used to connect two or more metallic parts like door, frames, cable trays together for equalizing the potential between them. It eliminates the localized voltage difference that may appear during faults or lightning surge, ensuring rise and fall of potential in all bonded components together. It improves personnel safety by nullifying potential difference between two metal parts and also reduces the electromagnetic interference.
Grounding bolts, lugs and clamps: Grounding bolts, lugs and clamps help secure mechanical and electrical bonding of conductors to the equipment. Corrosion free and tight joints are critical as loose or oxidised connection increases the impedance, reduces the fault current while promoting localised heating. Reduction in the fault current can also trigger relay mis operation. Hence, hardware with anti-corrosive treatment and proper crimped lugs are used to ensure long reliability.
Earth grid connection: All the grounding bus ultimately terminates to the grounding grid via grounding electrode conductor. This provides the final path for the fault current or leakage current to dissipate into the soil. Proper routing of the GEC with minimal bend and secure connection helps to maintain low impedance path while ensuring safe touch and step potentials.
Sizing of Equipment Grounding Conductor
The sizing of the conductor for equipment grounding is basically done via two methods
Table method
| Size of phase conductor (S) | Size of Equipment grounding conductor |
| S<16 mm2 | S |
| 16<S<35 | 16 mm2 |
| S>35 | Method 2 (Adiabatic Formula) |
Method 2: Sizing the area of the conductor according to the thermal minimum.
- S = Area of the conductor, mm2
- t= Time taken for the protective device to trip in seconds
- I = maximum grounding current in A
- K= material factor in As1/2/mm2.
The formula is S = I x √t / K
Example: Suppose the prospective fault current is 10000A and the time taken to clear the fault is 0.1s, the material of the conductor is chosen to be copper insulated pvc cable with material factor, k = 115 As1/2/mm2
Therefore, S = 10000 x √(0.1) / 115 = 27.49 mm2, Selecting the next standard size of copper as 35 mm2.
Equipment Grounding in different installations
Substations: In substations, all GIS enclosure, structures, gantries, transformer body and tanks, marshalling kiosks, cable trays are bonded together using GI strips. It is then connected to multiple points in the grounding grid to ensure minimum potential rise.
Industries: In industries, the motor control centre, the distribution boards and the variable frequency drives are connected to the grounding bus via equipment grounding conductors. The cable trays and the ladder racks are all bonded through out the long runs and terminated to multiple points in the grounding grid.
Building installation: In building installation, the ground LT panels, rising mains, the HVAC units, the elevator equipment are grounded. The metallic pipes of water and gas, ducts and cable armours are also bonded to maintain same potential and prevent shock during a fault.
Best practices of equipment grounding
Short grounding runs: It is essential to try and keep the equipment earthing conductor run small short without much bends exceeding 90 degrees as it will then act as inductor and increase the impedance.
Verify Continuity: It is essential to check every bonding for continuity, if the continuity is missing there would be risks of dangerous potential difference during faults resulting in shock hazards.
Protect the joints: All mechanical joints specially the bimetallic ones are to be protected with grease impregnated tape or mastic compound or bitumastic paints in order to prevent corrosion.
Document the layout: It is essential to prepare a document of the layout for the system to be tracible, helpful during expansion, retrofit and fault investigation. It also aids in for safety audits and compliance check.
This article is a part of the Safety and Earthing page, where other articles related to topic are discussed in details.
