To ground or not to ground is a decision that an Electrical Engineer needs to make when planning or modifying an electrical power distribution system. The decision is dependent on the requirement of the loads. Grounding in some form is generally recommended, although there are certain exceptions. Several methods and criteria exist for system grounding, each has its own purpose.
To assist the engineer in making decisions, the objectives of any grounding system are listed as follows:
- Reduce insulation level of power system equipment
- Power system equipment like transformer’s neutral must be grounded. This can decrease the operating voltage and insulation level of the equipment.
- Ensure personal safety
- A good grounding system of a substation can ensure that the touch and step voltages meet the standard voltage levels. Electric shock injuries result from contact with metallic components that are energized, whether intentionally or unintentionally. Effective equipment grounding practices can minimize these personal injuries.
- Eliminate electrostatic accidents
- Static electric current may create interference of electronic devices and generate fire near any flammable object. A good grounding system can release the static current to the earth which can prevent that type of incident.
- Reduce electromagnetic interference
- The normal operation of any electronic device is interrupted due to electromagnetic interference. This type of interference can be reduced by the good grounding system.
- Reduce cathodic protection current
- The voltage is usually induced in the pipeline at the same corridor of the transmission lines which can harm the utility operators. The cathodic protection is used in the pipeline to mitigate high touch potential and reduce the current due to a fault condition. Therefore, the cathodic protection system of the pipeline must be grounded to release the current into the earth.
- Detecting ground faults
- In the substation, there are many leakage breakers and other fault leakage protection devices used in the low voltage circuits. A high magnitude of the ground fault current is required to bring the protection device into action, if there is an earth fault in the circuit. Therefore, to meet this condition, the neutral point on the secondary side of the step-down transformer must be grounded.
The grounding conductor must also function to conduct the available ground-fault current (magnitude and duration) without excessive temperature rise or arcing. The use of a large cross-section grounding conductor is not enough. All parts of the fault circuit, including the terminations and other parts, must be capable of carrying the fault current without distress. The installation must also provide a lower impedance fault return path than other possible parallel paths that may have inadequate current-carrying capacity.
The equipment ground system is an essential part of the overcurrent protection system. The overcurrent protection system requires a low-impedance ground-return path in order to operate promptly and properly (see NEC). The earth ground system is rarely of low enough impedance and is not intended to provide an adequate return path. The impedance of the grounding conductor must be low enough that sufficient ground-fault current will flow to operate the overcurrent protective device and clear the fault rapidly.
Source:
- Methodology and Technology for Power System Grounding – Jinliang He, Rong Zeng, Bo Zhang
- Power Systems Grounding – Md. Abdus Salam, Quazi M. Rahman
- IEEE Std 142™ -2007, IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems