Earthing 101: The Basics

Earthing…. The unsung hero of any electrical system

Although being the most important aspect of the electrical system, sadly our understanding of the concept of earthing is mostly limited to the acceptable values of earthing pit, rating of NGR etc. Much heed is not given to the logical analysis behind the concept of earthing.

eg: A generator is provided with both body earthing and neutral earthing but a motor is only provided with body earthing. Why isn’t neutral earthing done for motors generally?

LV systems are generally solidly earthed while HV systems are indirectly earthed.... Again EHV systems are solidly earthed..... Why the difference??

What are the differences between TT, TN, IT systems? Which one is suited for which kind of application? Why and how is it better? Why not other systems?

What decides the acceptable value of the earth pit?

How to arrive at the value of the NGR resistance to be used?

All these and many more things related to earthing are just taken for granted without giving it a thought. Let’s dig deep into the concepts behind all of them.

This brings us to the first article in this series i.e. Earthing 101.

What is earthing? Why is it required??

Earthing of any equipment/system is connecting the neutral of the system and the exposed conductive parts of the installation to the earth. The charge of earth is considered neutral. Also any amount of charge added to the earth would not affect the charge of earth theoretically. So, the absolute earth potential is taken as the reference.

Earthing is used primarily for two purposes.

1. Protection against electric shock: If not properly earthed, humans or livestock coming in contact with the exposed conductive parts of an electric equipment can suffer from electric shock. This can be avoided by maintaining all parts of the system (except the live parts) at earth potential at all times i.e. by connecting them to earth. This is termed as “Equipment Earthing”.

2. Maintaining the neutral potential: Also, earthing is required to ensure the over-voltages in a system do not increase to such a value which can cause damage to the insulation. This can be achieved by earthing the neutral point of the system either directly i.e. solid earthing or indirectly i.e. through an impedance (NGR, NGT etc). This is termed as “System Earthing”.

 Now let's discuss about these two earthing schemes in detail.

Equipment Earthing: The basic objectives of equipment earthing are....

1) Avoiding dangerous electric shock voltages exposure: When there is an unintentional contact between a live part and the metal frame or structure that encloses it, the frame or structure tends to become energized to the same voltage level as that of the energized conductor. To avoid the appearance of this dangerous, exposed shock hazard voltage, the equipment earthing conductor must present a low impedance path from the stricken frame to the zero potential ground junction. The impedance should also be sufficiently low enough to accept the full magnitude of the line-to-ground fault current without creating an impedance voltage drop large enough to be dangerous.

2) Avoiding thermal distress: The earthing conductor must also function to conduct the ground fault current (both magnitude and duration) without excessively raising the temperature of the earthing conductor or causing the expulsion of arcs and sparks that could create a fire or explosion hazard to building or contents.

3) Operating the protection system: The total impedance of the fault circuit including the earthing conductor should also ensure the required current amplitude to cause operation of the protective system. A higher than necessary ground-circuit impedance would be acceptable if there is no impairment of the performance characteristics of the protection system.

System Earthing: The basic objectives of system earthing are...

1) Avoiding Over-voltages and Insulation Damage: System Earthing is designed primarily to ensure that the potential on each conductor is restricted to such a value as is consistent with the level of insulation applied. A fault on one phase of an unearthed or impedance grounded system places a sustained increased voltage on the insulation of ungrounded phases in a 3-phase system. This voltage is about 1.732 times the normal voltage on the insulation. This or any other sustained over-voltages on the unearthed system may not immediately cause failure of insulation but may tend to reduce the life of the insulation. (Refer Annexure-101A for the relation between earthing impedance & over-voltage and the maximum value of over-voltage.)

 2) Ensuring efficient and fast operation of protective devices: The resistance of system earthing should be such that, when any fault occurs against which earthing is designed to give protection, the protective gear should operate to make the faulty main or plant harmless to the rest of the installation. In most cases, such operation involves isolation of the faulty main or plant, for example, by circuit-breakers or fuses. In the case of underground systems, there is no difficulty whatever but consider the case of overhead line systems protected by fuses or circuit breakers fitted with overcurrent protection only. A low resistance value to be used for system earthing in this scenario to ensure the isolation of the fault.

 3) Avoiding thermal stress on the live parts of the system: While lower value of resistance for system earthing would be beneficial to the insulation of the system and to the quick & efficient operation of protective devices, too low a resistance could result in very high currents (during ground faults) causing excessive thermal stress on the live conductors. The resistance needs to be designed in such a way that the maximum possible fault current in the system (with the resistance earthing) would be lower than the thermal withstand capability limit of the live conductors.

Apart from these two purposes, earthing is sometimes necessary for the proper functioning of certain equipment. This is termed as "Functional Earthing". Some of the cases in which functional earthing is used:

• to complete the circuits of telegraph or telephone systems employing on-earth path for signalling purposes;
• to earth the power supply circuit and stabilize the potential of the equipment with respect to earth;
• for lightning-protective apparatus
• to earth screening conductors to reduce electrical interference to the telecommunication circuits.

Kindly leave your corrections, suggestions & feedback in the comments so that we can discuss.

Coming up....

Earthing 102: In-depth analysis of various system earthing schemes TN, TT, IT, their applications, pros and cons.

Annexure 101A: Relation between Earthing Impedance and Over-voltage

(Since Linkedin does not allow subscripts and superscripts, the derivation is attached herewith as images)

In some cases, Double Line to Ground(LLG) fault results in over-voltages that are slightly higher than the Single Line to Ground fault. But since the frequency of Single Line to Ground(LG) faults is comparatively much more, the insulation is often designed for Single Line to Ground fault scenario.