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Axis Basic Earthing System

What is TT Earthing System? Detailed Explanation

Scope

 

Over the last century, electrical safety standards have evolved into very sophisticated systems that encompass all components of a safe installation, including TT Earthing Systems and other types in the category. In Low-voltage (LV) Electrical Installations, the reference standard IEC 60364 is used to determine the actions to ensure the safety of employees and property.

 

Introduction

 

The IEC 60364 standard divides Earthing Systems into three categories: TT, IT, and TN. Because IEC develops International Standards for all electrical, electronic, and associated technologies and is the leading international organisation in its field, IEC 60364 is the apex level publication that informs the standards for LV Electrical Installations.

 

Axis Basic Earthing System

As a result, several national standards recognise IEC 60364's three types of earthing systems. The IEEE Wiring Regulations 17th Edition, also known as British Standard BS 7671: 2008, was published in January 2008 and is used in the UK and other countries. Indian Standard IS 732:1989 governs electrical installations in India (R2015).

 

Public Distribution Example Worldwide (LV Consumers) – LV Earthing System
Sr. No. Country Voltage LV Earthing System
1 India 230/415 TN-S
2 Germany 230/400 TT & TN-C
3 Belgium 230/400 TT
4 Spain 240/400 TT
5 France 240/415 TT
6 Great Britain 240/415 TT & TN-C
7 Italy 230/400 TT
8 Japan 100/200 TT
9 Norway 230/400 IT
10 Portugal 230/400 T
11 USA 120/240 TN-C-S

What is TT Earthing?

The connection to the supply source is directly connected to the earth, and the load end or installation metalwork is also directly connected. In this sort of Earthing System. As a result, in the case of an overhead line, the earth's mass will serve as the line's return path.

First: T stands for Direct connection to the Earth & Second: T stands for Terra meaning

 

Axis TT Earthing System

Axis TT Earthing System Earth

 

Because the power distributor only offers the supply neutral or protective conductor for the connection to the consumer, the neutral and earthing conductors must be separated during the installation. Small networks or sections downstream from TN and TT systems are where IT is usually deployed.

    

Significance of TT Earthing System and its graphical representation

 

This system is set up similarly to the TN-S system, except it does not provide users with their earth connection. Consumers must instead provide their dirt, such as by burying rods or plates underground to produce a low-impedance channel.

TT earthing using IEC 60364

TT Earthing Basic connection diagram using IEC 60364

 

TT Earthing Using 60364

TT Earthing application diagram using IEC 60364                        Source - megger.com

 

When TN-C-S setups aren't possible (in the gas station example) or rural regions where overhead poles provide power. Hence, people use TT systems. Where diverse soil types that may create external earth fault loop impedance values exist, shock protection techniques such as RCDs provide immediate supply cut-off.

 

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TT earthing main characteristics:

-The simple design and installation solution that gets power from the public LV distribution network directly.

-No continual monitoring is required (a periodic check on the RCDs may be necessary).

-Special devices, known as residual current devices (RCDs), provide protection and reduce the chance of fire when set to less than 500 mA.

-Each insulation fault causes a power outage, but the outage is limited to the faulty circuit by connecting the RCDs in series (selected RCDs) or in parallel (parallel RCDs) (circuit selection).

-Loads or sections of the installation that create large leakage currents during normal operation require additional precautions to avoid nuisance tripping, such as supplying the loads with a separation transformer or using appropriate RCDs.

 

Comparison of various earthing systems:

Sr. No. TT IT TN-S TN-C TN-C-S
Earth Fault Loop Impedance High Highest Low Low Low
RCD Preferred? Yes NA Optional No Optional
Need earth electrode at site? Yes Yes No No Optional
PE Conductor cost Low Low Highest Least High
Risk of broken neutral No No High Highest High
Safety Safe Less safe Safest Least safe Safe
Electromagnetic Interference Least Least Low High Low
Risks of safety High loop impedance Double fault Broken Neutral Broken neutral Broken neutral
Advantages Safe & Reliable Continuity of operation Safest Cost Safety & Cost

 

Importance of RCD in TT Earthing System:

-All exposed-conductive portions of the installation/loads must connect to a common earth electrode in a TT earthing system.

-The source system's neutral point is earthed outside of the influence area via the ground electrode. 

-As a result, the earth fault impedance consists primarily of two earth electrodes in series (i.e., the source and installation electrodes). The magnitude of the earth fault current is generally too small to operate overcurrent relays or fuses, necessitating the use of a residual current operated device as first-level isolation.

-A TT system's automatic disconnection is done by an RCD with a sensitivity of IΔn ≤ 50\Ra (where RA is the resistance of the installation earth electrode.

-IΔn is the RCD's rated residual operating current.

-The earth electrode's resistance for the installation is RA.

-The value of 50 V is substituted by 25 V for temporary supply (to work sites, etc.) and agricultural and horticultural premises.

 

Importance of RCD in TT Earthing System:

        Source - scirp.org

-The earth electrode of the substation neutral Rn has a resistance of 10 ohms.

-The earth electrode of the installation RA has a resistance of 20.

-Id = 7.7 A is the earth-fault loop current.

-The fault voltage Uf = Id x RA = 154 V which is serious, but in 50/20 = 2.5 A, which means that a 300 mA RCD will clear the fault in roughly 30ms without intentional time delay if a fault voltage exceeding occurs on an exposed-conductive-part.

-The RCD will close the circuit if there is a leakage current in the system. By detecting tiny leakage currents, they may be able to avert electrocution as part of the automatic disconnection of power, which switches off when a fault develops in the circuit rather than requiring human intervention.

 

TT Earthing AC Power Supply System:

 

The grounding protection can considerably reduce the risk of electric shock when the metal shell of the electrical equipment is charged (the phase line hits the shell or the equipment insulation is damaged and leaks).

Low-voltage circuit breakers (automatic switches) do not always trip. Leading to the leakage device's earth-leakage voltage exceeding the acceptable voltage, which is a harmful voltage.

Even a fuse may not be able to explode when the leakage current is minimal. As a result, a leakage protector is mandatory for safety. As a result, popularising the TT system is challenging.

The TT system's grounding device uses a lot of steel. Recycling it, as well as time and materials, is problematic.

Construction companies use TT. A unique protection line is utilised to reduce the quantity of steel used for the grounding mechanism when the construction unit borrows its power supply for temporary energy use.

Separate the new special protection line PE from the working zero-line N, which has the following characteristics:

  1. The grounding line and the working neutral line have no electrical connection.
  2. The working zero lines can have current in operation, but the protection line can't.
  3. The TT system is well suited to areas where ground protection is sparse.

 

Applications:

Remote areas use TT, where the cost of an additional PE conductor outweighs that of a local earth connection. Especially in older properties where safety might otherwise be jeopardised by the fracture of an overhead PE conductor by, say, a fallen tree branch. In most TN-C-S systems, TT supplies to individual homes are also seen when a property is deemed unfit for TN-C-S service.

Installations on a construction site, a farm, or a swimming pool may be improper unless additional measures, like an additional earth electrode, are adopted.

LV system earth.

All metallic enclosures and extraneous parts are at equipotential.

Protection against lightning due to low fault current.

Protection against the earth fault current due to low resistance path.

Protect the machine and human being against lightning and earth faults conditions.

 

Advantages of TT Earthing:

-Simplicity is key (very few calculations when installing)

-Extension that does not necessitate the calculation of lengths

-Fault currents are low

-There is very minimal upkeep

-When portable gadgets are given or if the earthing is poor, people's safety is compromised (with 30 mA RCDs)

-Operation with a low projected current source

-Ease of entry for conductors, smooth edges

-It is economical, safe and high graded in use

-Good withstanding capacity against harmonics supplied in the system

 

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