Lesson 6 of 21
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What is Earth Loop Testing

NAC-Trainer March 10, 2020

What are earthed and unearthed systems? What are the requirements of BS 7671? What are the advantages and disadvantages of the various types of earthing systems?

This lesson, which is based on IEE Guidance notes, is intended to provide information that it is hoped will prove helpful.

BS 7671 lists five types of earthing system and out of the 5, 3 are used commonly in the UK:

  • TN-S
  • TN-C-S
  • TT
  • TN-C (Uncommon in the UK)
  • IT (Uncommon in the UK)

What do they mean?

  • T = Earth (from the French word Terre)
  • N = Neutral
  • S = Separate
  • C = Combined
  • I = Isolated (The source of an IT system is either connected to earth through a deliberately introduced earthing impedance or is isolated from Earth. All exposed-conductive-parts of an installation are connected to an earth electrode.)

During an electrical fault on a circuit, a current will flow from the Line conductor towards Earth and in to the Neutral point of the supply company transformer.

This circuit (loop), which consists of all the elements within the loop (supply transformer winding, supply company phase conductor, main fuse, main switch, protective device, Line conductor, the fault, Earth conductor, main earth terminal, supply company earthing conductor) is the Earth Fault Loop. Its impedance is the measured value in Ohms. 

In short, Earth loop impedance testing – which is often simply called loop testing – is to verify that, if a fault occurs in an electrical installation, sufficient current will flow to operate the fuse or circuit breaker protecting the faulty circuit within a predetermined time. The objective is to make sure that the circuit is disconnected fast enough to prevent overheating and possibly a fire.

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