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Left: Phase-to-earth fault protection (resistance-earthed neutral at busbars); Right: Phase-to-earth fault protection (isolated neutral)
Left: Phase-to-earth fault protection (resistance-earthed neutral at busbars); Right: Phase-to-earth fault protection (isolated neutral)

Introduction to Electrical network protection guide

Among their multiple purposes, protection devices:

  • Contribute to protecting people against electrical hazards,
  • Avoid damage to equipment (a three-phase short-circuit on medium-voltage busbars can melt up to 50 kg of copper in one second and the temperature at the centre of the arc can exceed 10000 °C),
  • Limit thermal, dielectric and mechanical stress on equipment,
  • Maintain stability and service continuity in the power system,
  • Protect adjacent installations (for example, by reducing induced voltage in adjacent circuits).
In order to attain these objectives, a protection system must be fast, reliable and ensure discrimination. Protection, however, has its limits because faults must first occur before the protection system can react. Protection therefore cannot prevent disturbances; it can only limit their effects and their duration.

Furthermore, the choice of a protection system is often a technical and economic compromise between the availability and safety of the electrical power supply.

Protection system
Fiigure 1 – Protection system


The process for identifying the need for an UPS system, selecting, installing, and maintaining the UPS system are covered.

Covered topics: – Theory and principles of static and rotary UPS systems, design and selection of UPS, installation and testing of UPS, maintenance and operation of UPS systems, principles of static and rotary UPS, UPS system rating and sizing selection, operations/maintenance, batteries, troubleshooting, harmonic distortions, grounding, checklists, and acceptance testing.

Protection units continuously monitor the electrical status of power system components and de-energize them (for instance by tripping a circuit breaker) when they are the site of a serious disturbance such as a short-circuit, insulation fault, etc.

The choice of a protection device is not the result of an isolated study, but rather one of the most important steps in the design of the power system.

Based on an analysis of the behaviour of electrical equipment (motors, transformers, etc.) during faults and the phenomena produced, this guide is intended to facilitate your choice of the most suitable protective devices.

Designing power system protection

The design of protection for a power system can be broken down into two distinct steps:

  1. Definition of the protection system, also called the protection-system study,
  2. Determination of the settings for each protection unit, also called protection coordination or discrimination.

Definition of the protection system

This step includes selection of the protection components and a consistent, overall structure suited to the power system. The protection system is made up of a string of devices including the following (refer to Figure 1):

  1. Measurement sensors (current and voltage) supplying the data required to detect faults,
  2. Protection relays in charge of continuously monitoring the electrical status of the power system up to and including the formulation and emission of orders to the trip circuit to clear the faulty parts,
  3. Switchgear in charge of clearing faults, such as circuit breakers or combinations of switches or contactors and fuses.

The protection-system study determines the devices to be used to protect against the main faults affecting the power system and the machines:

  1. Phase-to-phase and phase-to-earth short-circuits,
  2. Overloads,
  3. Faults specific to rotating-machines.

The protection-system study must take the following parameters into account:

  1. Power system architecture and size, as well as the various operating modes,
  2. The neutral-earthing systems,
  3. The characteristics of current sources and their contributions in the event of a fault,
  4. The types of loads,
  5. The need for continuity of service.
Title:Electrical network protection guide – Schneider Electric
Size:480 kB
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Electrical network protection guide - Schneider Electric
Electrical network protection guide – Schneider Electric

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  1. mohamed
    Jun 21, 2018

    thanks for sharing

    Jan 30, 2018

    Thanks for what you have been doing by sending updated and related materials to my mail to download freely. I am highly impressed with this gesture..I will need materials on Electrical Drafting and Design.

  3. Girish R Nair
    Jul 27, 2016

    Your effort is incomparable. But I am trying to answer to some questions posted by your readers, as far as my knowledge. Let me know if I’m wrong in any points. Thanks a lot

  4. Tran Van Chuong
    Jul 10, 2016

    Tell me how to download this document,please?

  5. Osvaldo Rodrigues da Silva
    Aug 30, 2015

    you guys have a spreadsheet for short circuit calculation

  6. ali
    Jul 05, 2015

    Hi dear sir… We have the breaker to protect leakage and short circuit for tripping, I think this protective device it’s no need

  7. komichi
    Jun 16, 2014

    many thanks; we don’t forget your efforts; again thank you very much.

  8. Antonio Melgarejo
    Oct 06, 2013

    about distribution pipes of drinking water in buildings… Should be grounded?
    if a building does not they to groundend, is it good to decide to put their own home ground? Is it good turning the home into the fate of stray currents?

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