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Earthing systems (ES): Diagram 1a (left) - "TN-S system"; Diagram 1b (right) - "TN-C system"
Earthing systems (ES): Diagram 1a (left) - "TN-S system"; Diagram 1b (right) - "TN-C system"

Safety and installation standards

The IEC 60364 standard defines 3 types of earthing systems (ES):

  • TN system
  • TT system
  • IT system

Earthing systems (ES) characteristics are:

  1. An insulation fault has varying consequences depending on the system used:
    • Fault that is dangerous or not dangerous for persons
    • Strong or very weak fault current.
  2. If the fault is dangerous, it must be quickly eliminated
  3. The PE is a conductor.

The TT system combined with Residual Current Devices (RCD) reduces the risk of fire.

Defined by installation standards, basic principles for the protection of persons against the risk of electrical shocks are:

  1. The earthing of exposed conductive parts of equipment and electrical loads
  2. The equipotentiality of simultaneously accessible exposed conductive parts that tend to eliminate touch voltage
  3. The automatic breaking of electric power supply in case of voltage or dangerous currents caused by a live insulation fault current.

The IEC 60364 standard

Since 1997, IEC 364 is identified by a no.: 60 XXX, but its content is exactly the same.

Earthing systems (ES)

The IEC 60364 standard, in § 3-31 and 4-41, has defined and developed 3 main types of earthing systems (ES). The philosophy of the IEC standard is to take into account the touch voltage (Uc) value resulting from an insulation fault in each of the systems.

Protection using an Residual Current Device – RCD

Residual current device (RCD) - Wiring diagram
Residual current device (RCD) - Wiring diagram

Residual Curren rDevices RCDs with a sensitivity of 300 mA up to 30 A must be used in the TT system. Complementary protection using an RCD is not necessary for the TN or IT systems in which the PE is carried out using a conductor. For this reason, the type of protection using an RCD must be:

  • High Sensitivity (HS) for the protection of persons and against fire (30 mA / 300 mA)
  • Low Sensitivity (LS) up to 30 A for the protection of belongings.

This protection can be carried out by using specific measuring toroids that cover all of the live conductors because currents to be measured are weak. At the supply end of an installation, a system, which includes a toroid that measures the current in the PE, can even be carried out using high sensitivity RCDs.

AUTHOR: Schneider Electric experts

Title: Schneider Electric Low voltage expert guides n° 2 – Ground Fault Protection
Format: PDF
Size: 6.9MB
Pages: 60
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Schneider Electric Low voltage expert guides n° 2 - Ground Fault Protection

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  1. Aibrahimo
    May 12, 2014

    Is there any need to bring an earth cable along to a supply cable form a transformer to a DB, if the place (metalic containers) were i need the power is already earthed?

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