Scope of the grounding system

The grounding system for an electrical system creates the connection to earth and must meet the following requirements:

  • Discharge the voltage from touchable metallic system parts (chassis) to protect people from electric shocks

  • Discharge over-currents from direct lightning strikes to earth

  • Discharge induced currents from atmospheric discharges between two points of a power transmission line to earth

EMC performance and guidelines for the grounding system

System

EMC performance

Guidelines for use

TT

Conditionally suitable

Between the ground connection of the primary distribution network and the leakage current from the electrical system, that are caused by ground faults within the system. As with transient currents, potential difference occurs due to the leakage current from devices. These transient currents can lead to faulty couplings or even faults within the system.

A corrective measure here is a equipotential bonding conductor between the devices connected directly to earth. These measures basically convert the TT system into a TN-S system.

  • ELCB for personnel safety is required

  • Surge arresters should be installed (distributed over power transmission lines)

  • This type of network requires corresponding measures for devices with high leakage current potential that are located behind the ELCB in the outgoing direction

TN-C

TN-C-S

Bad

Because the functions and protective earth conductor are combined in TN-C and TN-C-S systems, current feedback occurs on the PEN conductors during normal operation. Current feedback from devices via the PEN conductors can lead to faulty couplings.

  • Ensure an unobstructed path for the PEN conductor when expanding the system!

  • Because of the high current in the PEN conductor this system is not permitted in areas of particularly dangerous sources.

  • If devices with high total harmonic distortion are operated in a system, this type of system is not recommended.

TN-S

Very good

The TN-S system is the best solution from an EMC point of view. The PE conductors have no power in normal operation.

  • Ensure an unobstructed path for the PE conductor when expanding the system!

  • A 500 mA ELCB must be installed for protection against fire.

  • Corresponding measures are required for devices with high leakage current potential that are located behind the ELCB in the outgoing direction.

IT

Bad

Note: The IT system is recommended as intrinsically safe for safety matters since no electric arcs can occur.

  • Ensure an unobstructed path for the PE conductor when expanding the system!

  • Filters for asymmetric interference currents cannot be installed.

  • Good EMC is only provided within systems (buildings) where all devices are connected to the same grounding device.

  • If circumstances dictate that the system must be divided to limit the cable lengths and leakage currents.

Recommended grounding system connection scheme

The following illustration shows a typical connection scheme for a grounding system:

A Lightning arrester down-lead

B Underground meshed earthing system with reinforcement at foot of the down-lead

C System ground connection, connected to the equipotential bonding strip, to which the PE conductor or PEN conductor are connected in turn

D Earthing system for a system section with integral metallic structures or additional ground connections (E)

E Interconnection between the lightning arrester down-lead and the earthing system as well as other metal structures in the vicinity

NOTE: A single, specifically laid ground connection is required for every electrical system and is in itself sufficient.