Earthing Arrangements

An earthing, or “neutral load” arrangement on an LV network is defined by two letters:

The first defines the earth connection of the transformer’s secondary (in most cases neutral)
The second defines the mases connection to earth

T  = Earthed
N  = Connected to neutral
I   = Insulated from earth

TT: “neutral to earth” load

Use of this type of load is generally stipulated by the electricity board. Should there be an insulation fault, all or part of the operational equipment is cut off.

Cut off is obligatory at first fault. The operational equipment must be fitted with instantaneous differential protection.

Differential protection can be general or subdivided according to the type and size of the installation.

This type of load can be found in the following contexts:
  • Domestic
  • Minor tertiary
  • Small workshops/processes,
  • Educational establishments with practical workshops, 

TT Earthing Schematic

TN: “neutral connection” load
This distribution principle is suited to all networks which have a cut off system at first fault.
Installing and operating this type of network is economical but requires rigorous general circuit protection. Neutral (N) and protective (PE) conductors can be common (TNC) or separated (TNS).

TNC arrangement
The protective and neutral conductor (PEN) must never be
sectioned. Conductors must have a section over 10 mm2
in copper and over 16 mm2 in aluminium, and must not  include mobile installations (flexible cables).

TN Earthing Schematic

TNS arrangement
A TNS network can be set up upstream of a TNC network, where as the opposite is forbidden. Neutral TNS conductors are generally sectioned, unprotected, and have the same sections as the corresponding phase conductors.

TNS Earthing Schematic

TNC-S arrangement
A TNC-S arrangement indicates distribution in which the neutral conductors and protection conductors are combined in one part of the installation and distinct in the rest of the installation.

IT: “insulated neutral” load
This neutral load is used when first fault cut off is detrimental to correct operation or personnel safety.

Implementing this type of installation is simple, but requires qualified personnel on-site to intervene quickly when faulty
insulation is detected, to maintain continuous operation and before a possible second fault leads to cut-off.

An overvoltage limitor is compulsory to enable overvoltage caused by HV installations (such as HV/LV transformer breakdown,
operations, lightning, etc.), to flow to earth.

Personnel safety is ensured by:
- Interconnecting and earthing of masses,
- monitoring first fault by IMD (Insulation Monitoring Device),
- using second fault cut off by overcurrent protection devices, or by differential devices.

This system can be found, for example, in hospitals (operating theatres), or in safety circuits (lighting) and in industries where continuity of operations is essential or where the weak default current considerably reduces the risk of fire or explosion.

IT Earthing Schematic

Socomec  have published an application guide for its range of industrial switching and protection systems, including very useful detailed design information on Earthing, LV distribution, overload currents, short-circuit currents, direct and indirect contact and various other topics. 

This article has been extracted from the guide "Application Guide Industrial Switching and Protection Systems 2011"

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