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,
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).
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.
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.
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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