Comparison

TT IT TN-S TN-C TN-C-S

Earth fault loop

RCD preferred? Yes N/A Optional No Optional

Need earth
Yes Yes No No Optional
electrode at site?

PE conductor cost Low Low Highest Least High

Risk of broken
No No High Highest High
neutral

Least
Safety Safe Less Safe Safest Safe
Safe

Electromagnetic
Least Least Low High Low
interference

High loop
Double fault, Broken Broken Broken
Safety risks impedance (step
overvoltage neutral neutral neutral
voltages)

Continuity of Safety
Advantages Safe and reliable Safest Cost
operation, cost and cost

Properties
Cost

 TN networks save the cost of a low-impedance earth connection at the site of each
consumer. Such a connection (a buried metal structure) is required to provide protective
earth in IT and TT systems.
 TN-C networks save the cost of an additional conductor needed for separate N and PE
connections. However, to mitigate the risk of broken neutrals, special cable types and lots of
connections to earth are needed.
 TT networks require proper RCD (Ground fault interrupter) protection.

Safety

 In TN, an insulation fault is very likely to lead to a high short-circuit current that will trigger an
overcurrent circuit-breaker or fuse and disconnect the L conductors. With TT systems, the
earth fault loop impedance can be too high to do this, or too high to do it within the required
time, so an RCD (formerly ELCB) is usually employed. Earlier TT installations may lack this
important safety feature, allowing the CPC (Circuit Protective Conductor or PE) and perhaps
associated metallic parts within reach of persons (exposed-conductive-parts and extraneous-
conductive-parts) to become energized for extended periods under fault conditions, which is
a real danger.
 In TN-S and TT systems (and in TN-C-S beyond the point of the split), a residual-current
device can be used for additional protection. In the absence of any insulation fault in the
consumer device, the equation IL1+IL2+IL3+IN = 0 holds, and an RCD can disconnect the supply
as soon as this sum reaches a threshold (typically 10 mA – 500 mA). An insulation fault
between either L or N and PE will trigger an RCD with high probability.
 In IT and TN-C networks, residual current devices are far less likely to detect an insulation
fault. In a TN-C system, they would also be very vulnerable to unwanted triggering from
contact between earth conductors of circuits on different RCDs or with real ground, thus
making their use impracticable. Also, RCDs usually isolate the neutral core. Since it is unsafe
to do this in a TN-C system, RCDs on TN-C should be wired to only interrupt the line
conductor.
 In single-ended single-phase systems where the Earth and neutral are combined (TN-C, and
the part of TN-C-S systems which uses a combined neutral and earth core), if there is a
contact problem in the PEN conductor, then all parts of the earthing system beyond the
break will rise to the potential of the L conductor. In an unbalanced multi-phase system, the
potential of the earthing system will move towards that of the most loaded line conductor.
Such a rise in the potential of the neutral beyond the break is known as a neutral
inversion.[9] Therefore, TN-C connections must not go across plug/socket connections or
flexible cables, where there is a higher probability of contact problems than with fixed wiring.
There is also a risk if a cable is damaged, which can be mitigated by the use of concentric
cable construction and multiple earth electrodes. Due to the (small) risks of the lost neutral
raising 'earthed' metal work to a dangerous potential, coupled with the increased shock risk
from proximity to good contact with true earth, the use of TN-C-S supplies is banned in the
UK for caravan sites and shore supply to boats, and strongly discouraged for use on farms
and outdoor building sites, and in such cases it is recommended to make all outdoor wiring
TT with RCD and a separate earth electrode.
 In IT systems, a single insulation fault is unlikely to cause dangerous currents to flow through
a human body in contact with earth, because no low-impedance circuit exists for such a
current to flow. However, a first insulation fault can effectively turn an IT system into a TN
system, and then a second insulation fault can lead to dangerous body currents. Worse, in a
multi-phase system, if one of the line conductors made contact with earth, it would cause the
other phase cores to rise to the phase-phase voltage relative to earth rather than the phase-
neutral voltage. IT systems also experience larger transient overvoltages than other systems.
 In TN-C and TN-C-S systems, any connection between the combined neutral-and-earth core
and the body of the earth could end up carrying significant current under normal conditions,
and could carry even more under a broken neutral situation. Therefore, main equipotential
bonding conductors must be sized with this in mind; use of TN-C-S is inadvisable in
situations such as petrol stations, where there is a combination of lots of buried metalwork
and explosive gases.
Electromagnetic compatibility

 In TN-S and TT systems, the consumer has a low-noise connection to earth, which does not
suffer from the voltage that appears on the N conductor as a result of the return currents and
the impedance of that conductor. This is of particular importance with some types of
telecommunication and measurement equipment.
 In TT systems, each consumer has its own connection to earth, and will not notice any
currents that may be caused by other consumers on a shared PE line

.
Regulations
 In the United States National Electrical Code and Canadian Electrical Code the feed from the
distribution transformer uses a combined neutral and grounding conductor, but within the
structure separate neutral and protective earth conductors are used (TN-C-S). The neutral
must be connected to earth only on the supply side of the customer's disconnecting switch.
 In Argentina, France (TT) and Australia (TN-C-S), the customers must provide their own
ground connections.
 Japan is governed by PSE law, and uses TT earthing in most installations.
 In Australia, the Multiple Earthed Neutral (MEN) earthing system is used and is described in
Section 5 of AS 3000. For an LV customer, it is a TN-C system from the transformer in the
street to the premises, (the neutral is earthed multiple times along this segment), and a TN-S
system inside the installation, from the Main Switchboard downwards. Looked at as a whole,
it is a TN-C-S system.
 In Denmark the high voltage regulation (Stærkstrømsbekendtgørelsen) and Malaysia the
Electricity Ordinance 1994 states that all consumers must use TT earthing, though in rare
cases TN-C-S may be allowed (used in the same manner as in the United States). Rules are
different when it comes to larger companies.
 In India as per Central Electricity Authority Regulations, CEAR, 2010, rule 41, there is
provision of earthing, neutral wire of a 3-phase, 4-wire system and the additional third wire of
a 2- phase, 3-wire system. Earthing is to be done with two separate connections. The
grounding system must also have a minimum of two or more earth pits (electrodes) to better
ensure proper grounding. According to rule 42, installation with connected load above 5 kW
exceeding 250 V shall have a suitable Earth leakage protective device to isolate the load in
case of earth fault or leakage.[10]

Application examples
 In the areas of UK where underground power cabling is prevalent, the TN-S system is
common.[11]
 In India LT supply is generally through TN-S system. Neutral is double grounded at each
distribution transformer. Neutral and earth conductors run separately on overhead
distribution lines. Separate conductors for overhead lines and armoring of cables are used
for earth connection. Additional earth electrodes/pits are installed at each user end to provide
redundant path to earth.[12]
 Most modern homes in Europe have a TN-C-S earthing system.[citation needed] The combined
neutral and earth occurs between the nearest transformer substation and the service cut out
(the fuse before the meter). After this, separate earth and neutral cores are used in all the
internal wiring.
 Older urban and suburban homes in the UK tend to have TN-S supplies, with the earth
connection delivered through the lead sheath of an underground lead-and-paper cable.
 Older homes in Norway uses the IT system while newer homes use TN-C-S.
 Some older homes, especially those built before the invention of residual-current circuit
breakers and wired home area networks, use an in-house TN-C arrangement. This is no
longer recommended practice.
 Laboratory rooms, medical facilities, construction sites, repair workshops, mobile electrical
installations, and other environments that are supplied via engine-generators where there is
an increased risk of insulation faults, often use an IT earthing arrangement supplied
from isolation transformers. To mitigate the two-fault issues with IT systems, the isolation
transformers should supply only a small number of loads each and should be protected with
an insulation monitoring device (generally used only by medical, railway or military IT
systems, because of cost).
 In remote areas, where the cost of an additional PE conductor outweighs the cost of a local
earth connection, TT networks are commonly used in some countries, especially in older
properties or in rural areas, where safety might otherwise be threatened by the fracture of an
overhead PE conductor by, say, a fallen tree branch. TT supplies to individual properties are
also seen in mostly TN-C-S systems where an individual property is considered unsuitable
for TN-C-S supply.
 In Australia, New Zealand and Israel the TN-C-S system is in use; however, the wiring rules
state that, in addition, each customer must provide a separate connection to earth, via a
dedicated Earth electrode. (Any metallic water pipes entering the consumer's premises must
also be "bonded" to the Earthing point at the distribution Switchboard/Panel.) In Australia and
New Zealand the connection between the Protective Earth bar and the Neutral bar at the
main Switchboard/Panel is called the Multiple Earthed Neutral Link or MEN Link. This MEN
Link is removable for installation testing purposes, but is connected during normal service by
either a locking system (locknuts for instance) or two or more screws. In the MEN system,
the integrity of the Neutral is paramount. In Australia, new installations must also bond the
foundation concrete re-enforcing under wet areas to the Protective Earth conductor
(AS3000), typically increasing the size of the earthing (i.e. reducing resistance), and
providing an equipotential plane in areas such as bathrooms. In older installations, it is not
uncommon to find only the water pipe bond, and it is allowed to remain as such, but the
additional earth electrode must be installed if any upgrade work is done. The incoming
Protective Earth/Neutral conductor is connected to a Neutral Bar (located on the customer's
side of the electricity meter's neutral connection) which is then connected via the customer's
MEN link to the Earth bar – beyond this point, the Protective Earth and Neutral conductors
are separate.