Earthing systems: TN-S, TN-C, TN-C-S, TT and IT

What the letters mean

IEC 60364, and national rules built on it such as BS 7671, describe a low-voltage system with two or three letters. The scheme probably comes from French (terre, neutre, isolé, séparé, combiné), but you only need to read it left to right. (The names are often written without hyphens, as TNS, TNCS or TNC; they mean the same thing.)

• First letter, the source. T: one point of the supply, normally the transformer’s star point, is connected directly to earth. I: no point is directly earthed; the system is isolated, or earthed only through a high impedance.
• Second letter, the installation’s exposed metalwork. T: connected to an earth electrode of the installation’s own, independent of the source earth. N: connected by a conductor to the source’s earthed point.
• Further letters, for TN only. S: neutral and protective functions run in separate conductors. C: they are combined in one conductor, the PEN. C-S: combined in part of the system, separate in the rest.

The arrangement matters because it decides the earth fault loop, the path a fault current takes from the source, through the fault and back. A low-impedance loop gives hundreds of amperes and a breaker or fuse clears the fault in a fraction of a second. A high-impedance loop gives a few amperes, and only a residual current device will notice. The diagrams below show each arrangement with the same fault: a line conductor touching a metal case.

TN-S

In TN-S the protective conductor is separate from the neutral all the way from the source. In older UK street supplies it was often the lead sheath or armour of the supply cable. TN-S is also the normal arrangement on a site with its own transformer, where the PE and the neutral leave the transformer separately. The typical maximum external loop impedance UK distributors declare for TN-S is 0.8 Ω.

TN-C

TN-C uses one PEN conductor for both jobs right through to the equipment. It saves a conductor, which is why distribution networks use it, but it has two drawbacks inside buildings. Neutral current flows in the same conductor that bonds the metalwork, and if the PEN breaks, every case connected to it can rise towards line voltage. RCDs also cannot work in TN-C, because the outgoing and returning currents both pass through the device, so a fault to the PEN is invisible to it. In the UK a consumer may not combine neutral and protective functions in one conductor inside an installation (ESQCR regulation 8(4)).

TN-C-S and PME

TN-C-S is the usual modern UK supply. The distributor runs a combined neutral and earth in the street and connects it to earth at many points along the network: protective multiple earthing (PME). At your cut-out the installation’s main earthing terminal is connected to that PEN, and from there neutral and protective conductors run separately. Fault loops are short and metallic: the typical maximum Ze for a PME supply is 0.35 Ω, the lowest of the UK arrangements.

The price is the PEN. If it breaks upstream, the installation’s earthed metalwork is connected through the loads to the line conductors and can rise to a dangerous voltage; the IET notes it can reach line voltage, and up to 400 V where a three-phase cable is involved. Main protective bonding keeps everything inside the building at the same potential, so the risk is greatest outdoors, where a person in contact with the metal also stands on true earth. Distributors may not offer a PME earth for some installations at all, such as caravans, boats and fuel filling stations.

TT

In TT the distributor provides no earth: the installation’s exposed metalwork is connected to its own electrode, usually a rod. Fault current must leave through that electrode, cross the soil and return through the supply transformer’s earth. In the UK, TT is used where the distributor does not offer a TN earth or where one may not be used, including the caravans, boats and fuel stations above, and for farms and other premises with a lot of outdoor wiring. Elsewhere it is the normal arrangement for public supplies in countries such as France and Spain, and in Japan.

The distributor’s side of a TT loop is typically up to 21 Ω before your electrode is added. The IET’s advice is to keep the electrode resistance as low as practicable, because values above about 200 Ω may not stay stable as the soil dries or freezes.

IT

In IT nothing at the source is connected solidly to earth. A first fault from a line conductor to earthed metalwork closes no low-impedance loop: only a small current flows back through the capacitance of the cables. With an illustrative 1 µF per phase at 50 Hz, that is about 0.22 A, which raises a 10 Ω electrode by only 2.2 V. IEC 60364-4-41 lets the installation keep running as long as RA × Id stays within 50 V, so supply continues while an insulation monitoring device raises an alarm. A second fault on another line conductor is a line-to-line short through the metalwork and must be disconnected, under the TT rules where metalwork is earthed in groups or the TN rules where it is all connected together.

That is why IT is chosen where an unplanned shutdown is worse than an alarm: operating theatres and other Group 2 medical locations (IEC 60364-7-710; Section 710 of BS 7671, rewritten in Amendment 4), process plants and mines. Distributing the neutral in an IT system is discouraged, because it makes the second-fault conditions harder to meet. Norway is the rare country with IT public supplies; an estimated 70% of households there are connected to IT networks.

The five arrangements compared

The fault loop and disconnection times

Under automatic disconnection of supply, the protective device has to clear an earth fault fast enough that touch voltage cannot hurt anyone. For 230 V final circuits, IEC 60364-4-41 and BS 7671 set 0.4 s in TN systems and 0.2 s in TT systems; distribution circuits get 5 s (TN) and 1 s (TT). BS 7671 applies the shorter times to final circuits up to 63 A with socket-outlets and up to 32 A for fixed equipment; IEC 60364 uses 32 A for both.

Whether a device can meet those times depends on the loop impedance, Zs: the external part Ze plus the line and protective conductors of the circuit (R1 + R2). Take a 25 m radial in 2.5 mm² twin and earth with a 1.5 mm² protective conductor, which adds 0.585 Ω at operating temperature, protected by a 20 A Type B circuit-breaker. That breaker needs Zs of 2.19 Ω or less to trip on its magnetic element within the time limit; the figure already allows for the supply running low, through BS 7671’s minimum voltage factor of 0.95. Fault currents below are at the nominal 230 V:

The TT row is why TT and RCDs go together. During that fault the metal case would sit about 189 V above true earth until something disconnects. You can check earth fault loop impedance for your own circuit, calculate prospective fault current at the origin, and size the circuit protective conductor so that it survives the fault until the device opens.

The role of RCDs

A residual current device compares the current going out on the line conductors with the current returning on the neutral. Any difference has leaked to earth, and above the device’s rating it trips. That makes RCDs indifferent to loop impedance, which is exactly what TT needs: the condition is RA × IΔn ≤ 50 V, so a 30 mA RCD tolerates an electrode of up to 1,667 Ω in principle. In practice the IET’s 200 Ω stability guide is the figure to design to.

In TN systems RCDs play a second role: additional protection. BS 7671 requires 30 mA RCDs on most socket-outlets up to 32 A and in special locations, as a backup in case basic or fault protection fails or a user is careless. They cannot be used in TN-C, and in IT systems the threshold has to be chosen with care, because the first-fault current is small and flows through the healthy phases’ capacitance.

PEN conductor rules

• A PEN in a fixed installation must be at least 10 mm² copper or 16 mm² aluminum, sizes chosen to make an accidental break unlikely.
• No device, single-pole or multi-pole, may be able to disconnect a PEN, because opening it would leave the metalwork connected to the line conductors through the loads.
• Once neutral and protective functions are separated, they may not be recombined downstream.
• In the UK the PEN belongs to the distributor; inside the consumer’s installation neutral and protective conductors must be separate.

Here is why a broken PEN matters, in a deliberately simple model: one installation with a 3,000 W load whose PEN has broken, with its earthed metalwork reaching true earth through a resistance RE. The metalwork rises to 230 V × RE / (Rload + RE): 122 V with RE = 20 Ω and 196 V with RE = 100 Ω. Holding it to 70 V would need RE of about 7.7 Ω or less. Real networks are worse, because neighbours on the same broken PEN add their loads, and that is why the UK rules lean on detection rather than on electrodes alone.

EV charging and the open-PEN problem in the UK

An EV charger outdoors combines both risk factors: a long run of connected metalwork, the car body, and a person standing on true earth. Since Amendment 1 of 2020, Regulation 722.411.4.1 of BS 7671 says a PME earth may not be used for a charging point outdoors, or one that could charge a vehicle outdoors, unless one of these measures is in place:

1. The charger is part of a three-phase installation whose load balance keeps the voltage between the main earthing terminal and true earth within 70 V if the PEN breaks.
2. An earth electrode connected to the main earthing terminal is low enough in resistance to hold that voltage within 70 V.
3. A device disconnects the vehicle from the line conductors and from protective earth within 5 s when the voltage between the protective conductor and true earth exceeds 70 V.
4. A device disconnects within 5 s when the voltage at the charger leaves the 207 to 253 V band, which is how many single-phase chargers detect a broken PEN.
5. An equivalent device, which may be built into the charging equipment.

The other way out is to make the charger TT, with its own electrode and its metalwork kept apart from the PME-earthed metalwork of the building. Open-PEN detection devices now have their own IET standard, IET 01, written to work with Section 722. The IET’s 2023 guidance on these devices works to the same regulation under Amendment 2, and the published summaries of Amendments 3 and 4 do not mention a change to it; check the current text before relying on an option. Because the voltage-band devices measure at the charger, a long or undersized cable can trip them on an ordinary low-voltage day; the IET recommends designing the charger circuit for no more than 3% voltage drop. See voltage drop for the numbers.

Mapping to NEC terms

North American codes describe the same physics in different words. A standard US service is TN-C-S in all but name: the utility’s multi-grounded neutral acts as a PEN up to the service, and the main bonding jumper splits it into a grounded conductor and equipment grounding conductors. The NEC has no TT option for ordinary premises wiring, because it does not accept the earth as a fault-current path. Its ungrounded and high-impedance grounded systems correspond to IT. More on the US side is in grounding and bonding.