TN-C-S
The difference at a glance
A PEN upstream, separate N and PE after the separation point.
Compare the five systems- 1 →Power supply
- 2 →Electrical panel
- 3Electrical appliance
On the left is where electricity comes from: a transformer in this diagram. On the right are the wiring and an electrical appliance in your home. A washing machine is shown as an example.
The incoming cable from the power supply enters the panel. It contains protective devices and conductor connection points. Separate cables then serve sockets, lighting and appliances; one such circuit is shown here.
Simplified single-phase illustration, not a wiring instruction. Protective devices and bonding are not shown in full.
Show contents
Four symbols, four roles
- L
- Line conductor: supplies electrical energy to the load.
- N
- Neutral conductor: carries load current back in the single-phase circuit shown.
- PE
- Protective conductor: connects exposed metal enclosures to the protective system; normally carries no load current.
- PEN
- Combined conductor: performs both the PE and N functions.
PEN: one conductor, two jobs
PEN is one shared conductor performing both the N and PE functions. Its name combines PE (protective conductor) and N (neutral conductor). PEN can carry the appliance’s normal operating current.
Before separation
After separation
N: carries operating current back from the appliance to the supply.
PE: connects the metal enclosure to the protective system and provides a path for current if insulation fails.
In TN-C-S, PEN separates at a designated point into two conductors: N for normal operation and PE for protection. They must not be joined again downstream. The illustration places this point at the panel intake for simplicity; in a real building it may be upstream of this panel.
If PEN breaks, both the operating circuit and protective connection are lost, and metal enclosures can become live. An electrician must design and carry out PEN separation; a DIY bridge at a socket is not a substitute.
How to read the letters
First letter — source: T = directly earthed; I = isolated from earth or earthed through a high impedance.
Second letter — enclosures: N = connected to the earthed source point; T = earthed through a local electrode.
Further letters — conductors: S = separate PE and N; C = combined PEN. C-S means combined first, then separate.
Compare the five systems
TN-S
N and PE stay separate from the source to the load.
During a fault: Fault current returns to the source through PE. A protective device must disconnect within the required time.
Technical details
For your plan: Fast circuit-breaker operation depends on fault-loop impedance and the breaker characteristic; it must be verified. RCD requirements also apply. Keep N and PE separate within the installation.
TN-C
A shared PEN combines protective and neutral functions.
During a fault: A broken PEN can leave connected metal enclosures at a dangerous voltage.
Technical details
For your plan: An RCD cannot be used in the TN-C section. PEN conductors have special requirements, generally including a minimum of 10 mm² copper or 16 mm² aluminium in fixed installations. Have existing installations assessed by an electrician.
TN-C-S
A PEN upstream, separate N and PE after the separation point.
During a fault: Downstream of the separation, fault current returns via PE and the upstream PEN to the source.
Technical details
For your plan: Separate PEN before any RCD. Never reconnect N and PE downstream. The location depends on installation design, national rules and network operator requirements; the main earthing terminal is not automatically the separation point.
TT
Metal enclosures are earthed through the installation’s local electrode.
During a fault: Fault current returns through electrodes and soil. It is often too small for a circuit-breaker to disconnect quickly.
Technical details
For your plan: RCDs normally provide fault protection. Their selection, arrangement and selectivity depend on the installation; a single central RCD is not a universal solution. Verify electrode resistance and disconnection conditions.
IT
The source is isolated from earth or earthed through a high impedance. Enclosures are still earthed.
During a fault: Operation may continue after a first fault under specified conditions. Insulation monitoring signals the fault, which must be located and cleared.
Technical details
For your plan: A second fault on another live conductor can require disconnection. IT needs coordinated protection and monitoring and is used for specialist applications.
Simplified single-phase illustration, not a wiring instruction. Protective devices and bonding are not shown in full.
What happens during a fault?
A closer look at the fault
The illustration shows the instant before protection disconnects, not a permissible operating state.
Simplified single-phase illustration, not a wiring instruction. Protective devices and bonding are not shown in full.
TN-S
① In TN-S, N and PE connect to the same earthed point at the transformer. Two separate conductors then run to the home: blue N and green/yellow PE. This common point is on the supply side; N and PE must not be reconnected in the home’s panel.
TT
Which system does my home have?
Check the documents: supply records, circuit diagrams and inspection reports.
Ask the network operator which system is provided at the supply connection.
Have an electrician verify the actual installation, including earthing, conductors and protective measures.
Do not identify a system by wire colours or conductor count alone. Do not remove covers or open sealed service equipment. If uncertain, select “unknown” in your plan.
What does this mean for your plan?
The PlanMyPanel wizard asks for the earthing system. The current rules require an incoming RCD for TT and unknown systems. This is a software planning assumption, not identification of the existing system. An electrician must complete the design and verification.
Language does not determine the applicable standards. Follow the national rules and network operator requirements for the installation’s location. This overview explains principles and does not replace verification of protective measures.
Frequently asked questions
How do I find out which earthing system my home has?
Check the connection documents, wiring diagram and inspection reports, or ask your grid operator which system is supplied at the connection point. An electrician confirms it on site — do not judge by wire colours and do not open sealed supply equipment.
Can I split the PEN conductor myself?
No. Where and how the PEN is split depends on the installation, national rules and the grid operator’s requirements, and a mistake can put dangerous voltage on metal enclosures. An electrician makes the split as part of the design, never as a jumper in a socket.
Is an RCD required in a TT system?
In TT the fault current returns through the ground and is often too small for a circuit breaker to disconnect quickly, so RCDs usually provide the fault protection. Which RCDs and where depends on the installation, and the earth electrode resistance must be checked.
What is the difference between TN-S and TN-C-S?
In TN-S, N and PE run as separate conductors all the way from the source. In TN-C-S they arrive combined as one PEN conductor and are split into N and PE at a defined point; after that point they are never joined again.
What is the IT system used for?
IT is used for special applications where the supply must keep running after a first insulation fault, such as hospital operating theatres or industrial processes. It needs insulation monitoring and coordinated protection and is not a typical choice for a home.