How to identify your earthing system

Why you would want to know — and where to stop

Your earthing system is the arrangement that gives the metalwork in your home a safe connection to earth, so a fault disconnects the supply instead of leaving something live. For V2L it matters because it decides how a floating V2L output is referenced when you switch the house onto the car — and a PME supply is the least forgiving case. This page is about what to look for, so you can have an informed conversation with an installer. It is not an instruction to open anything.

The three cues that tell you the system

Almost all UK domestic supplies are one of three types: TN-C-S (PME), TN-S or TT. The single most useful cue is where the main earthing conductor — the cable running to your main earthing terminal — actually connects.

• Earth connected to the supply neutral/PEN terminal at the cut-out → TN-C-S (PME). The supplier combines neutral and earth in one conductor (the PEN) and splits them at the cut-out; your earth is taken from that split. Often marked 'PME' or with a TN-C-S label. This is the normal arrangement for UK domestic supplies.
• Earth connected to the cable sheath or armour → TN-S. The supplier provides a separate earth all the way back to the substation, commonly via a lead-sheathed cable. The earth is a distinct connection to the sheath, separate from the neutral. Now comparatively rare.
• No supply earth at all — a local earth rod, with an RCD as the main protection → TT. The supplier provides only line and neutral; the installation makes its own earth with an electrode driven into the ground. The consumer unit usually carries a 'TT System — RCD Required' notice.

TN-C-S (PME) — earth from the cut-out

On TN-C-S the distributor runs a single combined neutral-and-earth conductor (the PEN) and splits it into separate neutral and earth at your cut-out. Your main earthing terminal is bonded to that split point. The tell-tale is the earthing conductor going to the supply neutral/PEN terminal, frequently with a 'PME' marking. The reason it is the least forgiving system for V2L is the open-PEN hazard: if that combined conductor breaks upstream, earthed metalwork in the house can rise toward 230 V relative to true earth.

TN-S — a separate supplier earth

On TN-S the supplier keeps neutral and earth separate the whole way from the substation, and provides a dedicated earth terminal — classically a connection to the lead sheath or armour of the service cable, often soldered. The cue is an earth conductor connected to the cable sheath, clearly distinct from the neutral, with no link between neutral and earth at the cut-out.

TT — your own earth electrode and an RCD

On TT the distributor provides only line and neutral; the installation makes its own earth via a local electrode (an earth rod). Because the fault path runs through soil, the earth-fault loop impedance is relatively high, so an RCD is essential as the means of disconnection. The cues are an earth rod with an earthing conductor to the main earthing terminal, no earth taken from the supply, and an RCD as the main protective device — usually with a 'TT System — RCD Required' label. TT is common with rural overhead supplies and is also a deliberate arrangement for some EV charge points.

Measurement as a cross-check, not a verdict

A competent person can measure the external earth-fault loop impedance (Ze) at the main earthing terminal as a cross-check. Indicative reference values are quoted as roughly 0.35 Ω for TN-C-S and roughly 0.8 Ω for TN-S, while TT can range from a few ohms to hundreds of ohms. But these are *typical declared* figures, not a definitive identifier — the supply type is settled by inspecting the connections and by the DNO's declaration, which the distributor is required to provide.

What this means before you go near V2L

Identification is the first step, not the design. Once the system is confirmed, the competent person decides how a floating V2L output is earthed on changeover — for an outbuilding, a TT island with its own electrode designed to BS 7430; indoors, an isolation transformer that keeps the single PME/MET earth. The contested floating-V2L-on-PME approach must be designed and proven by test by a competent person, and the vehicle manufacturer does not sanction this use. Knowing your system is what lets you brief an installer correctly — and recognise if one gets it wrong.