Your earthing arrangement and your supply decide what a charger survey has to check

Your home's electricity supply needs a path back to earth so that a fault clears safely. The way that path is built is called the earthing arrangement, and it was decided by the network operator when the property was connected. A charger survey checks it because the three arrangements found in UK homes behave differently when a charger is added, and one of them needs an extra part before a charger can go in at all.

The three types

• TN-C-S, often called PME (protective multiple earthing): the network operator combines the neutral and earth into one cable all the way to your property, then splits them apart inside your consumer unit. This is the arrangement on most homes built or rewired in the last few decades.
• TN-S: the earth and neutral run as separate conductors all the way back to the substation. It's more common on older properties, often where the incoming cable has a lead sheath.
• TT: the network operator supplies no earth at all. The property's earth comes from its own rod, driven into the ground outside.

Checking which one you have

Look at the earthing conductor where it enters near your meter, before it reaches the consumer unit. A single combined cable carrying both neutral and earth points to TN-C-S. A separate armoured cable running alongside the supply points to TN-S. A cable leaving the consumer unit and running to a rod outside points to TT.

Some meter cabinets carry a label from the network operator stating the earthing type directly, which saves the guesswork if it's there. What you can settle yourself is which pattern you can see. Confirming it with certainty, and measuring the earth loop impedance that goes with it, is a job for the electrician doing your survey.

Why it changes the charger installation

On a TN-C-S supply, a break in the network operator's combined neutral conductor can, in rare circumstances, raise the earth voltage at anything connected to it. Indoors that risk is managed by the wiring design, but a charger usually sits outside, often touched while the driver is standing on the ground, so the same fault carries more consequence. Many chargers include an open-PEN protection device built in to detect this condition and disconnect, and some installations instead add a separate earth electrode at the charging point. Which route applies depends on the charger and the installer's assessment.

On a TT supply, the charger needs its own earth electrode as a matter of course, because there's no earth coming from the network to begin with. Either way, the figure that settles it is the earth loop impedance test the electrician carries out at the survey.

A looped supply is an electricity connection that runs from the network into one house and then on into the next. It's common on older terraces and on some newer estates built to save on cabling. Nothing about living with it is a problem day to day, but it changes what a charger installer and the network operator need to check before a charger goes in.

Why it matters for a charger

A home charger pulls a steady load for several hours at a time, often overnight. On a looped supply, that load passes along the same cable your neighbour's supply uses before it reaches your property, or your supply continues on to theirs. The network operator (DNO, the company that owns and maintains the cables in the street) has rules about how much extra load a looped supply can take before the loop itself needs separating or reinforcing. This is a check an electrician raises at survey.

How to tell if your supply is looped

• Look at your meter and the cable entering your property. If you can see the same cable continuing visibly toward a neighbouring property, or a shared external box serving more than one house, that's a sign of a loop.
• Ask a neighbour whether their supply was ever described to them as shared, particularly if your houses were built or connected together.
• Check any paperwork from when the property was built or last had electrical work done. A looped supply is sometimes noted on old connection records.
• Where you can't tell either way, say so at survey. An electrician can trace the supply, and the network operator can confirm it from their own records on request.

None of this is something you're expected to diagnose with certainty from the outside. The point of checking beforehand is to walk into the survey already knowing what you've seen, so the time on site goes on confirming it.

What the network operator may need to do

If your supply turns out to be looped, the network operator may need to separate it, reinforce the cable, or fit additional equipment before a charger can be connected safely. This work is carried out by the network operator, and it can add time to the project. Your installer is the person who raises the application with the network operator and tells you what response has come back; it isn't something you can resolve directly yourself.