Glossary

Plain-English definitions for the terms used across the site. Defined on first use in the pages too.

V2L (vehicle-to-load): The EV acting as a standalone AC inverter, feeding portable loads through an adapter at the car's Type 2 inlet. No grid handshake.
V2H (vehicle-to-home): A communicating bidirectional charger feeding a home's wiring from the EV — distinct from a plain V2L socket; uses ISO 15118-20.
V2G (vehicle-to-grid): Bidirectional export from the EV in parallel with the grid; DNO-notifiable under ENA G98/G99.
PME (Protective Multiple Earthing): A TN-C-S supply where neutral and earth are combined (PEN) then split at the cut-out. The earth is taken from the supply neutral.
Open PEN: A broken combined protective-and-neutral conductor on a PME supply — it can raise exposed metalwork to a dangerous voltage. The reason BS 7671 §722.411.4.1 restricts PME for EVs.
TT: An earthing system where the installation provides its own earth via a local electrode; the supply gives only line and neutral. Protection relies on an RCD plus a low, stable electrode resistance.
TN-S: An earthing system with a separate earth conductor from the transformer; neutral and earth kept separate throughout.
ATS (automatic transfer switch): A self-sensing switch that transfers a load to a backup source on loss of the normal supply, and reverts when it returns. It has no command input.
Interlocked contactor changeover: A pair of contactors (KM1 grid / KM2 V2L) mechanically interlocked so they can never close together — the hard guarantee against paralleling.
Open-transition (break-before-make): A changeover that drops the load momentarily, swaps the source, then re-energises — never bridging the two sources.
RCD type (A / F / B): Residual-current device classes for the residual currents they can clear; power-electronic sources may need Type F/B or smooth-DC detection.
RDC-DD: A residual direct-current detecting device (BS IEC 62955) that detects smooth DC residual current so an upstream Type A/F RCD is not desensitised.
Isolation transformer: A double-wound transformer (BS EN IEC 61558-2-4) that galvanically separates the floating V2L output so the home side can be earthed in a controlled way.
Ra (electrode resistance): The resistance of a TT earth electrode to the general mass of earth; it must be low and stable enough for the RCD to disconnect reliably.