The terms, without the jargon.

The earth conductor that runs with the live conductors of a final circuit, so that a fault to exposed metal has a low-impedance path back to the source. Its size is not a guess: it is either the same as the live conductors up to 16 mm² or worked out from the adiabatic equation, whichever applies.

Used inRCBO-Protected SocketDistribution BoardEV Charge PointSocket Outlet (3-Pin UK Type G)

See alsoTT, TN-S and TN-C-SAdiabatic equationZs

How the installation is earthed, in the standard two-letter code: the first letter is the supply earth (T = one point directly earthed), the second is the exposed metalwork (T = its own earth electrode, N = connected to the supply earth). TN-S has a separate earth all the way back; TN-C-S (PME) combines earth and neutral in the distributor’s cable and splits them at your cut-out; TT relies on your own earth rod.

Used inEV Charger Dedicated CircuitEV Charge PointPortable Generator Set

See alsoPEN conductorRaEarth electrodeZe

A single conductor doing the job of both protective earth and neutral, used by the distributor on a TN-C-S supply up to the point where it splits at your cut-out. If that combined conductor breaks, every earthed metal part in the installation rises towards live — which is why a lost PEN on an EV charge point or a caravan supply has to disconnect all poles, earth included.

Used inEV charger circuit

See alsoTT, TN-S and TN-C-S

The resistance of the earth electrode itself, measured in ohms at the electrode with the supply disconnected. On a TT installation Ra has to be low enough that the whole earth fault loop still lets the RCD trip, because there is no metallic path back to the source to do the work.

Used inGenerator backup supply

See alsoEarth electrodeTT, TN-S and TN-C-SZs

A rod, plate, tape or buried foundation connection that puts the installation in contact with the general mass of earth. TT installations depend on one; TN installations may add one for an outbuilding, a generator or an EV charge point whose supply is not safe to export.

Used inDiesel Generator Backup SupplyEV Charge PointPortable Generator Set

See alsoRaTT, TN-S and TN-C-S

Connecting exposed metalwork and extraneous conductive parts — water pipe, gas pipe, structural steel — together so they cannot sit at different potentials during a fault. Main bonding joins them to the installation earth at the origin; supplementary bonding does the same locally in a bathroom or a swimming pool where the risk is higher.

Used inDistribution BoardVoltage & Continuity Tester

See alsoCPCTT, TN-S and TN-C-S

Compares the current leaving on live with the current returning on neutral and opens if they differ — the difference is current leaking to earth, possibly through a person. A 30 mA RCD is the standard additional protection for socket outlets and most final circuits, and it trips on leakage, not on overload.

Used inRCD-Protected Socket CircuitRCBO-Protected SocketEV Charger Dedicated CircuitAFDD-Protected Bedroom CircuitAFDD (Arc Fault Detection Device)Cooker Control Unit

See alsoRCD types AC, A, F and BRCBOIΔn

What waveform of leakage the device can actually see. Type AC detects smooth sinusoidal AC only and is being phased out; Type A also detects pulsating DC from electronics and is the normal choice; Type F adds mixed frequencies from variable-speed drives; Type B detects smooth DC as well, and is what an EV charge point or a PV inverter usually calls for.

Used inEV Charger Dedicated CircuitUnderfloor Heating ZoneDistribution BoardEV Charge Point

See alsoRCDIΔn

The leakage current at which an RCD is designed to trip — 30 mA for additional protection against shock, 100 mA or more for fire protection or for selectivity between devices. The device must trip at IΔn and must not trip at half of it, which is exactly what an RCD tester checks.

Used inAFDD (Arc Fault Detection Device)Distribution BoardResidual Current Device (RCD)RCD Tester

See alsoRCDRCD types AC, A, F and B

An MCB and an RCD in one module, so a fault on one circuit trips that circuit alone instead of the whole board. It also sidesteps the shared-neutral problem that makes a group RCD trip when you least expect it, which is why modern boards are mostly RCBOs.

Used inRCBO-Protected SocketEV Charger Dedicated CircuitAFDD (Arc Fault Detection Device)Distribution BoardEV Charge PointResidual Current Device (RCD)

See alsoRCDMCB

The overcurrent device in a modern board: a thermal element for overload and a magnetic element for short circuits, both in one module rated in amps. It protects the cable, not the person — that is what the RCD half of the circuit is for.

Used inProtected Lamp CircuitSingle-Lamp Switched CircuitTwo-Bulb Parallel CircuitTwo-Way Switching (Staircase Circuit)RCD-Protected Socket CircuitTimed Outdoor Lighting

See alsoType B, C and D curvesRCBOBreaking capacity

How much overcurrent the magnetic trip needs before it opens instantly. Type B trips at 3–5 times its rating and covers general sockets and lighting; Type C needs 5–10 times and tolerates the switch-on surge of motors, transformers and LED drivers; Type D needs 10–20 times and is reserved for heavy inrush such as a large motor starting direct-on-line or an X-ray set.

Used inDistribution Board

See alsoMCBInrush current

The largest fault current a protective device can interrupt safely without destroying itself — stamped on the device in kA, commonly 6 kA for domestic and 10 kA or more near a large transformer. If the prospective fault current at the board is higher than the device rating, the device can explode rather than clear the fault.

Used inAFDD (Arc Fault Detection Device)

See alsoProspective fault currentMCB

Samples the current waveform and looks for the signature of an electric arc — the crackle of a loose terminal or a damaged flex — then opens the circuit before the arc starts a fire. It sees the faults an MCB is too slow for and an RCD is blind to, because a series arc is just a normal load current as far as either of them is concerned.

Used inAFDD-Protected Bedroom CircuitAFDD (Arc Fault Detection Device)

See alsoMCBRCD

Designing protective devices so that the one nearest the fault operates and everything upstream stays closed. Get it right and a fault in a bedroom socket trips one RCBO; get it wrong and it takes out the whole house while the faulty circuit is still energised.

Used inAFDD (Arc Fault Detection Device)

See alsoRCBOType B, C and D curves

The current that would flow if live were connected solidly to earth or to neutral at a given point — highest at the origin of the installation, lower at the far end of a long cable. Every protective device in the installation must have a breaking capacity at least equal to the PFC at the point it is fitted.

Used inConsumer unit panel

See alsoBreaking capacityZs

The two-digit code on an enclosure: the first digit rates protection against solid objects — 2 keeps a finger out, 4 keeps a 1 mm wire out, 6 is dust-tight — and the second rates protection against water, from X4 (splashing from any direction) to X7 (temporary immersion). The two digits are independent, and neither says anything about impact, which is the separate IK code, so an outdoor fitting needs the rating that suits where it is rather than the biggest number on the box.

Used inJunction BoxBell PushTimer Switch

The total impedance of the path an earth fault current takes: the source, the live conductor out, and the CPC back. It has to be low enough that the fault current trips the protective device within the required time — typically 0.4 s for a 230 V final circuit — which is why every Zs reading is checked against a maximum value from the tables rather than judged on its own.

Used inRCD Tester

See alsoZeR1 + R2Prospective fault current

The part of the loop that belongs to the distributor — transformer, supply cable, cut-out — measured at the origin with the installation disconnected. Everything downstream adds to it: Zs = Ze + (R1 + R2).

Used inRCD Tester

See alsoZsR1 + R2TT, TN-S and TN-C-S

The resistance of the live conductor (R1) plus the circuit protective conductor (R2), measured end to end with the two joined at the far end. Added to Ze it gives Zs without a live loop test, which is how ring final circuits are verified and how long circuits are checked where a loop tester would struggle.

Used inRCD Tester

See alsoZsZeRing final circuit

A test at 500 V between conductors, and between each conductor and earth, with loads disconnected — reading in megohms. Anything at or below 1 MΩ on a 230 V circuit is a fail and needs investigating, and a reading taken through surge-protected or electronic equipment means nothing at all.

Used inDigital Multimeter

See alsoContinuityPolarity

Proving a conductor is complete end to end and that its resistance is what the cable length predicts. It is the first dead test on any circuit, because it finds the open CPC and the loose terminal that every later test would quietly misread.

Used inUnderfloor Heating MatClamp MeterDigital MultimeterVoltage & Continuity Tester

See alsoR1 + R2PolaritySafe isolation

Confirming that every switch and protective device is in the live conductor and that socket connections are the right way round. A lamp holder with the switch in the neutral works perfectly and still leaves the fitting live with the switch off.

Used inSolar PV with Battery StorageBell / BuzzerRCD TesterVoltage & Continuity Tester

See alsoContinuitySafe isolation

The full sequence: identify the circuit, switch off and lock off, prove the tester on a known source, prove the circuit dead, test the tester again on the known source. Skipping the re-prove step is how people test with a meter that stopped working halfway through the job.

Used inThree-Phase DOL Motor StarterSolar PV with Battery StorageContactor (3-pole)Distribution BoardMiniature Circuit BreakerElectric Motor

See alsoContinuityPolarity

The current a cable can carry continuously in its installed conditions without exceeding its temperature rating. It is not a fixed property of the cable: the tabulated value is corrected for ambient temperature, grouping with other cables, thermal insulation and the protective device, which is why 2.5 mm² can be rated 20 A in one place and 13 A in another.

Used inCable size calculator

See alsoVoltage dropAdiabatic equation

The volts lost along the cable because of its resistance, given as a percentage of the supply: 3 % for lighting and 5 % for other circuits from the origin. Long runs, undersized conductors and high currents all push it up, and the lamp dimming at the far end of a run is usually voltage drop rather than a fault.

Used inVoltage drop calculator

See alsoIz

The calculation that checks a conductor can survive a fault: it relates the cross-sectional area to the fault current, the disconnection time and a constant for the conductor material. If the CPC comes out smaller than the live conductors, the equation is what decides whether that is actually allowed.

Used inCable size calculator

See alsoCPCProspective fault current

Sizing a cable or a supply for what will realistically be on at once, not for the sum of everything plugged in. The rule of thumb that lets a 32 A ring serve a dozen sockets works because a house does not run every appliance simultaneously — and it is also the assumption a cooker circuit relies on, which is why a cooker control unit’s 13 A socket is not a spare socket for the kettle.

Used inCooker & Induction Hob SupplyCooker Control UnitEV Charge Point

See alsoIzRing final circuit

A 32 A circuit wired as a loop from the board and back to it, so every socket is fed from both directions and the current shares between two paths. It is the standard UK arrangement for general-purpose sockets, and it is the one circuit that needs its own set of tests to prove the ring is intact and no spur has been taken off it illegally.

Used inRCD socket circuit

See alsoRadial circuitR1 + R2

Every cable leaves the board and ends at the last point on the circuit — the opposite of a ring. Simpler to test and easier to protect, which is why radial circuits are the usual choice for kitchens, cookers, EV charge points and anywhere the load is known and fixed.

Used inRCD socket circuit

See alsoRing final circuit

The two conductors running between the two switches in two-way lighting, carrying either live or the switched live depending on which switch was last thrown. They are live conductors in every sense, so they need the same brown or marked sleeving as any other — and a two-way circuit is the classic place where an unmarked strap wire gets mistaken for a neutral.

Used inTwo-Way Switch

See alsoSwitched live

The conductor that leaves a switch and only becomes live when the switch is on — the feed to a lamp, a PIR’s output or a fan. Confusing it with a permanent live is the reason a fitting can appear dead at the switch yet still be live at the ceiling rose.

Used inContactor Motor StarterPIR Motion SensorOne-Way SwitchTimer Switch

See alsoStrapper

The brief surge a load takes when it switches on — a motor drawing six times its running current, a transformer saturating, an LED driver charging its capacitors. It is not a fault, but it is what decides whether the circuit can use a Type B breaker or needs Type C.

Used inUnderfloor Heating ZoneBell / BuzzerElectric MotorOne-Way Switch

See alsoType B, C and D curvesDOL starter

The simplest way to start a motor: a contactor puts the full supply straight across the windings. It gives maximum starting torque for the least equipment, and it also gives the largest inrush — which is why bigger motors use star-delta or a soft starter instead.

Used inThree-Phase DOL Motor Starter

See alsoInrush currentType B, C and D curves

An extra-low voltage system — 50 V AC or 120 V DC or less — that is electrically separated from earth and from higher-voltage circuits, so a single fault cannot give a shock. It is why garden lighting, bathroom fittings and bell transformers can be touched safely live, and why the separation has to be maintained right back to the transformer.

Used inBell / BuzzerBell Push

See alsoBonding

A PV module’s voltage with no load connected, and the figure that decides how many modules can go in series before the string exceeds the inverter’s maximum input voltage. It rises as the cells get colder, so a string sized on the standard test conditions can over-volt the inverter on a bright frosty morning.

Used inSolar PV Panel

See alsoBypass diode

A diode across each string of cells inside a PV module’s junction box. When part of the module is shaded the diode conducts around it, so the rest keeps producing instead of the shaded cells dissipating power as heat — the hotspot that permanently damages a module.

Used inSolar PV Panel

See alsoVoc