Inspection & Testing

Explore key terms

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• Continuity testing

  Low-resistance measurement proving that conductors which must be complete actually are. That means protective conductors back to the earthing terminal, bonding conductors to services, and the legs of ring final circuits. Performed dead with a low-ohm meter, it is the first step of the test sequence because everything after it assumes the earth path exists.

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• Insulation resistance

  The resistance between conductors that should be completely separated from each other. It is measured in megohms, line to neutral and each to earth, with a test instrument applying an elevated DC voltage. Healthy insulation reads very high; damaged cables, moisture or pinched conductors show up as a low reading long before they show up as a shock or a fire.

  Read the full Insulation resistance glossary entry
• Polarity

  Verification that conductors are connected the right way round throughout an installation. Single-pole switches and protective devices must sit in the line conductor, the centre contact of screw lampholders on line, and socket terminals wired as marked. A polarity fault can leave equipment live when switched off — which is why it is checked dead and confirmed again live.

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• ZsEarth Fault Loop Impedance

  The total opposition to current around the loop a fault current would travel. That loop runs from the supply, along the line conductor to the fault, and back through the protective conductor and the earthing. It is the supply's own part (Ze) plus the circuit's R1 + R2, the resistance of its line and protective conductors. A low Zs lets a big fault current flow, which trips the protective device quickly, so electricians measure it and compare it with the maximum allowed for that device.

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• ZeExternal Earth Fault Loop Impedance

  The portion of the earth fault loop that lies outside the installation — the supply transformer winding, the distributor's cables and the earth return path, measured at the origin with the installation's own parallel paths disconnected. Ze depends on the earthing system: a TT installation relying on an electrode in soil sees a far higher value than a TN supply with a metallic return.

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• EICRElectrical Installation Condition Report

  The report produced by a periodic inspection of an existing installation, recording its condition against the current edition of BS 7671. Observations are classified by urgency — C1 for danger present, C2 for potentially dangerous, C3 for improvement recommended, and FI where further investigation is needed — and a C1 or C2 makes the report unsatisfactory.

  Read the full EICR glossary entry