Lighting Circuit Fails Inspection Over Earth and Loop Impedance
You are finishing a small office fit-out. A new lighting circuit feeds twelve LED downlights through a mix of two-way switching and a dimmer, protected by a 10 A MCB. The circuit is wired in 1.5 mm² two-core plus earth, with the earth connected at the switchboard but the earth conductor cut short at three downlight positions and not connected to the metal luminaire bodies. The furthest point of the circuit measures a loop impedance of 4.9 Ω. The supply is a TN-C-S system with a declared maximum earth fault loop impedance for a 10 A MCB of 7.3 Ω at the origin, but the measured value at the far end includes the circuit impedance. The client wants the lights energised for a photoshoot the next morning.
1. Identify the non-compliances and explain the risk each presents.
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Two clear non-compliances exist. First, the earth conductor is not connected to the metal luminaire bodies at three positions. That defeats the protective earthing of Class I luminaires: if a fault energises a metal body, the fault current has no low-impedance path back to the source, the protective device will not operate promptly, and anyone touching the body receives a shock. It also means the loop impedance test at those points is meaningless because the protective conductor is discontinuous.
Second, the measured loop impedance of 4.9 Ω at the far end needs interpretation. The 7.3 Ω figure is the maximum at the origin for the device; the circuit's own impedance adds to it, so the far-end value must still allow the MCB to trip within the required disconnection time. Depending on the device's trip curve, 4.9 Ω at the far end may be acceptable or marginal, but it cannot be confirmed while the earth path is broken. The dimmer and LED load also warrant checking for compatibility, but that is a performance issue, not a safety non-compliance.
The installation cannot be energised and certified in this state. The photoshoot deadline does not override safety; energising a circuit with disconnected protective earths would expose the client and staff to electric shock.
2. What would you do to make the circuit compliant, and how would you verify it?
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Isolate and lock off the circuit. Extend and reconnect the protective earth conductor at all three downlight positions, using approved connectors and ensuring sound continuity to each metal luminaire body. Check every other luminaire position for the same defect, since three found suggests a pattern.
Re-test after repair: measure protective earth continuity from the switchboard to each luminaire body, expecting a low resistance reading consistent with the conductor length. Measure insulation resistance of the circuit, then measure earth fault loop impedance at the furthest point and at each luminaire position, confirming the values allow the 10 A MCB to disconnect within the required time. Confirm RCD protection is in place where required for the circuit and test it.
Check the dimmer is rated for the total LED load and is compatible with the drivers, since incompatible dimming can cause flicker or failure. Record all results on the certification, note the defect and rectification, and only then energise the circuit. Advise the client that the circuit is now safe but that the photoshoot may need to wait for the test results to be recorded.