Ghost Voltage on a Corridor Lighting Circuit
A trainee is troubleshooting a corridor lighting circuit in a school workshop. The circuit is 230 V, 50 Hz, wired in 1.5 mm² PVC conduit. A two-way switch controls a 20 W LED batten. With the switch off, a 10 MΩ digital multimeter reads 78 V between the lamp holder's live contact and the earthed metal frame. The trainee concludes that the switch or cable is leaking and wants to replace the switch. A 100 W incandescent test lamp connected across the same points does not glow at all. The DMM leads run about 2 m alongside the live feed, and the display jumps when the trainee moves the leads. The trainee also has a low-impedance voltage tester and a 500 V insulation tester. The school is open and the corridor must remain safe for students.
1. Explain how a 78 V reading can appear across an open switch with the lamp off, and why the 100 W lamp does not glow. Refer to input impedance and capacitive coupling.
Read the worked answer
A digital multimeter with 10 MΩ input impedance can form a capacitive divider with the small capacitance between the open live conductor and an adjacent live conductor or earthed metal. At 50 Hz, even tens of picofarads has an impedance of many megohms, so the meter can display tens of volts. The source is high-impedance: it cannot deliver current.
The 100 W lamp has a hot resistance of about 230²/100 ≈ 529 Ω. Connected across the same points, it loads the coupling capacitance heavily and collapses the voltage, so it does not glow. Moving the leads changes the coupling capacitance, which is why the display jumps. This is a classic ghost voltage, not proof of a switch leak.
2. What measurements would you make to decide whether the circuit is safe? Include insulation resistance and low-impedance testing, and state acceptance criteria.
Read the worked answer
- Isolate the circuit and prove dead. Disconnect the LED batten/driver so electronics do not affect the insulation test.
- Measure insulation resistance at 500 V DC: live-to-neutral and live-to-earth. A healthy 230 V circuit should be at least 1 MΩ; many standards require ≥1 MΩ, and a practical reading is usually much higher. A low reading indicates real insulation damage or moisture.
- Use the low-impedance voltage tester across the open switch with the supply on. It should read near 0 V; a real leakage or crossed conductor would hold voltage under load.
- Check switch continuity in both positions and verify polarity: the switch should be in the live conductor, not the neutral.
If insulation resistance is good and the low-impedance tester reads zero, the 78 V is pickup, not a fault.
3. The trainee wants to replace the switch. Give a reasoned decision and a safe course of action.
Read the worked answer
Do not replace the switch on the basis of the 78 V reading. The evidence points to capacitive pickup: high-impedance meter, no lamp glow and a reading that changes when leads move. Replacing the switch would waste time and would not remove the pickup.
Safe action: isolate and test as above. If insulation and polarity are correct, record the finding. If the LED batten glows faintly when off, fit a suitable load or neutral arrangement at the fixture, or use a driver designed for two-wire switching. Keep the metal frame earthed and verify earth continuity. If insulation resistance is below 1 MΩ or the low-impedance tester shows voltage, find and repair the leakage before re-energising.