Helpful video reference. This guide draws on Jordan Farley's Artisan Electrics video "MIND-BLOWING Electrical Fault Makes No Sense!", which documents a real UK fault investigation where voltage appeared in completely unexpected places. Jordan is a qualified UK electrician and managing director of Artisan Electrics, a Cambridge-based firm with over 341,000 YouTube subscribers. His walkthrough of an unexplained voltage showing on a ceiling is exactly the kind of puzzling real-world fault this guide explains.
1. What phantom voltage actually is
Phantom voltage, sometimes called ghost voltage, is a voltage reading produced by capacitive coupling between conductors, not by a genuine electrical source. It appears on isolated or dead conductors that run alongside live cables, particularly when those cables are in the same conduit, chase or back box.
Think of it like a form of electrical induction. The live cable acts as one plate of a capacitor; the isolated cable acts as the other. As the live conductor alternates at 50 Hz, it induces a small alternating voltage onto the dead conductor nearby. On a modern high-impedance tester, such as a non-contact voltage pen or a multimeter set to AC voltage, this induced voltage can read anywhere from a few volts to close to mains voltage, even though there is no real current available to cause harm.
2. Why modern testers are more likely to show it
Older voltage testers, particularly the direct-contact indicator screwdrivers once common on UK building sites, are low-impedance by design. They draw enough current through their neon bulb or resistance to collapse any phantom voltage immediately. If the voltage is real, the bulb lights. If it is phantom, it fades.
Modern electronic testers are deliberately high-impedance to avoid loading the circuit being tested, which makes them accurate and safe for delicate electronics. But that high input impedance also means they will faithfully report phantom voltages, because the tiny current from capacitive coupling is enough to drive the reading up without the tester pulling it back down.
Non-contact voltage pens are particularly prone to this. They are designed to detect the electric field around a live conductor without contact, so they will also detect the field around a conductor with phantom voltage. They are useful as a first check, but a positive reading from a non-contact tester alone is not sufficient proof that a circuit is live.
3. How to confirm whether the voltage is phantom or genuine
The definitive test uses a low-impedance voltage tester, which draws a small but meaningful current from the circuit under test. Genuine voltage has a current source behind it and will maintain the reading. Phantom voltage has no real source and will collapse as soon as the tester loads it.
Suitable low-impedance testers for UK domestic work include the Fluke T110, T130 and T150 series, the Martindale PD7600 and similar CAT III or CAT IV rated instruments. Many professional multimeters also include a dedicated LoZ (low-impedance) voltage setting alongside their standard Vac mode.
- Isolate the circuit at the consumer unit by switching off the relevant MCB or removing the fuse carrier.
- Test the isolated conductor with your standard tester and note the reading.
- Without changing the isolation, test the same point with a low-impedance tester.
- If the LoZ reading drops to zero or near zero, the original reading was phantom voltage. The circuit is dead and safe to work on.
- If the LoZ reading stays high, the voltage is real. Stop work and investigate.
4. Identifying the source of phantom voltage
If you have confirmed the voltage is phantom, it can still be useful to understand where it is coming from. Turn off adjacent MCBs one at a time while monitoring the reading on the isolated conductor. When you switch off the inducing circuit, the phantom reading should drop to zero. This tells you which live cable is sharing the route and creating the effect.
Phantom voltage is most common in:
- Back boxes where live ring final or radial cables are bundled alongside a switched-off lighting cable.
- Conduit runs or cable chases where several circuits travel together for some distance.
- Junction boxes in the ceiling void where lighting and power circuits pass through the same enclosure.
- Multicore cables with an unused core - the live cores capacitively couple into the unused core even inside a single cable.
5. When the reading is a genuine fault
If a low-impedance test shows that voltage persists on an isolated circuit, there is a real fault. Common causes in UK homes include:
- Broken or disconnected neutral. A missing neutral forces the circuit's return current back through the earth conductor, which can put the circuit protective conductor at a hazardous potential. Readings in this situation are genuine. See also the guide to fault-finding a broken neutral.
- Open PEN fault. In homes served by a TN-C-S (PME) supply, a break in the incoming combined protective earth and neutral (PEN) conductor puts the entire main earthing terminal at live potential, putting voltage on every earthed metal surface in the property. This is an emergency. See the guide to open PEN fault protection for what this condition is and what to do.
- Crossed or bridged circuits. A wiring error - often from a previous DIY job - can link two circuits so that isolating one does not remove voltage from the other. This is usually found at junction boxes or consumer units with non-standard wiring.
- Insulation failure. Damaged cable insulation allowing two conductors to touch. This kind of fault may be intermittent and will show up on an insulation resistance test. See the guide to testing insulation resistance.
When to call us
Phantom voltage is confusing when you encounter it for the first time, but confirming it with a low-impedance test takes a minute. A genuine voltage on an isolated circuit is a different matter - it means something in the wiring is not right, and tracking it down requires systematic testing. Richard carries calibrated test equipment and can trace and repair wiring faults in Sandwich and east Kent, usually in a single visit.
Unexplained voltage in your wiring?
Richard carries calibrated low-impedance test equipment and can distinguish phantom voltage from a real fault, and trace and repair whatever is causing it.
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