Ghost Voltage: Why Your Multimeter Reads 80 V on a Dead Wire

You isolate a circuit, lock it off, and put a meter across two conductors you know should be dead — and it reads 80 V. Most technicians read that as “still live somewhere” and go looking for a second feed that doesn’t exist, or worse, read it as confirmation that a component downstream is faulty when it never was. By the end of this article you’ll know exactly why a high-impedance meter does this, how to tell a ghost from a genuine fault in under thirty seconds with low-impedance mode, and how to prove a circuit is actually dead rather than assume it from one reading.

What Ghost Voltage Actually Is

Run two conductors together in the same conduit or trunking and you’ve built a capacitor, whether you meant to or not. Over a 20 m run, the capacitance between two adjacent conductors is typically a few tens to a few hundred picofarads. If one conductor is live at 230 V and the other is disconnected at both ends, that capacitance couples a small current into the floating conductor.

The numbers explain why the effect is so large. At 50 Hz, 100 pF has a reactance of roughly 32 MΩ. That reactance forms a voltage divider with whatever is measuring the floating conductor. A standard digital multimeter on AC volts has an input impedance of around 10 MΩ — high on purpose, so it draws almost no current and doesn’t disturb the circuit it’s testing. Put that 10 MΩ meter across 32 MΩ of coupling reactance fed from 230 V, and the divider lets a large fraction of the supply voltage appear on the meter. It is entirely normal to read anywhere from 30 V to 150 V on a conductor that is genuinely, safely dead.

Why Your Meter Reads It and a Motor Never Will

The current behind a ghost voltage reading is in the microamp range. That is nowhere near enough to do work: it cannot light a 2 W neon indicator, it cannot hold a contactor coil in, and it cannot deliver a shock. A relay coil, a solenoid, a motor winding — anything with a real, low-impedance path to earth — collapses that voltage to almost nothing the instant it’s connected, because a few hundred ohms or less swamps the microamp source completely.

This is the single fact worth holding onto: a voltage with no capacity to do work is not a live circuit. But that is a reason to prove it, never a reason to assume it. You confirm a ghost reading with a second measurement, every time — you don’t wave a meter near a wire, get a number under 100 V, and decide from memory that it must be capacitive coupling.

The Cure: Low-Impedance Mode

Most service-grade meters carry a low-impedance mode, marked LoZ or Low-Z. Switching to it drops the meter’s own input impedance to roughly 3 kΩ. That loads the coupling capacitance heavily — the microamp source cannot sustain any meaningful voltage across 3 kΩ, and a ghost reading collapses.

The rule that makes the two modes useful together: a real voltage barely moves between high-Z and LoZ; a ghost voltage collapses to under about 5 V. An 80 V reading in high-Z that drops to 1–2 V in LoZ is a dead conductor. A reading of 230 V in high-Z and 228 V in LoZ is a live conductor, and it will kill you exactly as readily as any other live conductor.

Two things will damage equipment if you get them wrong. First, never use LoZ on electronic control circuits, thermistor inputs, or 0–10 V signal lines — the 3 kΩ load pulls the signal down hard enough to damage the driving device, not just misread it. Second, LoZ does not replace a proving unit. It tells you whether a reading is real or induced; it does not confirm your meter is working correctly in the first place.

How to Prove a Circuit Genuinely Dead

Safe isolation under the OHS Act 85 of 1993 is a sequence, not a single reading, and skipping steps is how technicians get hurt on circuits they believed were already proven dead.

1. Prove the meter on a known live source — a proving unit, or a circuit you have already confirmed live. It should read the expected value within a few percent. This step exists because a meter that has already failed will read 0 V on anything, live or dead, and you have no way of knowing that without a reference reading first.
2. Test the conductor you intend to work on, in high-Z first, then in LoZ if the reading is ambiguous. A confirmed 0 V in both modes, or a high reading that collapses in LoZ, means dead.
3. Prove the meter again on the same known live source. If the meter failed between steps 1 and 2 — a flat battery, a blown fuse in the meter, a damaged lead — this step catches it. Without it, a meter that died mid-test reads 0 V on a genuinely live conductor and you believe it.

A live-work safety note belongs here: any time you are taking a reading on an energised panel rather than a proven-dead one, wear the correct PPE for the incident energy present, use a meter rated CAT III 600 V or better for plant panels and switchgear, and never treat a plausible-looking reading as a substitute for isolation when isolation is the safer option. Ghost voltage is a reason to look closer at a dead circuit, never a reason to skip proving one that should be live.

Worked Example: The Contactor Pole That Looked Like a Dead Compressor

A walk-in freezer compressor would not start. A previous technician measured 228 V at one contactor output terminal (T1) and 86 V at the other (T2), both against earth, both in the meter’s default high-impedance mode, and concluded the compressor itself must be open-circuit — a full supply on one leg and a partial reading on the other looked, at a glance, like voltage was reaching the load.

Switching both readings to LoZ told a different story: T1 held at 227 V, barely moving — a genuinely live terminal. T2 collapsed to 2.4 V — a ghost, coupled entirely from the adjacent live conductor in the same gland plate. T2 was dead, not partially live.

With the contactor energised, a further reading across the line and load terminals of the T2 pole itself — not to earth, but across the closed contact — read 226 V. Voltage across a closed switch means it isn’t actually closed electrically, whatever the armature is doing mechanically. Isolated, locked off, and proven dead by the same three-step sequence above, resistance across that pole read over-range; the other pole read 0.2 Ω. One contact face had burnt away.

Insulation resistance on the compressor windings came back over 500 MΩ, and the three winding resistances summed correctly with no shorted turns — the compressor was, and always had been, sound. The fault was a burnt contactor pole, a fraction of the cost of the replacement compressor that had already been quoted. The wrong diagnosis traced back to exactly one high-impedance reading taken on what turned out to be a dead conductor. If you want the full pole-by-pole method for confirming a contactor before you condemn it, see how to test a contactor properly — the same LoZ discipline applies before you touch a single pole. The same proving-dead sequence matters just as much before you handle a capacitor, which can hold a dangerous stored charge long after the supply is removed; see how to test a capacitor on an air conditioner for the discharge procedure.

Final Thoughts

Ghost voltage is not a fault and not a mystery — it’s the predictable result of a 10 MΩ meter measuring across a capacitively coupled conductor, and it resolves with one switch to LoZ and a reading that either barely moves or collapses toward zero. The discipline that actually protects you and your diagnosis isn’t memorising the 30–150 V range ghost voltages typically occupy; it’s never accepting a single reading as proof of anything, dead or live. Prove your meter, test the conductor in both modes, prove your meter again. That sequence is covered in full, alongside every other instrument error that produces a confident wrong answer, in the complete guide to electrical fault-finding.