Open Ground — 73 V Phantom Voltage

120 V Branch Circuit · GFCI Receptacle · Plug Tester Will NOT Trip

Note: This page provides general educational information only and does not interpret the Canadian Electrical Code or any legally adopted standard. Always consult the authority having jurisdiction for official requirements.

Hot (Black) Neutral (White) Ground — Intact Ground — Floating Capacitive Coupling

What’s Happening?

•The ground conductor is broken somewhere between the panel and the first device (GFCI receptacle).
•The ground wire downstream of the break is floating — it has no path back to the panel’s ground bus or earth.
•All devices on this circuit show “Open Ground” on a plug tester because the ground pin has no continuity to earth.

Why 73 V Between Ground and Neutral?

This is a phantom (ghost) voltage — not a real fault voltage. Here’s the mechanism:

•The hot conductor (120 V AC) runs right next to the floating ground wire inside the NM cable jacket.
•The two conductors act like plates of a capacitor. Energy transfers from hot to ground via capacitive coupling.
•Your DMM has 10 MΩ input impedance — so high that even tiny coupled currents (microamps) produce a readable voltage.
•The 73 V reading is typical — phantom voltages range from 30 V – 90 V depending on cable length, proximity, and meter impedance.
•A low-impedance meter (wiggy / solenoid tester) would read ~0 V, confirming it’s a phantom reading.

Why the Plug Tester GFCI Won’t Trip

The plug tester’s GFCI test button has an internal circuit that works like this:

•It connects Hot to Ground through a resistor, creating a deliberate ~6 mA ground fault.
•This imbalance between hot and neutral current should be detected by the GFCI’s current transformer (CT).
•But with the ground open, there is no return path for that 6 mA. Zero current flows.
•Since zero current flows through the ground, there is no current imbalance for the CT to detect → GFCI does not trip.

Note: The GFCI’s own TEST button (on the device face) uses a separate internal circuit that connects Hot to Neutral through a resistor downstream of the CT but upstream of the contacts, bypassing the ground entirely. That button WILL still work.

Troubleshooting Steps

•1. Verify phantom voltage by re-testing with a low-impedance (LoZ) meter. If it reads <5 V, it’s confirmed phantom.
•2. Confirm the GFCI still trips using its own TEST/RESET buttons (not the plug tester).
•3. Check ground continuity from the panel ground bus to each device ground terminal.
•4. Locate the break — check junction boxes, wire nut connections, backstab connections, and the cable itself for physical damage.
•5. Common break locations: backstab connections, wire nut joints, staple damage, rodent damage.

Safety Implications

•Equipment grounding is lost — metal enclosures, appliance frames, and device yokes are NOT grounded.
•If a hot wire touches a metal enclosure, it will become energized at 120 V with no path to trip the breaker.
•The GFCI will still protect against shock from hot-to-ground faults through a person (current returns via earth), but the plug tester cannot verify this.
•CEC Rule 10-400 requires effective equipment grounding — this circuit is non-compliant.

The Capacitive Coupling Math

For the curious — here’s why ~73 V makes sense:

•Two insulated conductors in NM cable (~1 mm apart) form a capacitor of roughly 50–150 pF/m.
•For a 15 m cable run: C ≈ 1–2 nF total capacitance.
•At 60 Hz, X_c = 1/(2π·60·1.5nF) ≈ 1.8 MΩ.
•With a 10 MΩ DMM in the circuit, the voltage divider yields V_gnd = 120 × (10MΩ / (10MΩ + 1.8MΩ)) × coupling_factor.
•With realistic coupling factors (0.6–0.8), this yields ~50–90 V on the meter. Your 73 V fits squarely in this range.