No handheld tool can promise that a spot in a wall is free of wiring. What you can do is lower the risk in three steps: know where cables usually run, check the spot with a tool that suits the job, and drill in a way that forgives a mistake. This guide covers all three, including what a phone can and cannot tell you.
These are habits of the trade, not guarantees. Older houses, renovations and previous owners all produce surprises.
| Tool | What it senses | Limit |
|---|---|---|
| Non-contact voltage tester | Voltage on a conductor, even with nothing switched on | Short range; made for testing at an outlet, switch or exposed cable, and often too weak through a finished wall |
| Electronic stud finder with AC warning | The electric field of energized wiring near the surface | A warning zone, not a precise line; shielded or deep cable can be missed |
| Wall scanner | Energized wiring, plus metal and other objects depending on the model | Best of the handheld options; still not a guarantee |
| Phone app in a wire mode | The 50/60 Hz magnetic field of current that is flowing | Sees nothing when no current flows, even if the cable is energized |
A phone has no voltage sensor. It has a magnetometer, and a cable only produces a magnetic field when current is moving through it. That gives a phone app one honest job here: confirming that a loaded cable is at a certain spot.
To use Wire mode in Ferrum:
Read the result one way only. A positive reading tells you a loaded cable is close. No reading does not tell you the wall is clear: the cable there may be idle, on a different circuit, or too deep. Never treat a quiet phone as permission to drill.
If you are cutting a large opening, working in a wall with a panel, or unsure about what you found, call a licensed electrician. That costs less than repairing a damaged circuit.
Only when current is flowing. An iPhone's magnetometer can pick up the 50 or 60 Hz magnetic field of a cable that is powering a heavy load. It cannot sense voltage, so an energized cable with nothing switched on is invisible to it.
No. A phone app or a tester can miss a cable that is idle, deep or on another circuit. A quiet reading is never proof that a wall is clear.
Most often straight up and down from outlets and switches, and sideways between outlets at roughly outlet height. Renovations and older houses often differ, so treat this as a guide, not a rule.
About Ferrum. Ferrum: Stud Finder & Metal is an iPhone app (iOS 17 or later) by 55DIGITS LLC that uses the phone's magnetometer to find the screws in a stud, steel behind walls and wiring that is carrying current. $4.99 once, no subscription, no ads, no data collected. It does not detect wood, copper, aluminum or plastic, and it does not measure depth.
Ferrum is not a substitute for a professional scanner where safety matters, for example when drilling near mains wiring or gas lines. Always verify by another means before drilling.