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I'm replacing a 240-volt split-phase electric baseboard heater. I've shut the power off at both the thermostat and the breaker box. Testing the wires with a non-contact voltage tester shows an intermittent "live" indication on the non-switched wire; it shows a continuous "live" indication on the hot side of a nearby outlet. Testing the wires with a multimeter shows 0V for switched-to-non-switched, switched-to-ground, and non-switched to ground; it shows 120V for hot-to-neutral and hot-to-ground in a nearby outlet.

Which should I believe? Is the NCV picking up a residual signal from the nearby outlet, or is the multimeter somehow ignoring the high-voltage wires?

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Trust the multimeter. No-contact testers should only be used for a preliminary test. If the no-contact indicates power a regular voltage tester or DMM should be used to verify.

A no-contact tester is susceptible to induced currents from wiring running in the same conduit or in close proximity to the wiring being tested.

Safety first though. Make sure circuits are de-energized before working in them.

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    I'd recommend also checking for voltage between the wires and any nearby grounded objects, including the floor you're standing on. If you only check voltage between wires in the same outlet, you can get an all-zero reading even if one of them is live and the rest are floating (e.g. because the ground failed or was never connected to begin with, and somebody miswired the switch so that it cuts off the neutral instead of the live wire). Sure, that should never happen, but you shouldn't trust your safety solely on the competence of the last person to work on the wiring (even if it was yourself!). – Ilmari Karonen Feb 29 '16 at 8:18
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    And if you want to be doubly sure, once you are convinced power is removed, tap the conductor onto earth. If you see a spark, stop. If you blow a fuse or an RCD, better than than get a shock. – abligh Feb 29 '16 at 9:29

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