Help troubleshooting a three‑way lighting circuit that behaves erratically

Trade Talk
1

In an older residence, there are two 3-way switches regulating a single light fixture, but the performance has become unpredictable following some modifications. What diagnostic tests can effectively reveal any miswiring of the traveler and common wires, as well as potential problems with the neutral and ground connections?

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2

Turn off power, then use a continuity tester or multimeter to verify traveler continuity between switches, confirm the common is correctly tied to hot or load, and check neutral/ground integrity.

3

Okay, here’s how to check your light switches: First, turn off the power. Take out the switches and label the common screw on each. After that, turn the power back on and use a multimeter or test lamp to find the hot wire and the one going to the light. As you test, flip the switches, and you should see the power switch back and forth. The common screw should only have power on one side. If you think the neutral and ground wires are bad, turn off the power. Check if the neutral wire goes all the way back to the panel and if the ground is connected everywhere with almost no resistance. Any power showing up on the neutral or ground when the power is on is a big warning sign.

4

Check for loose or miswired traveler connections, faulty switches, or shared-neutral issues; test with power off.

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Shut off power, identify common screws, and check traveler continuity with a multimeter. Verify commons link to hot or load, and ensure neutral and ground connections are intact and voltage-free.

6

To diagnose issues with your 3-way switches, follow these steps:

Diagnostic Tests
-Continuity Test: Use a multimeter to check continuity between the common terminal and traveler terminals of each switch. Flip the switch and verify that continuity changes between the common terminal and each traveler terminal.
-Voltage Test: Measure voltage at each switch and the light fixture with the circuit breaker on. You should read approximately 120 volts between the hot wire and neutral wire at the light fixture.
-Grounding Test: Verify that the ground wires are properly connected to the ground terminals in the electrical boxes and at the light fixture.