ELECTRICAL BASICS · DRILL 2 OF 3
3 phase voltage: how to check itWhere to put the probes, and what the reading means.
Tap a terminal for the red lead and one for the black lead. Phase to phase, phase to ground, the 24 V DC supply, then a full live–dead–live test.
Most meter mistakes aren’t about the meter. They’re about where the probes go. Measure across two different circuits and the number means nothing. Test the line side of an OFF disconnect and it’s still live. Trust a zero without proving the meter and a broken meter can fool you.
The panel below has the incoming line (L1–L3), the disconnect, the load side (T1–T3), ground (PE) and a 24 V DC supply, laid out like the starter panel in Level 1.
Three-phase and 24 V DC quick reference
| Red lead | Black lead | 480 V system, power ON |
|---|---|---|
| L1 | L2 (or L2–L3, L1–L3) | About 480 V AC, phase to phase |
| L1, L2 or L3 | PE (ground) | About 277 V AC, phase to ground (wye system) |
| +24 V | 0 V | 24 V DC (reversed: −24 V) |
| L1 | +24 V | Meaningless: two separate circuits |
Line side vs load side
A disconnect opens what’s below it: the load side (T1–T3) and anything fed from it. The line side (L1–L3) comes in above the switch and stays live with the handle OFF, until the feeder upstream is shut off and locked out.
The live–dead–live test
- Live: test the meter on a known live source (a proving unit or a known live circuit). It must read correctly.
- Dead: test every point: each phase to each other phase, each phase to ground, and any control supply fed from the load side.
- Live: test the meter on the known source again. If it still works, the zero readings were real.
Use a meter rated for the voltage and category of the equipment (for example CAT III 600 V or higher for 480 V panels) and the PPE your site requires.
Common questions
What voltage should I read on a 480V 3 phase system?
On a 480Y/277 V wye system, about 480 V phase to phase (L1–L2, L2–L3, L1–L3) and about 277 V from each phase to ground. Delta systems read differently phase to ground, so know which system you’re on.
How do you check 3 phase voltage with a multimeter?
Set the meter to AC volts and measure L1 to L2, L2 to L3 and L1 to L3. The three readings should be close to each other. Then measure each phase to ground. A reading far off the others points to a missing or weak phase.
What is the live dead live test?
Prove your meter on a known live source, test every point for zero, then prove the meter on the known source again. It shows the meter worked before and after, so the zero readings are real.
Is the line side dead when the disconnect is OFF?
No. A disconnect only opens what’s below it, the load side. The line side (L1–L3) above the switch stays live until the feeder upstream is shut off and locked out.
Why can’t I measure from L1 to the 24 V DC supply?
They’re separate circuits: 480 V AC power and 24 V DC control. A reading between them means nothing. Measure within one circuit: phase to phase, phase to ground, or + to − on the DC supply.
YOUR NEXT STEP
Try it on a real panel
Level 1 puts you at a motor starter panel: lock it out, prove zero energy with the meter, wire the control circuit and run the motor. It's free.
Training practice only. Always follow your site's written energy control procedure, your employer's authorization rules and the meter maker's instructions on real equipment.