Three-phase field metering

On a suspected wye:

• Three L-N ≈ equal
• Three L-L ≈ equal and ≈ 1.732 × L-N

On a delta:

• Three L-L ≈ equal
• No normal L-N set unless a high-leg or corner-grounded variant exists — know your service

Record all six (or three) voltages every time you open a dispute about “low voltage.”

Clamp A, B, C. On a balanced motor they should be close. A large gap means:

• Single-phasing (one lost supply leg)
• Unbalanced supply voltage
• Unequal tap or high-resistance connection
• Different single-phase loads on a shared feeder

Use a rotation meter before coupling a pump. ABC vs ACB is not cosmetic — it is direction. Document rotation before teardown so reconnect matches.

A three-phase analyzer gives kW, kVAR, kVA, PF, and sometimes harmonics. That closes the loop with P = √3 EI PF. Average-sensing meters on distorted VFD waveforms can lie — prefer true RMS / power quality tools when drives dominate.

• One line current near zero on a three-phase motor feeder (lost phase)
• Motor growling, high currents on remaining legs, rapid heating
• Voltage present on a floating leg through motor windings can fool a careless voltage-only check — confirm with clamps under load

• CAT-rated meters for the environment
• Keep fingers behind guards; use PPE
• Never open a CT secondary under load
• Discharge caps in PF banks before handling

Expected sets:

• 208Y/120: L-N~120, L-L~208
• 480Y/277: L-N~277, L-L~480
• 480 Δ: L-L~480, no standard 277 L-N

Unbalance rule of thumb: investigate when phase currents differ sharply on a motor that should be balanced — “one leg half the others” is never OK.

High-impedance meters show voltage on open conductors coupled by capacitance. Load the circuit or use a low-Z mode; confirm with clamps.

1. L-L voltages balanced
2. Three currents balanced under load
3. Rotation correct
4. Overload settings match FLA

That set catches most “mystery three-phase motor” calls.

Situation. Motor “has voltage on all three” but overheats. Clamps show 0 A on leg B, high A and C. Contactor finger on B is burned open; voltage on B at the motor is phantom from winding coupling.

What happened. Voltage-only troubleshooting missed single-phasing. Current metering caught it immediately.

Applied lesson. On three-phase motors, amps under load beat voltage alone.

### Teaching pause — say this out loud

Before you leave this lesson, explain the main idea to an imaginary first-month helper in under one minute. If you need the book open to do it, reread How it works once more. Field diagnosis only helps after the concept is yours.

Also sketch the key diagram from memory (triangle, wye/delta, filter shape, or charge curve — whichever this lesson used). Labels beat artistic skill.

### Why this lesson matters on Monday morning

Three-phase field metering is not trivia. You will meet it when a meter reading looks “impossible,” when a replacement part is almost right, or when a helper asks why the book uses √3 or lead/lag. Master the model here so the next call is pattern recognition, not panic.

Common Monday uses: verify a nameplate against clamps, explain a PF or capacitor change to a customer, or catch a miswired series/parallel or wye/delta assumption before energizing.