Three-Phase Power
Definition
Three-phase power uses three live conductors whose voltages are 120° apart, usually with a neutral, and it is the standard supply for high-power AV distribution. The voltage between two phases is √3 (about 1.73) times the voltage between a phase and neutral. IEC 60038 lists 230/400 V at 50 Hz, used in Europe and many other countries, and 120/208 V and 277/480 V at 60 Hz, used in North America: 400 V between phases for 230 V to neutral in Europe, 208 V for 120 V in the US. When linear loads are balanced across the three phases, their currents cancel in the neutral. Switch-mode power supplies, LED drivers and dimmers draw third-harmonic currents that add in the neutral instead, so the neutral current can exceed the phase current, and IEC 60364-5-52 requires the neutral to be sized for the harmonic current in that case.
In the field
Fixtures and amplifiers are spread across L1, L2 and L3 so that each phase carries a similar current. An unbalanced supply overloads one phase and its protective device while the other two carry little load, and a phase that trips during a show is often traced to poor balancing. With LED and switch-mode loads, the neutral current is measured as well, because it can remain high even when the phase currents are balanced.
Sources
- IEC 60038 (Wikipedia, pointeur secondaire) : 230/400 V et 400/690 V à 50 Hz ; 120/208 V, 277/480 V et 347/600 V à 60 Hz · en.wikipedia.org
- IEC 60038:2009, IEC standard voltages : valeurs normalisées des tensions nominales des réseaux d'alimentation · webstore.iec.ch
- Electrical Installation Guide (Schneider Electric), Sizing the neutral conductor : harmonique 3 additionnée dans le neutre ; dimensionnement selon l'IEC 60364-5-52 · electrical-installation.org
- Mathematics of three-phase electric power (Wikipedia, pointeur secondaire) : tension composée = √3 × tension simple ; annulation des courants dans le neutre en charge équilibrée · en.wikipedia.org