Textbook lesson 25

Three-Phase Supply Fundamentals

Line/phase values, phase rotation, balanced and unbalanced loads.

Training safety note: This is study material, not permission to perform electrical work. Practical activities must be done on approved training equipment or under licensed supervision with the current rules and workplace procedures.

In this lesson

Learning outcomes
Core theory
Trade application
Worked example
Workshop task
Fault-finding notes
Revision questions and answers

Learning outcomes

Explain the purpose of this topic in everyday electrical work.
Identify the circuit conditions that must be checked before relying on a reading.
Apply the relevant calculation, test or drawing interpretation in a supervised training scenario.
Recognise common apprentice mistakes and unsafe assumptions.
Record evidence in a form that another tradesperson can understand.

Core theory

Alternating current is not simply DC that changes direction. The voltage and current vary sinusoidally, and the timing relationship between them matters. Resistive loads draw current in phase with voltage. Inductive loads such as motors and transformers cause current to lag. Capacitive loads cause current to lead. This phase relationship affects power factor, voltage drop and heating.

RMS values let electricians compare AC to an equivalent DC heating effect. A 230 V RMS supply has a higher peak value, but the RMS value is what is used for most power calculations. Understanding this prevents confusion when apprentices see different values on oscilloscopes, labels and calculations.

Three-phase systems are used because they deliver smoother power and efficient motor operation. The relationship between line and phase values depends on star or delta connection. A neutral conductor carries the imbalance in a star-connected system, so harmonics, load balance and borrowed neutrals must be understood before assumptions are made.

Key relationships

S = √3 × V_L × I_L

For balanced three-phase loads, apparent power is calculated from line voltage and line current. Apprentices should be careful not to mix phase voltage and line voltage without knowing the connection type.

Worked example

A balanced three-phase load draws 12 A from a 400 V supply. Apparent power is approximately 1.732 × 400 × 12 = 8.3 kVA. If power factor is 0.85, real power is about 7.1 kW.

Textbook depth: phase rotation, balance and neutral current

Three-phase supply uses three active conductors separated by 120 electrical degrees. Balanced three-phase loads draw equal current in each phase, and in a star system the neutral current is low because the phase currents cancel vectorially. Unbalanced single-phase loads connected across phases and neutral create neutral current.

Phase rotation matters for three-phase motors. Reversing two phases reverses motor direction. Apprentices must never change phase sequence casually; pumps, fans, compressors and conveyors can be damaged or create hazards if run backwards.

Calculation: A balanced 400 V three-phase load draws 20 A at 0.9 power factor. Apparent power is √3 × 400 × 20 = 13.86 kVA. Real power is 13.86 × 0.9 = 12.47 kW.

Trade application

On site, this topic is rarely isolated. It connects to safety, drawings, protection, cable selection, terminations, testing and documentation. A good apprentice does not ask only “does it work?” They ask whether it is correctly supplied, correctly protected, correctly controlled, mechanically sound, suitable for the environment, and verifiable by inspection and test.

When using this material, build a notebook of standard methods. For each topic, write the normal value, the likely fault value, the test points, the instrument setting, and the action to take if the result is abnormal. This becomes a practical diagnostic map rather than a collection of memorised definitions.

Workshop practical

Use a training transformer or simulator to compare resistive, inductive and capacitive AC loads. Record supply voltage, load current, apparent power and real power where instruments allow. Draw the phase relationship as a simple phasor diagram.

Evidence to collect: labelled sketch, predicted readings, actual readings, explanation of differences, supervisor feedback and one improvement to your method.

Fault-finding notes

Confirm the complaint or task requirement in plain language.
Compare the installation against the drawing, label or expected circuit arrangement.
Prove whether supply is present at the correct point and under the correct condition.
Divide the circuit into smaller sections instead of testing random points.
After repair, test the protective measure, not just the load operation.

Common apprentice mistakes

Mistake	Why it matters	Better habit
Measuring voltage without a reference plan	The reading may be real, induced, back-fed or meaningless without a return path.	State the exact two points being measured and the expected value first.
Assuming a device is faulty because it is not operating	The fault may be supply, control, protection, return path, settings or mechanical load.	Prove each section of the circuit in sequence.
Recording only pass/fail	Future workers cannot see whether results were strong, marginal or abnormal.	Record actual values, conditions and instrument details.

Assessment standard

The assessor is looking for correct RMS language, recognition of phase angle, correct use of apparent/real/reactive power terms, and safe interpretation of single-phase and three-phase diagrams.

Revision questions

What should be proven before this task is attempted on real equipment?
Which measurement would best confirm the main idea of this lesson?
What reading or symptom would make you stop and ask for supervision?
How could a poor termination change the behaviour of this circuit?
What information should be recorded for handover or assessment evidence?