Three-Phase Terminology: Phase vs Line?

‘Phase’ and ‘Line’ —How They are Defined

©2019 L A Waygood

Two fundamental terms which frequently cause confusion, when studying three-phase alternating-current systems are: ‘phase‘ and ‘line‘. This is hardly surprising, because the terms are often used quite incorrectly, not only in the field, but very often, unfortunately, in textbooks too!

Often, for example, we will hear someone referring the three conductors which hang from a distribution line as ‘phase conductors‘ or as ‘phases‘. This is completely incorrect. The correct terminology is ‘line conductors‘, or ‘lines‘.

The three energised conductors that connect a three-phase load to its three-phase supply are called ‘lines‘. The voltage between any two line conductors are termed ‘line voltages‘ and the current that passes along each line conductor is termed a ‘line current‘. The terminals of a three-phase source (generator or transformer) or of a three-phase load, to which line conductors are connected are termed ‘line terminals‘.

Phases‘, on the other hand, are connected between any pair of line terminals. Whether ‘star’ (‘wye’) or ‘delta’ connected, an alternator’s or transformer’s three windings, and three load impedances, are ‘phases‘. The voltage appearing across any phase is termed a ‘phase voltage‘, and the current passing through any phase is termed a ‘phase current‘.

In a delta-connected system, even though the line voltage is numerically-equal to the corresponding phase voltage, we must always retain the correct terms, according to WHERE those voltages are being measured.

In a star-connected system, even though the line current is numerically-equal to the corresponding phase current, we must always retain the correct terms, according to WHERE those currents are being measured —as illustrated below:

Identifying Lines

Line conductors and line terminals are identified in accordance with national standards. In Europe, for example, the colours brown, black, and grey are used to identify lines and line terminals. Other countries use different colours. Internationally, however, it is common to use the letters A, B, and —on circuit diagrams (as above), for example. Using letters has the additional advantage that high-voltage lines (or terminals) can be identified using upper-case letters: A, B, and C, while low-voltage lines (or terminals) can be identified using lower-case letters: a, b, and c.

The British Standard covering transformers, for example, uses the letters A, B, and C, to identify high-voltage transformer terminals, and the lower-case letters, a, b, and c, to identify low-voltage terminals.

It’s important to understand that these colours or letters are used to identify line (conductors) or terminals, NOT phases.

Identifying Phases

Phase windings or load impedances are not, themselves, identified. Instead, they are identified in terms of the line terminals between which they are connected. For example, in the case of a delta-connection, the phase winding connected between line terminals A and B, is identified as Phase A-B; the phase winding connected between line terminals B and C, is identified as Phase B-C; the phase winding connected between line terminals C and A, is identified as Phase C-A. The same applies, if we use colours, instead of letters: e.g. ‘phase brown-black’, etc.

In the case of a star connection, each phase is identified in terms of the line terminal to which it is connected, and the neutral point — i.e. Phase A-N‘, ‘Phase B-N‘, and Phase C-N.

It makes no sense whatsoever to refer to, for example, the ‘brown phase or phase A, etc., as those colours or letters indicate lines (or terminals), not phases.

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