直流電
Direct current mains supplies are only of historical interest in the UK but the colour coding was red for live and black for earthed (regardless of the polarity). Hardly any loads were polarity sensitive when direct current systems were introduced (principally incandescent lighting, heating systems or series direct current motors) and it was considered more important to identify the live wire than the polarity. In later years of direct current supplies, however, much more equipment became sensitive to polarity, such as many domestic radios &amp; television sets. Where all three wires were available, the historical colour code was red (positive), black (middle) and white (negative). The negative line changed to yellow in 1964, and then to blue in 1966.