Complex sentence: Hardly any loads were polarity sensitive when D.C. systems were introduced (principally incandescent lighting, heating systems or series D.C. motors) and it was considered more important to identify the live wire than the polarity.
Split into simpler sentences: Hardly any loads were polarity sensitive at this time (principally incandescent lighting; heating systems or series D.C. motors). It was considered more important to identify the live wire than the polarity.