The IEC 61992 series specifies requirements for d.c. switchgear and controlgear and is intended to be used in fixed electrical installations with nominal voltage not exceeding 3 000 V d.c., which supply electrical power to vehicles for public guided transport, i.e. railway vehicles, tramway vehicles, underground vehicles and trolley-buses. Part 3 of IEC 61992 specifies requirements for d.c. disconnectors, switch disconnectors and earthing switches for use in indoor fixed installations of traction systems.