The various protection, control and annunciator units of the SPACOM and REF, REM, REC and REX products are linked together via the SPA bus, which physically is posed of fibre-optic cables.Two types of fibre-optic cables are used, i.e.plastic core cables and glass-fibre core cables.
The plastic core and glass fibre cables have different optic specifications. Plastic-core cables are used for shorter distances, i.e. under 30 m, whereas glass-fibre cables can be used for distances up to 1000 m. The transmission distance is the distance between two adjacent bus connection modules, which means that the total length of the cable loop can be longer than the transmission distance given above.
The best advantage of a data bus using optical fibres is that a disturbance-free communication is obtained. Light signals transmitted via opto-fibres are completely immune to electrical and electromagnetic interference. Being isolators the fibre-optic cables also prevent current from flowing between the SPACOM and REF, REM, REC and REX products of a system because the different devices may be on different potential levels. This means that the conventional electrical pilot wires of a substation switchgear installation, to a great extent, are replaced with fibre-optic cables which transfers measured values, control commands, status and event data, etc.