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 composed of fiber-optic cables. Two types of fiber-optic cables are used, i.e. plastic core cables and glass-fiber core cables.
The plastic core and glass fiber cables have different optic specifications. Plastic-core cables are used for shorter distances, i.e. under 30 m, whereas glass-fiber cables can be used for distances up to 1000 m.