Flat cable sleeve cores are made of ferrite material A5, RMS-303 or K5B RMS-303. It is an ideal sleeve core for flat cables and it is used in telecommunications, electronic and broadband applications. Applications and Benefits of Flat Cable Sleeve Cores Supplied in ferrite material A5, RMS-303 or K5B RMS-303. It is a suitable sleeve core for flat cables. Applied in telecommunications, broadband and electronic industries.