This mathematical ARL quantity subtracts the shortest link between two distribution rooms from the total ring length. If a is the shortest length between two distribution rooms, and b, c and n each have longer lengths, then the averaged ring length is ARL = (a + b + c + n) - a.
This calculation basis, which is the basis for the lobe length determination, takes into account the worst case imaginable, namely that the shortest connecting piece between two distribution spaces fails. The shortest cable length must therefore be subtracted from the total length. The ring length to be traversed would be in the case of a failure of the shortest connection (a): Ring length = 2 x (b + x + n).