To efficiently evaluate three faults of coaxial cable breakage

unsuccessful three phase transposition and water ingress in the cross-bonded box

a cable line model is established according to the actual operating line. Current modulus K and unbalance coefficient J are presented by calculating the grounding current. They are used as evaluation parameters for evaluating typical faults in the cross-bonded box. It is shown that when the coaxial cable is ruptured

the value of K of faulty phase is 0 while the value of non-faulty phase is smaller than 1. When three phase transposition is unsuccessful

two values of K are almost equal and close to standard value 1

but the other value of K is about one time higher than the value of the normal condition. When wateringress happens in the cross-bonded box

the value of K does not change obviously

but the value of J for water ingress in one box is much bigger than that for water ingress in two boxes. The evaluation criterion is on the basis of monitoring the current in grounding line at the starting position of cable system. These two parameters of the criterion can be used to effectively diagnose the typical faults in the cross-bonded box on whole line without the influences of the changes of load current and grounding resistance. This diagnostic method is simple

easy to operate

and highly accurate

and it can also be applied to a long cable system.