Location of Cable Faults

TALK “LOCATION OF CABLE FAULTS”
given by Mr. S.J. Little – Member.
19th March, 1982.
The meeting was attended by 29 members.
Mr. Little defined a fault as a failure of a cable to transmit without interruption. Faults are usually caused by insulation failures creating shorts to earth. There are some 30/50 cable faults annually in Worthing.
Experience is extremely important in the matter of locating cable faults coupled with technical aids which have steadily been improved.
An early method was the Murray loop comprising a centre zero galvo, a length of piano wire and a battery, and based on the Wheatstone Bridge. It still required the digging of several holes until the precise location of the fault was found. But labour was cheap and the method was acceptable.
Transformer high voltage breakdown testing was introduced in the
1930’s.
After the war the Post Office developed with Cossor a radio system using a CR tube, putting a pulse into the cable to provide a travelling wave which, in hitting the fault, returned. It operated on a resistance to earth of less than 40 ohms.Next was the impulse current equipment using an impulse generator- transformer., rectifier and capacitor. Providing the type of cable is known the nature of the fault is indicated on a CR tube.
A good practical rule to use in the event of a failure is to walk the streets of the area affected and look for road digging by other authorities; 50% of all cable faults are due to damage by other authorities.
Faults in low voltage distribution cables in built up areas are more difficult to find than those in High voltage transmission lines.
Future equipment will probably use the present impulse current system combined with computer analysis.
After a question session, a vote of thanks was given by Mr. Beeken.
Mr. Little’s description of the strides made in fault location over the years made fascinating listening. He had obviously gone to a lot of trouble to ensure our understanding and enjoyment of his address.