IEEE 1234:2007 pdf free download – IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
4.3 Precautions
Many cable fault locators and trouble-shooters use a high-voltage dc test (see IEEE Std 400TM [B11j ) aspart of their standard fault-locating procedure. In the late seventies, it became apparent that dc testing mayexacerbate cable defects in service-aged extruded dielectric insulation lacking tree-retardant properties.Such cables may ultimately fail sooner than they would have if dc testing had not been performed.Therefore, proof testing of service-aged cables with extruded dielectric insulation lacking tree-retardantproperties is not recommended.
lf dc proof testing of service-aged cables should become necessary for a justifiable reason,the cablemanufacturer should be consulted for the maximum dc maintenance test value. For example,testing ofcables by qualified cable fault-locating and trouble-shooting personnel,in critical service areas such ashospitals,continuous-process industries, and cold-storage units, still provides the advantage of identifyingdeteriorated cables prior to their failure,and enabling repairs/replacement to be done，under plannedconditions, without sudden interruption of service.
4.4 Grounding
Cables can only be considered de-energized and grounded when the conductor and the concentric shield areconnected to the system ground at the test site, and if possible at the far end of the cable.
When fault-locating on a defective cable, installation, or system, a single system ground at the test site isrecommended (see Figure 1).The shield or concentric conductor of the faulted cable is connected to systemground. If this connection is missing, deteriorated, or has been removed, it must be replaced at this time.Asafety ground cable must connect the instrument case with system ground. If the test instrument is anHV device, the safety ground cable should be at least a braided or stranded #2 copper cable.Only after thesafety ground cable is in place should the test cable be connected to the center conductor and concentricshield; the center conductor-to-ground connection can then be removed.
Should a local ground be advisable or required for the test equipment,the case ground must remainconnected to the system ground in order to maintain an acceptable single ground potential.
5.Cable system fault characteristics
Cable faults vary, and even similar faults may exhibit different symptoms in different environments, cablesystems,cable types,or applications.To be able to more readily diagnose a fault and select the properoperating procedure，cable fault locating can be divided into direct radial distribution and networkdistribution categories. Cable fault-locating procedures in radial distribution can be tailored to a particularproblem and are casily controlled (see Figure 2).In Network Distribution, fault-locating procedures dependon many interrelated parameters that make solving a particular problem more complex.
5.2 Network distribution
Network cable systems form the backbone of most three-phase underground distribution systems in areasof high load density and maximum reliability requirements.A network cable system is characterized bycircuits with multiple branches and service taps. It is installed within a duct/manhole system.Transformerprimarices are connected either directly to the cable via oil-filled termination boxes and preformed elbows,or through sets of disconnect switches. The secondaries of network transformers, fed from multiple primarycircuits, are paralleled.Each transformer secondary can be isolated via a network protector.
5.2.1 Safety consideration
Network cable systems require mention of several additional safety issues，since the secondaries oftransformers are tied to a common bus. With transformer primaries connected in a delta configuration, aprimary cable could be energized via a closed network protector due to a faulty master relay within theprotector. To avoid backfeeding of transformer primaries and cable, all network protectors must be lockedin the open position before connecting fault-locating equipment. After verifying the status of all protectors,the primary cables must be checked for voltage and must be grounded.