EZ-Thump 12 kV, model v3, cable fault location system

There are five stages in determining cable fault locations:

• fault classification – identifying the type of fault
• pre-location – determining the distance to the fault
• route tracing – determining the route of the cable
• pinpointing – identifying the exact position of the fault
• cable identification – identifying which of several cables is faulty

Perhaps not from a theoretical point of view, but from a real-world point of view, the TDR should fit the cable/application.

Pre-location is used to indicate the distance to the fault. While modifying the fault to create conditions more suitable for a particular pre-location technique may occasionally be necessary, it is always best to pre-locate the fault with conditions as found. Several recognised methods of pre-location assist rapid, accurate, and safe location of faults. These include:

• Pulse echo (low voltage pre-location)
• Arc reflection (high voltage pre-location)
  • Arc Reflection Plus (ARP)
  • Differential Arc Reflection (DART)
• Impulse current (high voltage pre-location)
• Voltage decay (high voltage pre-location)

The results obtained with these techniques will allow the approximate location of the fault to be determined. Still, the accuracy of the results is affected by many factors, including changes in cable types, cable size, and joints, which affect the velocity factor of the cable under test. The lay of the cable is a vital factor as any results obtained with pre-location relate to the actual length of the physical cable, which may be very different from the length of the cable route!

Pinpointing is the identification of the fault’s exact location. Pinpointing is carried out directly over the cable. The most common technique relies on detecting acoustic and electromagnetic signals emitted at the fault location when the cable is being surged by a surge generator (thumper). A sensitive ground microphone and electromagnetic pickup, used in conjunction with an amplifier, detect these signals.

If you can charge the cable, you can ‘thump’ it, and that’s exactly what the pinpointing function of the EZ-Thump does. Accurate pinpoint fault location of the typical high-resistance/flashover faults is achieved using the ‘thunder and lightning’ method, whereby the 500 J surge generator (thumper) and an acoustic/electromagnetic receiver are used.

There are many techniques, including:

Basic tests

• DC test to determine flashover voltage
• Sheath fault test
• VLF test to determine flashover voltage

Pre-location

• Pulse reflection measurements
• TDR measurements
• ARM (arc reflection method)
• ARM Plus
• ARM power burning
• Decay plus (ARM – igniting the fault using a dc generator)
• Decay (travelling wave method, oscillation method)
• Current catching (ICE)
• Three-phase current catching (ICE)
• ICE Plus (low-voltage networks only)
• High voltage bridge method (pre-locating sheath faults)
• Voltage-drop method (pre-locating sheath faults)

Fault conversion

• Burning
• Performance burning

Route tracing

• Line location
• Line routing

Pinpointing

• Audio frequency generator (twist field and minimum turbidity methods)
• Shock discharges (acoustic field method, acoustic pinpointing)
• Pinpointing sheath fault

Cable and phase identification

• Phase identification earthed
• Phase identification and phase determination on live systems

The maximum circuit length that the EZ-Thump can test depends on the cable type, but as a rule of thumb, we generally say 3 km - giving 1.5 km for either end. In some circumstances, it can do more.

The EZ-Thump weighs just 33 kg and is compact enough to fit inside an average-sized car. It’s ideal for difficult-to-reach locations, such as rural and inner city areas, because you can easily transport it.

After you have confirmed the exact fault location by pinpointing, you must excavate the cable so that the fault can be confirmed visually. The fault is sometimes evident because of external signs such as cracks, breaks, burning, and general damage. However, there may often be no visible damage with the fault contained inside an apparently sound cable.