Limitations of the TEC-X35
The X35 detects the magnetic field generated by the high voltage breakdown of the fault. The X35 also relies on the fact that when a fault is broken down by the Test Set, the Test Set current supplied on the main conductor returns by both earth ground and the sheath of the cable. If the sheath of the cable is not properly bonded to earth ground frequently, the Test Set current will return only on the sheath or neutral of the cable. This will create an equal and opposite current on the feeder which will prevent a magnetic field from being generated. Without the generation of a magnetic field, the X35 will not be able to detect and display the Test Set pulses.
This condition generally occurs on direct buried "jacketed" concentric neutral type cable where grounding is infrequent and the neutral is not in contact with the earth. The Thumper current will return only on the neutral of the cable and not disperse into the earth, preventing a magnetic field from being generated. Therefore the X35 is only designed to operate on either a distribution type system where frequent feeder bonding is provided or the cable neutral is in direct contact with the earth ("unjacketed" concentric neutral type cable). Jacketed URD cable is generally only earth bonded at transformers limiting localization to between transformers only. However, with direct buried unjacketed cable, the X35 will generally "pin-poinnt" the fault.