Typical Cable Fault Situations

Cable Construction:During the introduction of direct buried underground residential distribution, secondary service cable designs changed several times. Construction began with insulated copper conductors and bare neutral cables. For economic reasons, construction changed to aluminum conductors with either a bare copper neutral or tin coated copper concentric neutrals and then finally to all insulated aluminum conductors. The SPITFIRE is used to locate faults in any of these cable designs.

Fault Types:When a fault occurs in an insulated aluminum conductor service cable, generally, one customer will call and complain of low voltage. This helps the fault locating personnel to quickly pinpoint the location of the fault. On the other hand, because the copper takes a long time to corrode and the leakage current burns away the cable insulation on the adjacent cables, a short circuit generally occurs between conductors. This will result in a major fault where the primary fuse fails and many customers are out of service.

To better understand the difference in the mode of failure of copper or aluminum conductors with damaged insulation, in an ac circuit, a copper conductor first plates away from the conductor on one half of the cycle and then plates back to the conductor on the other half of the cycle. This happens because the surface being affected can plate in both directions due to the surface of the copper conductor remaining conductive.

With an aluminum conductor, the aluminum conductor plates away from the conductor on the first half of the cycle. It cannot plate in the opposite direction during the next half of the cycle because the aluminum conductor becomes insulated with aluminum hydroxide. Consequently, the conductor transforms quickly into insulating aluminum hydroxide, resulting in low voltage to the customer.
Locating Faults:When fault locating insulated secondary service cables using a remote ground return, the fault is generally pinpointed by making a single "X" on the ground.

When fault locating secondary cable with bare copper neutrals, the bare copper neutral, not a remote ground, must be used as your ground return to pinpoint the fault. However, the better way to locate this type of fault is to begin with the use of the remote ground. While looking for the fault you may identify many fault locations. Mark each one for future reference. Fault locate the entire cable run using the remote ground, return to the transmitter, remove the BLACK transmitter lead from the remote ground and connect it to the bare copper neutral. Return to each of the locations that were previously marked. The signal will only appear at the fault location. All of the other previously marked locations will have no signal.

There will, however, be times when only a single fault location will occur using either ground return method to pinpoint the fault.

The SPITFIRE fault locating system was designed especially to accurately pinpoint cable faults in any low voltage direct buried cable system.

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