What do you do with a cable fault that won’t "THUMP"?

Kevin Wallace - Application engineer

When you work in the role of technical support you tend to get exposed to some very challenging situations. That has been this Application Engineer's experience. When an Engineer throws in the towel and picks up his mobile to ask for help you know that he has exhausted all options. After all ‘real’ men don't stop to ask for directions!

One of the most typical challenges that we receive from our customers is the elusive cable fault that will not ‘thump’ to pinpoint the fault. This is a typical problem with faults within a splice, blow-outs with either burnt back conductors or neutrals and bolted (solid short) faults. In all three situations the problem lies within the gap distance between the conductor and neutral.

The sole basis of pinpointing and high voltage radar techniques is to create an arc or high voltage breakdown at the point of the fault. The breakdown that is established reduces the impedance of the fault allowing the cable radar to reflect a pulse off the arc that is produced. In the case of splice faults or burnt-back conductors or neutrals, the distance between the conductor and neutral (gap) may be excessive.

When a proof test or high potential test is performed the meters indicate a fault to ground; excessive leakage current and low to no voltage can be sustained. This test confirms that the cable is faulted by the measurement of excessive leakage current, but does not indicate the condition of the gap that the surged voltage must bridge.

When performing an Arc Reflection test a reflection from the fault is never captured because a suitable arc cannot be established across the excessive gap. In the situation of a What do you do with a cable fault that won’t "THUMP", bolted fault or solid short the problem is that there is NO gap. When this fault is surged we experience a rush of current through the short circuit, but no arc and therefore no ’thump‘.

So how are these elusive faults found? The first task will be to localise the fault. The Megger MTDR is a very powerful tool with many tricks up its sleeve.

If the problem is a bolted fault (solid short), the MTDR Time Domain Reflectometer will clearly show a negative reflection at the point of the failure. Simply measure the distance to this reflection to obtain an approximate location of the failure. A blow-out (open circuit) will produce a significant positive reflection. Simply measure to this event to localise the fault. The splice is more challenging.

If the Arc Reflection technique does not render results, the MTDR can also perform the Surge Pulse (Impulse Current) fault locating technique. This technique takes full advantage of the thumper discharged energy and will typically produce results even with excessive gaps.

How do you pinpoint a fault that will not thump? That used to be a problem until during an underground workshop a seasoned cable fault veteran taught this Application Engineer a new trick.

The fault can be pinpointed by using the receiver portion of a cable locator/tracer kit much like the Megger L1070. Apply breakdown surges with a surge generator, like the Megger PFL40. At the point of the fault the peak deflection measured on the tracer's receiver will jump off scale. Now that's a neat trick!