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Basic Causesof Cable Failure

Although it may seem like more, there are actuallyonly four basic causes of field failures (open cut or

underground) in trailing cables and drag cables.They are:

Mechanical Damage Exceeding Recommended Bend Radius Current Overload Excessive Tension

Individually, or in combination, these can result insignificant downtime. Awareness of symptoms, orother symptoms of problems, can aid cable users indetermining the problem and how to correct it. After

one of the four basic causes occurs, the cable iseither immediately rendered unusable or a series ofsubsequent problems begins which make it appearthat the cable is at fault.

Mechanical DamageObvious Condition: Outersheath is usually torn orcrushed open and has roughedges or abrasion marks

leading up to the opening.Obscure Condition: The sheath may have little or nomarks on the outside, but the conductor insulationinside is ruptured – either partially or totally.If this is not a total electrical failure, it will lead toleakage current, nuisance tripping, and downtime.Figure 1 shows mechanical damage that washidden beneath a small mark on the outer sheath.

Possible Causes: Sharp rocks, roof falls, sharpedges on shuttle car reeling devices, run-over.

Corrective Action: Cable handlers, machine helpers,and other operations personnel need to be madeaware of the sometimes delicate nature of softcopper stranding and the rubber materials inside thecable. Developing an appreciation for theproduct capabilities and limitations will go a longway toward reducing mechanical damage.

Exceeding Recommended Bend Radius

Mechanical damage to innercomponents also occurs whenthe cable is bent in a radius farsmaller than the manufacturers’recommendations. When drag-ging cables, be sure to work allkinks, knots, and loops out of

the run. Otherwise, the loop will become taut aroundthe cable and end up one-time the cable diameter(Figure 2 ) instead of the normal twelve- to sixteen-times the cable diameter. As a result of exceedingAmerCable’s bend radius recommendations, damageto all conductors and insulation is imminent. Smalldiameter ropes can cut the sheath and/or squeezethe core until insulation damage occurs.

Possible Causes: Poor dragging procedures / deployment technique.

Corrective Action: Training on proper bendingradius. Use of large diameter ropes or slings canreduce handling damage.

Kinks Are AvoidableKinks begin as loops, caused in most cases by“pulling” rope from a stationary reel and cannever be satisfactorily removed. Kinks resultin permanent “weak spots.”

Never “pull out” a loop, always “unfold” it.To unwind rope straight without looping,“roll” the reel or coil.

Figure 1

Figure 2

Incorrect Unreeling Technique 

Correct 

Unreeling Technique 

Incorrect Unlooping Technique 

Correct Unlooping Technique 

16

Mining CablesMining Cables

AUSTRALIA

Tiger® Brand is a registered trademark of AmerCable Incorporated.

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Mining CablesMining Cables

AUSTRALIA

Basic Causesof Cable Failure

Current Overload

Condition: Cable insulation carries a 90° Celsiusrating, but sheathing compounds have notemperature rating. Sheathsare compounded for thehighest mechanical strength,since this is its primaryfunction. If powerconductors are run at 90°Cin free air and no greaterthan the rated amperage, thecable will perform for its anticipated life.

Problems arise when the cable is:1) wound up on a reel without the proper

derating factor applied,

2) stacked in a “pile,”

3) direct buried without increase of conductorsize, or

4) run at maximum amperes and voltage drop.

In these cases, the power conductor can reachtemperatures up to 200°C (392° Fahrenheit). This

melts the tin coating anddarkens the conductorscolor. The sheath vitrifiesand cracks open. Figure 3 

shows the changes in thestrand and Figure 4 showsthe changes in sheathingafter high heat.

Possible Causes: When no longer in free air, thecable cannot dissipate heat created by the energized

power conductors into the atmosphere. Heat buildsup inside the cable; the conductors surpass theirtemperature rating, and the sheath heat-ages at arapid rate.

Corrective Action: Always perform amperagecalculations prior to ordering cable for new orrebuilt equipment. Be sure to derate for multiplelayers of cable on the shuttle car reel. Whenupgrading the horsepower of mining equipment,a larger conductor size becomes necessary. On

particularly long runs of cable, calculate voltagedrop prior to ordering cable. High current at lowvoltage can also overheat a cable.

Excessive Tension

Condition: Excessive tension can manifest itselfrapidly, but most of the time the damage ishidden. When cables are operated somewhat abovemanufacturers’ limits, the flexible stranded powerconductor’s start a fatigue process of high wear atintimate points of contact. This is particularly truewhere the center bunch’s outer wires intersect thesix-bunch layer’s inner individual wires. Undernormal tension and wear, a little “dust” is the by-product. Under higher tension, “notching” occurs,as shown in Figure 5 . Individual wires literally

abrade in two against each other. Once severalwires fatigue-break, the ends continue to flexagainst surrounding wires and an exponential

growth rate of brokenwires results.

Possible Causes: Shuttlecar reel tensioning is thesingle biggest cause. Whenthe cable does not want torise up off of wet mine

floors, operators increasetension on the reel. For drag cables, expediencyprecipitates long lengths being dragged.

Corrective Action: Keep the shuttle car reel set sothat 4.5 to 6 meters cable is suspended betweenthe shuttle car and the mine floor during reeling andde-reeling. Keep the tie-point back in a crosscut andnot in the main haulage way. This spreads thetension of the “dynamic reel reverse” over4.5 to 6 meters of cable instead of 1 to 1.5 meters.

When dragging cable, a rule-of-thumb is to notexceed 60 meters when pulling by one rope orsling. Add extra slings and pull the cable up inloops of 60 meters.

If you want a more exact calculation to determinethe maximum length of cable that should bedragged from its center-point, use this formula:

L = –––––––––

L = Total Maximum Length of Cable (m)f = 0.5 (coefficient of friction)W = Total Weight of Cable

(kg/m)

Figure 3

Figure 4

Figure 5

T

f x W x 10

17Tiger® Brand is a registered trademark of AmerCable Incorporated.