Underground Cable Fault Detector

Introduction

More than 3 million miles of electrical cables are strung overhead across the country. Add to that at least 180 million telephone and cable TV lines, and it's no wonder hurricanes, tornadoes, fires and ice storms are wreaking havoc on the electrical systems each year, causing utility outages that last days, weeks and longer. Power outages over extended periods present major health and safety concerns and economic losses. Concerns about the reliability of overhead lines, increases in their maintenance and operating costs, and issues of public safety and quality-of-life are leading more and more utilities and municipalities to the realization that converting overhead distribution lines to underground is the best way to provide high-quality service to their customers.

For utility companies, undergrounding provides potential benefits through reduced operations and maintenance costs, reduced tree trimming costs, less storm damage and reduced loss of day-to-day electricity sales when customers lose power after storms. Creative funding options are often available to make the goal of undergrounding a reality.

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The underground cable system is very important for distribution especially in metropolitan cities, airports and defense service.

Advantages And Disadvantages Of Underground Cable System

Advantages

This includes aesthetics, higher public acceptance, and perceived benefits of protection against electromagnetic field radiation (which is still present in underground lines), fewer interruptions, and lower maintenance costs. Failure rates of overhead lines and underground cables vary widely, but typically underground cable outage rates are about half of their equivalent overhead line types. Potentially far fewer momentary interruptions occur from lightning, animals and tree branches falling on wires which de-energize a circuit and then re-energize it a moment later.

Primary benefits most often cited can be divided into four areas:

Potentially-Reduced Maintenance and Operating Costs:

• Lower storm restoration cost
• Lower tree-trimming cost

Improved Reliability:

• Increased reliability during severe weather (wind-related storm damage will be greatly reduced for an underground system, and areas not subjected to flooding and storm surges experience minimal damage and interruption of electric service.
• Less damage during severe weather
• Far fewer momentary interruptions
• Improved utility relations regarding tree trimming

Improved Public Safety:

• Fewer motor vehicle accidents
• Reduced live-wire contact injuries
• Fewer Fires

Improved Property Values:

• Improved aesthetics (removal of unsightly poles and wires, enhanced tree canopies).
• Fewer structures impacting sidewalks

Disadvantages

The main disadvantage is that underground cables have higher initial cost and insulation problems at high voltages. Another main drawback is that, if a fault does occur, it is difficult to locate and repair the fault because the fault is invisible.

Types Of Fault In Underground Cables

The most common types of fault that occur in underground cables are,

• Open circuit fault.
• Short circuit fault.
• Earth fault.

1. Open circuit fault

When there is a break in the conductor of a cable, it is called open-circuit fault. The open-circuit fault can check by a megger. For this purpose, the three conductors of the 3 core cable at far end are shorted and earthed. Then resistance between each conductor and earth is measured by a megger. The megger will indicate zero resistance in the circuit of the conductor that is not broken. However if a conductor is broken the megger will indicate an infinite resistance.

2. Short-circuit fault

When two conductors of a multi-core cable come in electrical contact with each other due to insulation failure, it is so-called as short-circuit fault. Megger can also be used to check this fault. For this the two terminals of a megger are connected to any two conductors. If the megger gives a zero reading it indicates short-circuit fault between these conductors. The same is repeated for other conductors taking two at a time.

3. Earth fault

When the conductor of a cable comes in contact with earth, it is called earth fault or ground fault. To identify this fault, one terminal of the megger is connected to the conductor and the other terminal connected to the earth. If the megger indicates zero reading, it means the conductor is earthed. The same procedure is repeated for other conductors of the cable.

The common methods of locating faults are :

1. Sectionalizing: This procedure risks reducing cable reliability because it depends on physically cutting and splicing the cable. Dividing the cable into successively smaller sections and measuring both ways with an ohmmeter or high-voltage insulation resistance (IR) tester enable to narrow down search for a fault. This laborious procedure normally involves repeated cable excavation.
2. Time-domain reflectometry (TDR): The TDR sends a low-energy signal through the cable, causing no insulation degradation. A theoretically perfect cable returns that signal in a known time and in a known profile. Impedance variations in a "real-world" cable alter both the time and profile, which the TDR screen or printout graphically represents. One weakness of TDR is that it does not pinpoint faults
3. Murray loop test: It is a bridge circuit used for locating faults in underground or underwater cables. It uses the principle used in the potentiometer experiment. One end of the faulted cable is connected through a pair of resistors to the voltage source. Also a null detector is connected. The other end of the cable is shorted. The bridge is brought to balance by changing the value of RB.