Electrical cables are essential to the operation of countless systems, from utility grids to industrial equipment. However, when something goes wrong inside a cable, like a short, break, or other discontinuity, finding the precise location of the fault can be complex and difficult.
That’s where cable fault testing comes in. Advancements in technology of locating equipment combined with a logical progression of diagnostics and measurement has helped reduce downtime and loss of revenue due to cable faults. But how does this equipment work, and what’s the proper procedure for diagnosing and locating the fault?
How Do Cable Fault Testers Work?
Cable Fault Testers combine a combination of electrical principles and specialized tools (to detect, locate, and pinpoint faults in underground or hidden cables. Electrical signals are sent down the cable and reflections or responses are analyzed.
The process is generally achieved in two main steps:
- Initial Testing to Estimate Fault Location
- Pinpointing the Fault
It’s important to follow the steps in this order to prevent possible further damage to the cable.
Let’s break each one down.
Initial Testing to Estimate the Fault Location
Before deploying locating tools, technicians begin with basic continuity and resistance checks using low-voltage equipment. These checks confirm or rule out the presence of a fault in the cable. The technician must avoid high-voltage testing at this stage to prevent altering the fault characteristics, which could hinder and impede localizing the fault.
Once a fault is confirmed, a Time Domain Reflectometer (TDR) is used. Due to its speed and efficiency, portability, and accuracy in calculating fault distance, this is one of the most common and effective tools for locating faults. It provides a reliable, practical and straightforward way to quickly identify fault type and pinpoint its distance.
Time Domain Reflectometer
A time domain reflector, or TDR, functions by delivering a low-energy electrical pulse through the cable and measuring the time it takes for the signal to reflect back. When the pulse encounters a change in impedance, which is usually caused by a short, a break, or discontinuity, the pulse reflects from that point back to the device. By measuring the time delay and knowing the signal’s transmission speed, the TDR can calculate the approximate distance to the fault.
The TDR does more than just measure reflection time, however. It also analyzes the amplitude, timing, and shape of the returning pulse and uses this data to determine the type and severity of the fault.
When your TDR can’t Locate the Fault
In many cases, particularly with intermittent or high-resistance faults, the TDR alone isn’t sufficient.
At this point, additional techniques can be utilized:
- Arc Reflection Method: Using this method, a high-voltage pulse is sent down the cable, creating a temporary arc at the fault point. This arc reshapes the impedance in a way that the TDR can detect, allowing it to estimate the distance to the fault.
- Pulse Current Technique: This technique is the typical method for locating faults in extremely long cables. This method also uses a high-voltage pulse to create a flashover, or spark, at the fault. The flashover generates transient signals which are characterized by fast risetime, short duration energy pulses that reflect and travel back through the cable.
These techniques are used to estimate the distance to the fault. Once that is determined, the next step is physically pinpointing its location using a high-voltage surge generator.
Pinpointing the Fault Location
Surge Generator (Thumper)
Commonly known as a “thumper”, a surge generator feeds high-voltage pulses at predetermined intervals into and through the faulty cable. Once these pulses reach the fault, a flashover occurs, resulting in a loud popping sound, or “thump”. This physical reaction also generates an electromagnetic field, which will also aid in locating the fault.
Locating the “Thump”
In order to pinpoint the fault location once the ‘thump” is detected:
- An acoustic-magnetic surge wave receiver is used in conjunction with the surge generator, especially for buried or obstructed cables. This tester utilizes a ground sensor or earth-piercing spikes, a receiver/display unit for visual readings from the electromagnetic field created by the “thump,” and headphones for auditory fault noise.
- Technicians methodically move the receiver along the cable path, listening for the loudest “thump” and measuring the strongest electromagnetic signals.
- The technician then zeroes in on where the “thump” and the signals are most intense, which is the precise location of the fault. At this point, the cable can be unearthed with minimal disturbance, and then repaired or replaced.
Cable faults greatly affect the performance of the cable, and can arise due to several reasons, such as insulation or wire defect, weakness in the cable, or other disturbances. Cable fault testing may seem like a complex science, but with advanced tools such as TDRs, surge generators, and high-voltage testers, technicians can now quickly diagnose, locate, and repair faults, maximizing efficiency and minimizing costly damage.
JM Test systems is your one-stop shop for cable fault detection equipment rental. Visit our page today for more information or to request a rental quote.
FAQ
What is cable fault testing and why is it important?
Cable fault testing is the process of detecting, locating and diagnosing faults, such as shorts, breaks, or discontinuities in electrical cables. This procedure is critical in minimizing downtime, reducing costs associated with repairs, and preventing safety hazards.
What are the most common cable faults?
The most common types of cable faults are low resistance faults, high resistance faults, and open circuit faults
What are the most likely reasons for cable failure?
The most likely reasons for cable failure are ageing, degradation of cable sheath, inappropriate cable application, and excessive internal heating of cable
Should I use a TDR first, or just begin by using a surge generator (thumper)?
A TDR should be used first to verify a fault in the cable. You must avoid high-voltage testing at this stage to prevent altering the fault characteristics, which could hinder and impede localizing the fault.
What equipment is used to pinpoint the exact fault location after estimating it with a TDR?
A Surge Generator, or “thumper” is used to send high-voltage pulses that cause a flashover, or spark, at the fault point, creating a distinct electromagnetic signal, or “thump”.
