How to effectively identify and locate power cable faults
Power cable faults can take on many different forms, but some are easier to identify than others. Permanent faults on simple networks, like street lighting systems where the cable path is known, are often easier to detect. On the other hand, finding faults in power cables is rarely straightforward and can become very expensive - especially for underground cables. In the UK, digging a single hole in a city street can cost tens of thousands of pounds, and excavating buried cables can reach a staggering £4 million per mile.
So, what’s the solution? A structured approach to diagnosing and locating cable faults using modern testing equipment is far more efficient and cost-effective. Let’s walk through the process.
Step 1: Initial testing
The first stage of fault detection is relatively simple. Carry out continuity and low-voltage resistance checks to confirm that there is a fault. Avoid the temptation to use a high-voltage insulation test at this point, as this could change the fault’s characteristics, making it more challenging to locate later.
Step 2: Using a time domain reflectometer (TDR)
Once a fault is confirmed, the next step is to localize it using a time domain reflectometer. A TDR sends a brief, low-voltage pulse through the cable and measures the time it takes for the pulse to be reflected back from the fault. This helps estimate how far the fault is from the test point. TDRs are especially effective for identifying open or short-circuit faults.
It’s a good practice to store a reference trace before conducting further tests. This allows comparison between live and recorded data, helping to identify any changes in the fault's condition.
Step 3: Dual-channel TDRs
Some TDRs, known as dual-channel TDRs, can test two cable phases simultaneously. This makes it easier to compare a faulty circuit with a working one. Dual-channel TDRs, like the Megger CFL535G, can even test live circuits, eliminating the need for extra blocking filters.
While basic TDRs are compact and cost-effective, they can detect a significant percentage of faults. They are an excellent investment for smaller projects where purchasing advanced equipment might not be feasible.
Step 4: When the TDR can't find the fault
If the fault is higher-resistance, a TDR will not be able to detect the issue. One solution is to “condition” the fault by burning it to change its characteristics. However, caution is required with this method – it could introduce complications during future testing.
Step 5: Arc Reflection and Impulse Current Methods
If basic TDR techniques don't work, more advanced methods like the arc-reflection technique may be used. This method involves sending a high-voltage pulse down the cable, which creates a temporary arc at the fault site. The arc behaves like a short-circuit fault, allowing the TDR to locate it.
Another method, the impulse current technique, uses a high-voltage pulse to trigger a flashover at the fault. The transient signals generated during the flashover travel along the cable, and their time intervals can be analysed to estimate the distance to the fault.
Step 6: Pinpointing the fault
Once the distance to the fault is determined, the next challenge is to pinpoint its exact location. This process involves using a surge generator - commonly referred to as a thumper - which sends high-voltage pulses through the cable. At the fault location, these pulses cause a flashover, which creates an audible noise, or thump, and an electromagnetic field. These can be detected using specialized receivers.
For buried cables, a shockwave receiver is often used. The operator moves this device across the cable path until the sound and electromagnetic field are strongest, indicating the fault's location.
Step 7: Special cases - cables in ducts and short-circuit faults
Locating faults in cables placed in ducts can be tricky, as sound travels through the duct and makes it harder to pinpoint the exact location. In such cases, it may be more cost-effective to replace a section of the cable than dig up a buried one.
Additionally, not all faults will produce a thump. For example, short-circuit faults do not cause flashovers, meaning there is no sound or electromagnetic field to detect. In these situations, a TDR combined with a cable route tracer can help determine the fault's distance, but pinpointing its exact location becomes more difficult.
Why invest in cable fault location equipment?
While locating faults on power cables can be challenging, using a combination of modern testing instruments and a structured fault-finding approach can make the process significantly easier. Given the potential downtime and losses associated with cable faults, investing in the latest fault location equipment is a smart, cost-effective decision.
Our range of state-of-the art cable fault detection tools provides everything you need to resolve cable faults quickly and safely - from smaller fault location tools to stand-alone fault locating systems that allow you to locate and identify a range of faults from one simple user interface.
