Finding and fixing cable faults

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Within any company, a defective electrical cable has the power to cause significant disruption. Heightened safety risks and interruptions to business operations mean that detecting and resolving these problems is crucial – especially when high voltages are involved. Dan Wagner, operations manager at Smith Brothers, explores the methods, apparatus and expertise involved in specialist cable fault testing.

As with many technical issues, it always seems to be the case that cable faults occur at the most inconvenient of times – when an interruption to the power supply is not just an annoyance, but can also have significant financial implications. For instance, an outage in the middle of a production line can create havoc for a manufacturer, whilst downtime for even a few hours can be very costly for a busy office. And of course, with any electrical infrastructure, safety is of paramount concern.

But for cable faults that occur below the ground, accurately locating the defect and carrying out the necessary repairs can be difficult – not to mention disruptive where power supply is concerned. Technical expertise and specialist equipment are therefore essential, to ensure the issue can be found and dealt with efficiently.

Underlying issues

A cable fault might occur due to a number of underlying problems – including water ingress, mechanical damage, poor materials, subpar jointing practice, partial discharge or age-related deterioration – many of which often aren’t immediately apparent. Such issues can impact all types of electrical cable, no matter the age or voltage. And it’s only once the power supply starts being affected that it becomes clear there’s an underlying problem.

Arising from these complications, the most common are short circuit faults – which can be phase-to-phase or phase-to-earth – and open circuit faults, where a break in the cable occurs. A combination of the two can also take place and in many instances, the defect starts as a minor point of damage to the cable or joint. This is then exacerbated by a gradual ingress of water, causing a breakdown in insulation resistance to the point where flashover occurs – triggering the protective device and cutting off the mains supply.

In some instances, such faults clear once the protection has kicked in and are therefore known as transient faults. Although these may sound like more temporary glitches, they can be extremely disruptive. In fact, detecting such intermittent problems tends to be more difficult than identifying a perpetual issue, resulting in an increased chance of the defect recurring in the future – often leading to a permanent fault.

Where the outer casing of the cable has broken, this is classified as a sheath fault. As with open and short circuit faults, this can lead to a far greater issue if water ingress occurs. However, if detected early enough, such external damage can be repaired, preventing possible cable failure in the future.

Finding and fixing faults

Whilst the causes and catalysts behind cable faults are well-known, finding and repairing them is a complex process that requires advanced equipment and technical expertise. Where there is suspected circuit damage, it’s therefore vital to enlist an electrical cabling specialist to locate the fault, fix the problem and restore the power supply.

Following a call-out, engineers will review any protection that has been activated and conduct an insulation resistance test. Depending on the results of this initial assessment, they may attempt to re-energise the system entirely or in part, to re-establish mains supply where possible. They will then walk the length of the circuit route, inspecting the visible cabling, joints and terminations for signs of failure and additionally looking out for potential sites of cable damage – including new structures or recent excavations, for example.

The next stage is to run preliminary tests to determine the characteristics of the fault. The observations from these are then used to inform which specialist equipment needs to be employed to locate it. Firstly, the general site of the fault is pre-located, then a more exact position pinpointed within this identified area, before excavations are conducted and further visual inspections carried out.

Portable TDRs (Time Domain Reflecto-meters) are usually employed at this stage for both open and short circuit faults. Combining advanced high voltage surge wave and arc reflection technology, this apparatus has a receiver that utilises acoustic and electromagnetic pinpointing, enabling the fault to be located within inches. Using such equipment, circuit lengths of up to three miles can be tested, making it invaluable for large-scale connections and infrastructure.

Although less technologically advanced, High Resistance Fault Locators are also widely used in the pre-location phase, for particularly difficult-to-find high resistance defects. Incorporating bridge technology – a tried-and-tested method that has been used for many years – this apparatus remains pertinent in the identification process for high resistance faults.

After the fault position has been identified, the necessary repairs to the cabling, joints and terminations are then carried out by the engineers. Once rectified, insulation resistance testing and continuity assessments can be conducted, to confirm all issues are resolved before the circuit is re-energised. VLF (very low frequency) testing is also often performed at this stage, to ensure any further cabling problems have been identified prior to the supply being restored.

Speed and safety

Cable fault call-outs are often urgent, so a swift response is essential to minimise disruption – especially in industrial or manufacturing settings. After safety, the next priority is to get systems back up and running, so seeking assistance from specialist engineers with experience in the latest and fastest fault finding techniques is vital.

No one can predict when a cable fault will occur and by the time symptoms appear, the underlying issue will inevitably have worsened over time. But whilst reactive response is essential in such instances, there are preventive measures that can be taken to deter the need for emergency assistance.

To reduce the requirement for critical fault finding and repairs, enlisting a contractor to conduct routine tests and carry out the necessary upkeep on cabling circuits is a good idea. Not only does proactive preventive maintenance mean that minor issues are spotted before they turn into larger problems, it can also be a cost-saving option for businesses in the long run – minimising possible downtime and removing the need for urgent call-outs, tests and repairs. 

Five essential fault finding steps 

1. Isolate and identify

Where there are multiple cables, determining the defective one immediately is crucial to protecting workers and decreasing disruption. In most cases, the cable fault will be permanent, meaning protection devices at either – or both – ends of the cable will have been triggered. Safety is of paramount importance, so once the faulty cable has been detected, ensuring it is isolated and earthed is the main priority.

2. Diagnose

Once the correct cable has been identified, tracing the circuit route and working out the phase where the fault has occurred – and whether this is of high or low resistance – is the next step. This test will determine what technique and equipment is needed to diagnose and locate the fault. If below 100 Ohms, a low voltage pulse from a TDR (time domain reflectometer) can usually be employed, whilst faults above 100 Ohms require bridge technology or shock discharge from an impulse generator.

3. Pre-locate

Efficiency is key to the pre-location phase, as this enables the fault position to be determined to a small percentage of the overall cable length and significantly reduces pinpointing time. For low resistance faults, pre-location might be all that’s needed to find the precise position of the defect, whilst high resistance ones require additional testing methods such as arc reflection.

4. Pinpoint

Pre-location allows engineers to find the fault position with a 5% margin of error, but greater accuracy is essential for the safe and swift rectification of cable defects. Following this initial locating phase with acoustic methods – alongside a shock discharge generator – therefore enables the fault site to be identified within a 0.1% distance.

5. Repair and re-energise

Once the exact fault location has been pinpointed, excavation can take place, to enable the necessary repairs and safety testing to be conducted. After the fault has been fixed and signed off by the contractor, the cable can then be re-energised and the power supply reinstated.