Craig Collinson, operations director at Smith Brothers, explores the methods and expertise required 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.
So, when it comes to identifying and fixing cable damage, acting fast to reduce risk and disruption is essential.
Underlying issues
A cable fault might occur as a result of 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 affect all types of electrical cable, no matter the age or voltage.
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 this may sound like a temporary glitch, it can be extremely disruptive. Detecting such intermittent problems is difficult – increasing the likelihood of issues recurring in the future and often leading to a permanent fault.
Where the outer casing of the cable has broken, this is classified as a sheath fault and – as with open and short circuit faults – 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.
Testing, testing
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 Reflectometers) 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.
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 secondary 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.
To prevent the need for such an emergency response, enlisting a contractor to conduct routine tests and carry out the necessary upkeep on cabling circuits is a good idea. Not only does proactive preventative 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.
