An infrared camera can show that a connection is hot, but heat alone does not tell a planner whether to fix it tonight or at the next shutdown. Thermographic severity criteria answer that question. They take the temperature rise of an anomaly, its delta-T from a reference, and map it onto a scale of repair urgency so a warm image becomes a ranked action. This guide explains how delta-T is measured, how standard severity tables convert those ranges into priorities, and why this step is what turns thermography from a diagnostic curiosity into a maintenance planning tool.
Thermographic severity in one line: Thermographic severity criteria are the rules that rank an infrared anomaly by its delta-T, the temperature difference between the hot spot and a reference component or ambient, and map that difference onto a scale of repair urgency. By assigning each finding to a severity band, from monitor-only up to immediate action, the criteria turn a temperature reading into a prioritised maintenance decision so that limited repair effort is spent on the anomalies most likely to fail first.
The foundation of every severity scheme is delta-T rather than absolute temperature. Delta-T is the difference between the temperature of the anomaly and a reference, and choosing the right reference is the whole game. The preferred reference is a similar component operating under similar conditions, such as the same terminal on another phase of the same circuit, because it shares the same ambient and, ideally, the same load. Comparing a hot lug to its identical sibling isolates the fault from everything the two have in common, so the number that comes out reflects the defect and not the weather.
When no comparable component exists, the reference becomes the ambient air temperature, and the delta-T is the rise above ambient. This is common for single components with no matching neighbour, but it is a weaker comparison because ambient does not account for load or normal operating warmth, so severity tables generally set different, more conservative thresholds for ambient-referenced readings than for component-to-component readings. Knowing which reference was used is essential to reading a delta-T correctly, and a good survey record always states it.
Delta-T also has to be read in the context of load, because heating rises steeply with current. A thirty-degree rise measured at half load is not the same fault as a thirty-degree rise at full load; the first will be far worse when the circuit is fully loaded. Careful practice records the load at the time of measurement and, where possible, normalises the delta-T toward expected full-load conditions before applying the criteria. Without that context, a low-load reading can quietly under-rank a serious defect.
A severity table is simply a set of delta-T bands, each tied to a recommended response. A widely referenced example is the table associated with NETA, the InterNational Electrical Testing Association, which groups anomalies by their temperature difference into several priority levels. At the low end, a small rise indicates a possible deficiency that warrants further investigation or monitoring at the next survey. A moderate rise indicates a problem that should be corrected as scheduling permits. A larger rise indicates that repair should happen soon, and the highest band calls for immediate action because failure may be near.
Because the strength of a comparison differs, these tables provide separate threshold columns for the two reference methods, with lower delta-T numbers triggering a given priority when the reference is a similar component and higher numbers required when the reference is only ambient. This reflects the fact that a component-to-component comparison is more trustworthy, so a given rise means more. A facility typically adopts one recognised scheme and applies it consistently, so that a delta-T of a given size always lands in the same band regardless of which inspector took the reading.
The exact numeric boundaries vary between published schemes and are sometimes tailored to an organisation's own risk tolerance, so the value of a severity table lies less in its precise figures than in its consistency. What matters is that everyone measuring anomalies uses the same rules, so that the ranking is repeatable and defensible. This document deliberately does not quote specific threshold figures, because they differ by standard, reference method, and organisation; the working principle is that increasing delta-T maps to increasing urgency, split by whether the reference is a like component or ambient.
The purpose of severity criteria is to make the leap from image to schedule. Once each anomaly carries a severity band, the maintenance planning process can treat findings differently in a principled way: immediate-action findings drive same-day or emergency work orders, high-priority findings are slotted into the next available outage, moderate findings are bundled into planned maintenance, and monitor-only findings are logged to be re-measured at the following survey. Without this ranking every hot spot would compete equally for attention, and the connection most likely to fail could sit behind a dozen trivial ones.
Severity also enables trending, which is where the real predictive value lies. The same anomaly measured over successive surveys produces a sequence of delta-T values and severity bands, and a finding that climbs from a low band to a higher one is deteriorating on a schedule the survey history reveals. A stable low-severity finding can safely be watched, while one that is accelerating between surveys deserves earlier intervention than its current band alone might suggest. Reading severity as a trajectory rather than a single label is what separates good reliability programmes from those that merely file reports.
Centralising this ranked data is where a monitoring platform helps. In distributed operations such as oil and gas facilities, thermographic findings come from many sites and many surveys, and a cloud SCADA and asset-monitoring platform such as Merobix can hold each finding against its specific asset with its delta-T, reference basis, load, and severity band. Keeping that alongside the live electrical data the platform already collects lets a planner sort the whole fleet's anomalies by severity, watch each one's trend across surveys, and generate work orders directly, so the severity criteria drive action rather than gathering dust in a standalone report.
A component-to-component delta-T compares the hot spot to a similar part under similar conditions, such as the same terminal on another phase, which cancels out shared ambient and load and gives the most trustworthy reading. An ambient delta-T compares the hot spot to the surrounding air, used when no matching component exists, and is weaker because it ignores normal operating warmth and load. Severity tables set separate, more conservative thresholds for ambient-referenced readings for this reason.
Raw temperature is distorted by ambient conditions and load, so the same connection reads differently on a hot day than a cold one. Delta-T removes the shared conditions by comparing the anomaly to a reference, so the number reflects the fault itself. This makes the ranking consistent across seasons, sites, and inspectors, which is exactly what a repeatable priority scheme needs.
No. The exact numeric boundaries differ between published schemes and are sometimes adjusted to an organisation's own risk tolerance, and they also differ between the component and ambient reference methods. What matters is that a facility adopts one recognised scheme and applies it consistently, so a given delta-T always lands in the same severity band. The universal principle is simply that a larger temperature rise maps to greater repair urgency.
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