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FLIR Systems’ views on applying severity criteria in infrared maintenance inspections

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article image Thermal image of a structure is used for Predictive Maintenance

FLIR Systems  has explained that people need to be acquainted with the installation to decide whether a hot-spot is a real problem or not.

Defining and applying severity criteria for infrared maintenance inspections

Thermal imaging has evolved into a valuable diagnostic tool for Predictive Maintenance. By detecting anomalies often invisible to the naked eye, thermography allows corrective action to be taken before costly system failures occur.

Finding out what the problem is, the scale and consequences of a possible shut-down of a production line, analysing, reporting and taking connective action, are only a few of the questions that can rise when using infrared thermography. This document provides the answers to frequently asked questions regarding this matter.

Finding hot spots is one thing, but assessing them is another issue; inspection results have to be classified and put into some kind of formal structure to allow a decision about how, and especially when to take corrective action.

The Infrared Training Center, an independent infrared training institute which also provides certification, suggests the following severity criteria for electro-technical components and installations. Many thermographers take it as a guideline.

Class A: A very serious anomaly that requires immediate attention

Class B: A serious anomaly that requires attention as soon as possible

Class C: An anomaly that requires monitoring and a check-up at the earliest convenient time.

The shutdown problem: It has to be determined whether the hot spot is of strategic importance or not. Severity criteria fulfil the need to prioritise findings in such a way that the serious problems are dealt with first.

Direct, immediate repair of all detected anomalies in a plant are virtually impossible, as this would be too costly and inefficient to do.

The task of a thermographer and his customer or boss, is to keep a plant running in a profitable mode with as few interruptions as possible and at the lowest cost, while maintaining a safe operation in terms of worker, property, and environmental safety.

This explains why a classification of faults is a logical consequence and is by all means mandatory.

Temperature limits define action

Preventive maintenance is based on comparative evidence. An infrared camera operator gathers quantitative and qualitative information about scanned objects.

Quantitative information consists of a scanned object’s temperature, while qualitative information applies to discovering, describing and locating the faults.

Both are compared to normal temperature levels or function modes, within a certain range.

Measuring temperature levels becomes necessary to be able to decide how to deal with the problem once it is found. However, it is assumed that the thermographer is acquainted with the apparatus he is inspecting, knows that different types of components with different functions, as well as identical components with varying functions that can have different temperature limits.

So a temperature limits table is clearly not enough. And there are indeed no universally applicable severity criteria for a multitude of components. Of course, if an object is too hot, it is usually a sign that something should be done about it. But not always immediately and not necessarily a repair.

It will, however, certainly require attention, which can be defined as a stadium that alerts the concerned people, urging them to start the process of a short- or medium-term solution planning.

Their outcome may of course be an immediate repair, but if it is not, some other actions may be decided such as, among others: lessening the load, applying additional cooling, ordering spare parts and labour, scheduling a time for shutdown and repair that will impact production the least, or doing additional measurements with other techniques for verification.

Developing criteria and guidelines

‘Immediate repair’ and ‘attention’ may be good indicators, but they are only a first step towards defining levels of urgency.

There are three main sources of information to determine severity criteria, namely, rules set by standards organisations, available literature about the inspected materials (operation manuals, construction plans etc.), and previous field experience (previous inspection reports, thermographer’s experience in the sector).

Organisations will have to implement and consistently continue the following actions in order to be successful:

  • Keeping up to date with what standards organizations and others publish: relevant standardisation organisations are ISO (International Organisation for Standardisation), IEC (International Electrotechnical Commission), NETA (International Electrical Testing Association), IEEE (Institute of Electrical and Electronics Engineers, Inc.), DIN (Deutsches Institut für Normung), and other supranational or national standardisation bodies.
  • Studying available literature about the inspected materials and components: manufacturer's websites and manuals are an excellent source of information about materials and components for the thermographer.
  • Documenting experience from the field: these data are of crucial importance to determine, adjust or even re-assess severity criteria. It is necessary to preserve and store previous inspection reporting and keep it available in various media.

Infrared camera manufacturers like FLIR Systems offer various software suites for easy, clear and transparent predictive maintenance inspection reporting.

As a rule, criteria and guidelines should be kept alive by the accumulation of experience, gathered by thermographers.

Work of a thermographer

A thermographer can determine the severity of a problem correctly with a thorough knowledge of the installations. The thermographer should be able to interpret the anomaly with regard to its impact on the entire operation or machinery unit.

Similar or identical hot spots on two identical components do not necessarily have identical consequences.

An in-depth knowledge about the surveyed objects, empowers the thermographer to decide about the severity criterion.

This is a fact that is not often recognised, but good thermographers do understand this. They know that identical or different types of components with different functions have different temperature limits and they use their knowledge and experience to come up with reasonable solutions and reporting, within the framework of the customer’s guidelines, when available.

In addition, it should be pointed out that severity criteria are not only important as structure for classification of faults but also as a formalized communication tool between the thermographer and his customer, who is often expert and decision-maker about further measures and actions.


Severity criteria are guidelines. They are useful indicators but they are insufficient as they should be developed for each type of industry, company, operation, and even material and component. The application of severity criteria should be a continuously evolving, dynamic process that takes new experience into account.

Much depends, as always, not only on the user skills of the thermographer, but also on his/ her knowledge of and expertise in a relevant business sector.

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