Loose and corroded electrical connections are among the most common causes of overheating, unplanned outages, and electrical fires in commercial and industrial installations. Infrared cameras let maintenance teams find these faults early, without de-energising the circuit or making physical contact. This article explains the physics behind connection heating, what a thermal image actually reveals, and how to run a reliable infrared inspection that catches problems before they escalate.
How Loose and Corroded Connections Generate Excess Heat
Every electrical connection has some resistance. When that resistance increases, whether through a loosened bolt, oxidised contact surface, or corroded terminal, the joint converts more electrical energy into heat. The relationship follows a simple formula: power dissipated as heat equals the current squared multiplied by the resistance (P = I²R). Double the resistance at a connection carrying 200 A, and you quadruple the heat generated at that point.
Corrosion is particularly insidious. Copper oxide and aluminium oxide are poor conductors, so even a thin layer of corrosion across a contact face raises resistance significantly. Where dissimilar metals meet, galvanic corrosion accelerates the process. Aluminium-to-copper joints in older switchboards are a classic culprit.
Thermal cycling compounds the problem. As a connection heats and cools with load changes, the metals expand and contract at slightly different rates. Over months and years, this loosens bolted joints that were torqued correctly at installation. The loosened joint now has a smaller effective contact area, which raises resistance further, which generates more heat. It is a self-reinforcing failure mechanism. Left unchecked, temperatures can climb high enough to melt insulation, damage bus bars, or ignite surrounding materials.
How Infrared Cameras Detect Connection Faults
All objects above absolute zero emit infrared radiation proportional to their surface temperature. An infrared (IR) camera contains a detector array, typically a microbolometer, that converts this radiation into a visible thermal image. Each pixel in the image represents a temperature measurement, so the camera produces a complete thermal map of whatever it is pointed at.
On a thermal image of a distribution board, a healthy connection appears at roughly the same temperature as the adjacent conductor. A failing connection shows up as a distinct hotspot, often 10°C to 80°C or more above its surroundings, depending on load and severity. This visual contrast makes faults obvious even before they cause visible damage.
It is worth distinguishing a thermal imaging camera from a spot pyrometer (infrared thermometer). A spot pyrometer gives you one temperature reading at a single point. A thermal camera gives you tens of thousands of simultaneous readings across a scene, which means you can quickly scan an entire panel and spot anomalies you would never find with a single-point device. For electrical maintenance, the camera wins hands down.
Camera resolution matters. A higher-resolution detector (say 320×240 pixels versus 160×120) lets you identify smaller hotspots from a greater distance. For most LV and MV electrical work, a 320×240 camera from manufacturers like Fluke or FLIR is typically more than adequate.
Conditions for a Reliable Infrared Scan
You cannot just point a thermal camera at a panel and expect meaningful results. The physics of I²R heating means that a high-resistance joint only produces a detectable temperature rise when sufficient current flows through it. Most industry guidance recommends that equipment should be operating at a minimum of 40% of its rated load during the scan. Below that threshold, a failing joint may not produce enough heat to distinguish it from the background.
Scheduling matters. Survey a factory distribution board at 3 a.m. on a Sunday when production is shut down, and you will likely miss faults that would be glaringly obvious during a weekday peak.
Emissivity is the other variable that catches people out. Emissivity describes how efficiently a surface emits infrared radiation, on a scale from 0 to 1. Oxidised copper has an emissivity around 0.6 to 0.7, which is workable. But a freshly polished copper bus bar can have an emissivity below 0.1, making it nearly invisible to the camera or causing wildly inaccurate temperature readings. The thermographer must adjust the emissivity setting on the camera for the material being measured, or use a reference marker (a patch of high-emissivity tape or paint) to get a reliable absolute temperature.
|
Material |
Typical Emissivity |
Notes |
|
Oxidised copper |
0.6-0.8 |
Common on aged bus bars; reasonable for direct measurement |
|
Polished copper |
0.02-0.07 |
Highly reflective; requires emissivity correction or reference marker |
|
Oxidised aluminium |
0.2-0.4 |
Lower than copper; care needed with absolute temperature readings |
|
Painted surfaces / insulation |
0.9-0.95 |
Easy to measure accurately |
|
Steel (oxidised) |
0.7-0.9 |
Enclosures, mounting hardware |
Environmental factors also influence accuracy. Wind, direct sunlight, and ambient temperature swings can all mask or exaggerate thermal anomalies. Indoor scans are generally more reliable. Where outdoor scanning is necessary, a competent thermographer accounts for these variables in the report.
Classifying Hotspot Severity
Finding a hotspot is only the first step. The next question is how urgent is it? The most widely used method compares the temperature of the anomaly to a reference point, typically a similar component under similar load, and calculates the temperature difference (delta-T or ΔT). Larger delta-T values indicate more serious faults.
Several classification systems exist, including those referenced by NETA (InterNational Electrical Testing Association) and various insurance bodies. While specific thresholds vary, a commonly applied framework looks something like this:
|
Delta-T (above reference) |
Severity |
Recommended Action |
|
1°C, 10°C |
Minor |
Monitor; schedule repair at next planned outage |
|
11°C, 20°C |
Intermediate |
Repair at earliest convenience; increase monitoring frequency |
|
21°C, 40°C |
Serious |
Repair as soon as possible; assess risk of continued operation |
|
> 40°C |
Critical |
Immediate action required; consider de-energising the circuit |
Context matters. A 15°C rise on a lightly loaded circuit could indicate a worse underlying fault than a 25°C rise on a circuit running near its rated capacity. A competent thermographer considers load, ambient conditions, and the type of equipment when classifying severity, rather than relying on delta-T alone.
Common Causes of Electrical Connection Failure
Understanding why connections fail helps you target your infrared inspections more effectively. The usual suspects include:
- Under-torqued or over-torqued bolted joints, both of which reduce effective contact area.
- Thermal cycling, as described above, which gradually loosens fasteners.
- Vibration, particularly in industrial environments near rotating machinery or in mobile substations.
- Corrosion from moisture, chemicals, or atmospheric contaminants, especially in coastal or industrial locations.
- Dissimilar metal joints (e.g. aluminium conductors landed on copper terminals) without appropriate bimetallic connectors.
- Poorly made crimps, where the wrong die size, insufficient compression, or incorrect conductor preparation leaves voids in the barrel.
Any of these mechanisms produces the same end result: increased resistance at the joint, which an infrared camera will pick up as heat under load.
Infrared Inspection of Switchgear and Distribution Boards
Most electrical thermal surveys focus on switchgear, distribution boards, motor control centres, and bus duct connections. These are the locations where the highest number of bolted and crimped connections exist, and where faults are most likely to develop.
Scanning energised panels requires care. Opening a panel door exposes the operator to energised conductors and to the risk of arc flash. Where possible, many sites now fit infrared windows, transparent ports made from materials like calcium fluoride or crystal polymer that transmit infrared radiation while keeping the panel sealed. The thermographer scans through the window without opening the enclosure, which significantly reduces risk. For more on this topic, including common misconceptions about the arc rating of IR windows, see this article on arc-rated infrared windows.
Where IR windows are not installed, appropriate PPE and safe systems of work must be in place before any panel is opened for scanning. The Health and Safety Authority (HSA) in Ireland and HSE in the UK both provide guidance on safe working near energised electrical equipment.
Standards and Qualifications for Electrical Thermography
Thermographic inspection is only as good as the person behind the camera. ISO 18436-7 defines three levels of competence for condition-monitoring thermographers. Level 1 covers data collection and basic analysis. Level 2 covers interpretation, diagnosis, and reporting. Level 3 covers programme management and advanced analysis. For routine electrical surveys, a Level 2 qualified thermographer is typically the minimum competence you should look for.
On the testing side, BS EN 16714 (non-destructive testing, thermographic testing) provides a general framework, while IEC 62991 specifically addresses the use of thermal imaging for predictive maintenance of electrical installations. In practice, many organisations also reference NETA guidelines or manufacturer application notes when setting up their inspection programmes.
Calibration of the thermal camera itself is essential. An out-of-calibration camera may systematically over- or under-report temperatures, which directly affects severity classification. Cameras should be calibrated to traceable standards at the interval recommended by the manufacturer, typically every 12 to 24 months. For more detail, the thermal imaging FAQs page covers common questions about camera accuracy and calibration.
FAQs
Can infrared cameras detect faults inside fully enclosed panels?
Not through solid steel or aluminium enclosures. The metal blocks infrared radiation. You either need to open the panel (with appropriate arc flash PPE and safe system of work) or install infrared windows that transmit IR radiation while keeping the enclosure sealed and personnel protected.
What minimum load is needed for a meaningful thermal scan?
Industry guidance typically recommends at least 40% of rated load. Below that, a failing connection may not generate enough heat to produce a visible anomaly. Schedule scans during normal operating conditions rather than during shutdowns or periods of low demand.
How often should electrical thermal imaging surveys be carried out?
Annual surveys are common for general commercial and industrial installations. Critical infrastructure, such as data centres or hospital switchboards, often benefits from six-monthly or even quarterly scans. Your insurer may also specify a minimum frequency as a condition of cover.
Conclusion
Infrared cameras give electrical maintenance teams a fast, non-contact way to find failing connections before they cause outages, equipment damage, or fires. The technique is straightforward in principle but relies on correct load conditions, emissivity awareness, and a qualified thermographer to deliver reliable results. If you need a professional thermal imaging survey of your electrical installation, or want to discuss camera options and training for your own team, get in touch with Powerpoint Engineering's thermography team.