How to Read Thermal Survey Results Correctly

A thermal image showing a bright connection inside a distribution board can look alarming, but colour alone does not establish a fault. Knowing how to read thermal survey results means putting every image in its operating context: the electrical load at the time, the component type, the temperature difference, and the condition seen on site.

For homeowners, facilities managers and industrial duty holders, a well-presented thermographic survey can identify developing issues before they become outages, damage or fire risks. It is also a useful maintenance tool for prioritising work. It should not, however, be treated as a substitute for electrical inspection, testing or competent engineering judgement.

Start with the survey scope and conditions

Read the front of the report before examining the thermal images. It should state what was surveyed, which areas or assets were inaccessible, the date and time, the equipment used and the surveyor’s findings. A report that does not define its scope can create a false impression that an entire installation has been assessed.

Thermal surveys are only meaningful when electrical equipment is operating under a representative load. A lightly loaded circuit may not reveal a loose termination that becomes hot during peak demand. Conversely, a circuit running unusually hard during a temporary process or seasonal peak may produce temperatures that would not normally occur.

Look for recorded load data where available. For a three-phase board, this may include the current on each phase and the neutral. For motors, pumps or mechanical plant, it may describe the duty cycle and whether the equipment was running normally. If load was low, the report should say so and may recommend a further inspection under more suitable operating conditions.

Ambient temperature and airflow matter too. A cabinet in direct sunlight, a warm plant room or a panel mounted beside a heat source can affect readings. These factors do not make thermal imaging unreliable, but they do affect how results should be interpreted.

How to read thermal survey images

A thermal image is a map of surface temperature, not a direct view inside a cable, terminal or breaker. The colour palette is chosen to make temperature differences easier to see. White, yellow and red areas are often hotter than blue or purple areas, but the colour scale can be adjusted for each image. Never compare colours between two images without checking the temperature scale shown alongside them.

The key figure is usually the temperature difference, often called delta T. This compares the suspected item with a similar component under similar conditions, or with the surrounding reference temperature. A terminal at 55°C may be entirely different in significance depending on whether adjacent terminals are at 50°C, 25°C or 90°C.

A useful report will identify:

• the asset and exact location, such as distribution board designation, circuit reference or plant item;
• the maximum apparent temperature and the relevant reference temperature;
• the electrical load and operating state during inspection;
• a visible-light photograph to confirm what is being viewed; and
• a clear recommendation with a priority or timescale.

The visible image is more than a convenience. It helps distinguish a hot cable gland from a hot terminal, confirms phase identification and allows maintenance teams to locate the item safely. Without it, remedial work can become slower and more uncertain.

Compare like with like

The strongest thermal findings are comparative. In a three-phase system with broadly balanced loading, one phase connection substantially warmer than equivalent connections on the other phases can indicate a loose or deteriorating termination, contact resistance or overload. A warm component may also be normal if it is carrying more current than its neighbours.

This is why temperature alone does not provide a universal pass or fail threshold. Components have different designs, materials and permitted operating temperatures. A fuse, contactor, cable termination and motor bearing should not be judged against the same number. The surveyor should assess the pattern, the loading and the manufacturer’s information where relevant.

Separate likely faults from misleading heat patterns

Not every hot spot is an electrical defect. Thermal cameras measure infrared radiation from surfaces, and some materials are difficult to assess accurately. Shiny metal busbars, terminals and cable lugs can reflect heat from the surveyor, nearby equipment or the sun. Their apparent temperature may be misleading because of low emissivity and reflected temperature.

An experienced surveyor manages this by changing viewing angle, using appropriate camera settings, checking comparable surfaces and, where safe and suitable, measuring an adjacent high-emissivity surface. The report should record any limitations rather than presenting an uncertain reading as a confirmed fault.

Common findings that warrant further investigation include loose or corroded connections, overloaded circuits, deteriorated switchgear contacts, phase imbalance, failing fuses and abnormal motor or bearing temperatures. The correct response depends on the defect. A high-resistance termination may require isolation, examination, re-termination and torqueing to the manufacturer’s requirements. An overloaded circuit may need load redistribution, circuit alteration or a wider capacity review.

Do not ask a maintenance operative simply to tighten a connection while equipment remains energised. Electrical work must be planned, safely isolated where required and completed by a competent person. The cause also needs to be addressed. Re-tightening a terminal without checking conductor condition, loading and the connection method can leave the underlying issue unresolved.

Read the priority rating with judgement

Most reports classify findings by urgency, often using categories such as immediate, urgent, planned or monitor. The language differs between providers, so use the report’s own definitions rather than assuming that a colour code has a universal meaning.

An immediate finding may involve a severe abnormal temperature, evidence of insulation damage, arcing risk or a critical asset where failure would have serious consequences. It may require prompt isolation or an emergency repair. An urgent item may be safe to manage briefly but should be rectified within a defined timescale. Lower-priority observations can often be monitored or addressed during planned maintenance, particularly where the temperature difference is modest and operating conditions explain it.

Criticality is as relevant as temperature. A moderate anomaly on a supply serving life-safety equipment, a production line, an airport system or a rail-related installation can carry greater operational risk than a similar anomaly on a non-critical local circuit. A sound report connects the technical observation to the likely safety and continuity impact.

For commercial and industrial premises, turn recommendations into an action record. Allocate a responsible person, target date, work order and evidence of completion. Following repair, a repeat thermal image under comparable load provides useful confirmation that the abnormal pattern has been removed.

Use thermal findings alongside electrical maintenance

Thermography is particularly effective as part of a planned preventative maintenance programme. It can be undertaken with equipment energised and often identifies deterioration that cannot be seen during a visual inspection. Yet it has limits. It cannot verify protective-device operation, insulation resistance, earthing arrangements, conductor continuity or every defect hidden within an installation.

For that reason, a thermal survey should sit alongside appropriate inspection and testing, manufacturer maintenance instructions and site-specific risk assessments. In many workplaces, the findings will also inform maintenance duties under the Electricity at Work Regulations 1989. For fixed wiring, it does not replace the inspection and testing needed to assess compliance with BS 7671 where that is required.

Trend data adds real value. If the same connection rises in temperature between surveys under similar load, action should not wait for a dramatic image. Equally, an item that appears warm once but remains stable after load changes and closer assessment may not justify unnecessary replacement. Good maintenance is proportionate, evidence-led and recorded.

Questions to ask before approving remedial work

Before committing to repairs, establish whether the finding has been verified under suitable load and whether the proposed work addresses the cause rather than the symptom. Ask what safe isolation, testing and post-repair checks will be carried out. Where production, tenant services or critical infrastructure are involved, agree the outage plan and contingency arrangements before work begins.

For larger estates, it is worth standardising asset labels, panel schedules and report references. Clear identification makes each future survey easier to compare and reduces delays when an engineer needs to locate a reported component. SJB Smart Electricals approaches electrical surveys with this practical link between findings, safe remedial work and long-term maintenance planning.

The most useful thermal report is not the one with the brightest images. It is the one that gives you enough reliable evidence to make the next safe, proportionate decision – whether that is urgent repair, planned investigation or a repeat survey at a more representative load.