Qualitative vs. Quantitative Thermal Analysis in Electrical Thermography

Understanding the Critical Differences for Certified Thermal Electrician™ Professionals

Thermal imaging has become an important condition-assessment method for electricians, power-quality specialists, and facility maintenance teams. For professionals pursuing or holding the Certified Thermal Electrician™ credential, understanding the distinction between qualitative and quantitative thermal analysis is fundamental to professional electrical thermography.

NFPA 70B provides the electrical-maintenance framework within which infrared thermography can be used to evaluate electrical equipment. The current edition recognizes temperature differences between similar electrical components under similar loading and between electrical components and ambient air temperature.

Where thermographic suggested-action criteria are being applied, ANSI/NETA MTS provides the technical ΔT comparison and suggested-action framework used throughout the Certified Thermal Electrician™ program.

1. What Is Qualitative Thermal Analysis?

Pattern-Based, Comparative Electrical Interpretation

Qualitative thermal analysis focuses primarily on thermal patterns, gradients, symmetry, asymmetry, and relative differences within the scene. The initial purpose is generally to identify anomalous thermal behavior that deserves closer evaluation.

One of the strongest electrical applications of qualitative analysis is comparing similar components performing similar functions under comparable loading and environmental conditions.

Key Attributes of Qualitative Analysis

• Pattern recognition between comparable conductors, phases, breakers, terminations, or other components.
• Identification of thermal gradients, asymmetry, and anomalies.
• Useful for locating areas that deserve further quantitative measurement or electrical investigation.
• Comparisons between similar surfaces can reduce some uncertainty associated with absolute temperature measurement, but emissivity, reflections, angle, focus, distance, and environmental conditions still matter.
• Particularly useful during route-based inspections where many similar electrical components can be compared efficiently.

Common Qualitative Indicators in Electrical Systems

• Localized spot heating — may be consistent with increased connection resistance or another localized source of heating and warrants further investigation.
• Heating along a conductor — may be associated with loading, conductor characteristics, installation conditions, harmonics, environmental effects, or other factors.
• Phase-to-phase thermal asymmetry — may indicate differences in loading or electrical behavior that should be verified with appropriate electrical measurements.
• Diffuse equipment heating — can indicate that additional investigation of the equipment, load, ventilation, internal components, or operating conditions is appropriate.

A thermal pattern does not independently prove a loose connection, improper torque, overload, harmonics, corrosion, defective breaker, insulation failure, or another specific root cause.

Those conditions should be confirmed using appropriate electrical measurements, physical inspection, manufacturer information, equipment ratings, maintenance history, and other available evidence.

2. What Is Quantitative Thermal Analysis?

Measurement-Based Thermal Evaluation

Quantitative thermal analysis uses radiometric temperature measurements to determine actual target temperatures and temperature differences as accurately as practical under the inspection conditions.

Quantitative measurements require the thermographer to account for the variables that materially affect infrared temperature measurement.

Key Attributes of Quantitative Analysis

• Requires an appropriate emissivity value for the target surface.
• Requires consideration of reflected apparent temperature (RAT).
• Requires proper focus and sufficient target resolution.
• Requires consideration of distance, viewing angle, target size, IFOV/MFOV, surface condition, atmospheric effects, and environmental conditions.
• Allows the thermographer to calculate a valid temperature difference (ΔT) when an appropriate reference condition exists.
• Can support comparison with applicable manufacturer temperature limits or equipment ratings when the measurement location and manufacturer criteria actually correspond.

A thermal camera measures infrared radiation from an observed surface. It does not directly measure conductor ampacity, internal conductor temperature, electrical resistance, current, torque, or the temperature of an inaccessible internal connection.

ANSI/NETA MTS Suggested-Action Criteria

The comparison method must be established before the ΔT result is classified. Similar-component comparison and component-to-ambient comparison use different temperature-difference ranges.

Similar Components Under Similar Loading

There is no Orange tier in the CTE™ visual mapping for the similar-component comparison because the applicable NETA comparison does not contain a separate intermediate tier.

Component-to-Ambient Comparison

Celsius is the controlling technical ΔT value. Fahrenheit can be shown as a converted convenience, but it should not be treated as a separate severity scale.

Examples of Quantitative Applications

• Measuring ΔT between similar breakers or terminations operating under sufficiently comparable loading.
• Measuring component-to-ambient ΔT when that is the appropriate comparison method.
• Evaluating whether a measured surface temperature exceeds an applicable manufacturer limit where the manufacturer has established a limit for that measurement location.
• Evaluating thermal differences on neutral conductors while separately measuring current and harmonic content when those conditions are relevant.
• Performing pre- and post-maintenance thermography under sufficiently comparable operating conditions.
• Establishing historical thermal information for later comparison while recognizing that trending is supporting evidence rather than a separate NETA comparison method.

3. Applying the Comparison Methods to Electrical Examples

The correct action tier comes from the applicable comparison method and measured ΔT. Thermal appearance alone does not establish the cause of the condition.

Example: Similar Components Under Similar Loading

Three comparable phase connections are operating under sufficiently similar load and conditions:

• Phase A = 42°C
• Phase B = 40°C
• Phase C = 44°C

The temperatures are relatively close, but the thermographer should still consider loading, surface characteristics, measurement quality, and the overall thermal pattern before drawing conclusions.

Example: Major Similar-Component Difference

Assume comparable components are operating under sufficiently similar loading:

• Phase A = 42°C
• Phase B = 40°C
• Phase C = 84°C

Using an appropriate representative reference temperature, Phase C has a ΔT far greater than 15°C. Under the similar-component NETA comparison, that places the result in the major discrepancy — repair immediately tier.

Within the CTE™ visual system, that NETA result maps to Red.

The temperature difference identifies a serious thermal discrepancy. It does not by itself prove that the connection is loose, improperly torqued, oxidized, overloaded, or internally defective.

Example: Parallel Feeders

One parallel conductor is measured 18°C hotter than comparable conductors. Before applying a similar-component classification, the thermographer must verify that the conductors are actually suitable for like-for-like comparison, including loading and operating conditions.

If the comparison is valid, a ΔT greater than 15°C falls into the major discrepancy — repair immediately tier for similar components.

The thermal result warrants investigation of current sharing, terminations, conductor condition, installation, and other possible causes.

Example: Feeder Breakers

A feeder breaker is measured 12°C warmer than an otherwise comparable breaker.

If both breakers are truly similar components operating under comparable loading and conditions, a 12°C ΔT falls within the 4–15°C probable deficiency — repair as time permits similar-component tier.

Current measurements, loading, conductor connections, breaker condition, ambient effects, and manufacturer information should then be evaluated as appropriate.

Example: Component-to-Ambient

Assume an electrical component is measured at 58°C and ambient air temperature is 30°C.

The component-to-ambient ΔT is:

58°C − 30°C = 28°C ΔT

Under the component-to-ambient NETA comparison, 28°C falls within the 21–40°C tier — monitor until corrective measures can be accomplished.

Within the CTE™ visual system, that result maps to Orange.

4. Combining Qualitative and Quantitative Thermography

Professional electrical thermography normally uses qualitative and quantitative techniques together rather than treating them as competing approaches.

1. Observe the thermal pattern.
   Identify unusual symmetry, asymmetry, gradients, or localized heating.
2. Determine whether a valid comparison exists.
   Prefer similar components under comparable loading where that comparison is available. Use component-to-ambient when that is the appropriate method.
3. Control measurement variables.
   Address emissivity, reflected apparent temperature, focus, distance, target size, viewing angle, environmental conditions, and other significant variables.
4. Measure and calculate ΔT.
   Record the area of concern and the appropriate reference temperature.
5. Apply the applicable technical criteria.
   When using ANSI/NETA MTS, apply the suggested-action tier associated with the actual comparison method.
6. Evaluate supporting evidence.
   Consider load, equipment ratings, manufacturer limits, historical information, electrical measurements, asset condition, thermal pattern, and other relevant evidence.
7. Document the finding and recommendation.
   Clearly distinguish the measured thermal condition from suspected cause and from the professional recommendation.

Important Documentation Items

• Equipment and component identification.
• Thermal and corresponding visual images.
• Area-of-concern temperature.
• Reference temperature.
• Calculated ΔT.
• Comparison method used.
• Relevant operating and load conditions.
• Emissivity and reflected apparent temperature where relevant.
• Distance, target size, focus, and other measurement limitations where relevant.
• Applicable ANSI/NETA suggested-action tier when that framework is being used.
• CTE™ visual mapping, when used.
• Supporting observations and recommended follow-up.

5. Why Certified Thermal Electricians Must Understand Both Methods

Electrical thermal behavior can be influenced by I²R heating, connection resistance, circuit loading, harmonic current, equipment construction, heat transfer, environmental conditions, and operating history.

Because of this, professional electrical thermography requires both pattern recognition and reliable quantitative measurement.

Qualitative Analysis = Finding the Pattern

• Efficiently identifies thermal anomalies and asymmetry.
• Useful for scanning large numbers of electrical components.
• Helps identify the areas that deserve closer quantitative evaluation.
• Particularly useful when valid like-for-like electrical comparisons are available.

Quantitative Analysis = Measuring the Condition

• Produces measured temperatures and ΔT values when measurement variables are properly controlled.
• Allows the applicable ANSI/NETA suggested-action tier to be determined when a valid comparison exists.
• Supports professional documentation and maintenance decision-making.
• Supports historical comparison when inspection conditions are sufficiently comparable.

To learn more about the Certified Thermal Electrician™ Program, visit:
https://ThermalElectrician.com