Qualitative vs. Quantitative Thermal Analysis in Electrical Thermography

Understanding the Critical Differences for Certified Thermal Electrician™ Professionals

Thermal imaging has become a foundational diagnostic method for electricians, power-quality specialists, and facility maintenance teams. For professionals pursuing or holding the Certified Thermal Electrician™ credential, the distinction between qualitative and quantitative thermal analysis is central to performing NFPA 70B–aligned electrical inspections.

NFPA 70B (2023 Edition) integrates both analysis types into its infrared thermography guidance, particularly in Chapter 9: Infrared Thermography, where the standard outlines severity criteria, temperature rise guidelines, and inspection methodology. Understanding both approaches ensures accurate diagnostics, safety, and defensible maintenance decisions.

1. What Is Qualitative Thermal Analysis?

Pattern-Based, Comparative Electrical Interpretation

Qualitative thermal analysis focuses on visual thermal patterns, gradients, and relative temperature differences—not absolute temperatures. NFPA 70B recognizes this method in §9.2.1 (Visual Temperature Differences), where it emphasizes comparing components under similar loading and environmental conditions.

Key Attributes of Qualitative Analysis

• Pattern recognition between conductors, phases, or terminations.
• Identification of thermal gradients and anomalies.
• Less sensitivity to emissivity and background reflections than strict temperature measurement.
• Essential for quickly locating electrical defects during energized inspections.

NFPA 70B Guidance on Qualitative Analysis

• §9.2.1 – Recommend comparing similar components under similar loads.
• §9.2.3 – Qualitative scanning is recommended for anomaly detection.

Common Qualitative Indicators in Electrical Systems

• Spot heating – high-resistance termination or torque issue.
• Linear heating along a conductor – overload or harmonic distortion.
• Phase imbalance – one phase operating significantly hotter than others.
• Diffuse heating – internal device or insulation deterioration.

2. What Is Quantitative Thermal Analysis?

Temperature-Corrected, Measurement-Based Evaluation

Quantitative thermal analysis focuses on determining accurate temperatures using emissivity correction, reflected temperature compensation, and proper camera configuration. NFPA 70B references temperature-based severity assessment in §9.2.5, including “temperature rise above similar components” and “temperature rise above ambient.”

Key Attributes of Quantitative Analysis

• Requires correct emissivity settings for the target material.
• Requires accurate reflected apparent temperature (RAT) entry.
• Considers distance-to-target, convection, and surface condition.
• Allows comparison to manufacturer specifications, insulation classes, and NEC-related ampacity limits.

NFPA 70B Severity Criteria (Table 9.2.5)

Table 9.2.5 in NFPA 70B uses temperature rise (ΔT) to classify severity when comparing similar components or when referencing ambient:

Examples of Quantitative Applications

• Determining whether a termination exceeds allowable lug temperature rating.
• Comparing conductor temperature to insulation rating (e.g., 60°C, 75°C, 90°C per NEC 310.16).
• Evaluating breaker case temperature against manufacturer specified maximums.
• Assessing neutral heating due to harmonic loads.
• Performing pre- and post-maintenance thermal trending.

3. NFPA 70B Severity Levels – Expanded Electrical Examples

NFPA 70B provides the severity framework, but real electrical systems require interpretation that a Certified Thermal Electrician™ can provide. Below are practical examples of each severity level as applied to real-world electrical equipment.

Severity Level 1: Low Temperature Rise (1°C – 10°C)

“Monitor Condition” per NFPA 70B

• Slightly Warm Breaker Case
  A breaker under moderate load shows a small rise over ambient or adjacent breakers.
  Action: Document and monitor during future thermographic inspections.
• Mildly Warmer Neutral Conductor
  Uniform, mild heating from light harmonic activity or small load imbalance.
  Action: Note for trending; no immediate intervention required.
• Slightly Elevated Lug Temperature
  A termination is a few degrees warmer than adjacent connections.
  Action: Include in routine inspection logs.

Severity Level 2: Medium Temperature Rise (11°C – 20°C)

“Schedule Maintenance” per NFPA 70B

• Overheated Termination With Stable Pattern
  ΔT in the 12–18°C range, indicating torque loss or early oxidation.
  Action: Schedule correction at next planned outage.
• Breaker Heating Above Ambient but Below Critical
  ΔT in the 15–18°C range, suggesting internal resistance.
  Action: Evaluate load and schedule maintenance.
• Feeder Conductors Under High Load
  Consistent linear heating, ΔT 12–17°C.
  Action: Review loading conditions and conductor sizing.

Severity Level 3: High Temperature Rise (> 21°C)

“Immediate Action Recommended” per NFPA 70B

• Loose or Failing Lug Connection
  Concentrated hotspot 25–60°C above similar lugs.
  Action: Immediate shutdown and repair recommended.
• Overloaded or Failing Breaker
  Internal heating with ΔT often > 40°C.
  Action: Replace or service immediately.
• Harmonic Overheating on Neutral Conductors
  Neutral significantly hotter than phases, ΔT 25–40°C or higher.
  Action: Investigate nonlinear loads and consider neutral upsizing.
• Bus Bar or Switchgear Joint Failure
  Sharp, localized hotspot, often > 50°C above ambient.
  Action: Treat as imminent failure; shut down and repair.

Special Case: ΔT Above Similar Components

• Example 1: A = 42°C, B = 40°C, C = 84°C → ΔT = 44°C.
  Action: Immediate intervention required.
• Example 2: Parallel feeders; one 18°C hotter than the others.
  Action: Schedule maintenance.
• Example 3: Feeder breaker 12°C hotter than identical unit.
  Action: Evaluate loading and plan repair.

4. NFPA 70B Combined Methodology: Qualitative + Quantitative

NFPA 70B effectively endorses a three-step thermographic inspection process:

1. Qualitative Scan – Identify abnormal heating (per §9.2.1).
2. Quantitative Evaluation – Measure ΔT, compare to Table 9.2.5, and determine severity.
3. Documentation – Record findings, loading, ambient conditions, and corrective action (per §9.3).

Documentation requirements include:

• Load conditions (§9.3.3).
• Ambient temperature (§9.3.3.1).
• Emissivity and RAT settings (§9.3.2).
• Component identification.
• Severity category and recommended action.

5. Why Certified Thermal Electricians Must Master Both Methods

Electrical thermography is governed by I²R heating, load diversity, torque quality, harmonic content, and insulation limits. Because of this, neither qualitative nor quantitative analysis alone is enough for NFPA 70B–aligned electrical maintenance.

Qualitative = Rapid Fault Detection

• Fastest way to identify dangerous anomalies and asymmetries.
• Ideal for route-based inspections.
• Less affected by emissivity errors when comparing similar components.

Quantitative = Severity Classification

• Enables NFPA 70B–aligned ΔT-based severity ranking.
• Provides defensible documentation for insurance and legal purposes.
• Supports predictive maintenance and long-term trending.

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