The T4 temperature class (135°C) is better for applications with lower heat exposure, while T6 (85°C) is suitable for environments requiring stricter temperature control, making T4 more versatile for a wider range of conditions.
When working with electrical equipment in hazardous areas, understanding temperature classes is critical for safety. The T-class rating system (T1-T6) defines the maximum surface temperature equipment can reach without igniting surrounding flammable gases or dust. This article explains the key differences between T4 and T6 classifications and why higher ratings provide greater safety margins.
What Are Temperature Classes (T-Class Ratings)?
Temperature classes categorize electrical equipment based on the maximum surface temperature it can reach during operation. These ratings ensure devices won’t ignite explosive atmospheres in hazardous locations. The system ranges from T1 (highest max temp) to T6 (lowest max temp):
| Temperature Class | Maximum Surface Temperature |
|---|---|
| T1 | 450°C |
| T2 | 300°C |
| T3 | 200°C |
| T4 | 135°C |
| T5 | 100°C |
| T6 | 85°C |
Why Temperature Ratings Matter
Every flammable substance has an autoignition temperature (AIT) – the minimum temperature required to ignite without a spark or flame. For example:
- Gasoline: 280°C
- Methane: 537°C
- Hydrogen: 500°C
Equipment must maintain surface temperatures below these thresholds. A gas heater rated T6 (85°C max) is safer for hydrogen environments than a T4 device (135°C max) because hydrogen’s AIT is 500°C.
T4 vs T6: Key Differences
1. Temperature Thresholds
T4 allows surface temperatures up to 135°C, while T6 limits temperatures to 85°C. This 50°C difference significantly impacts safety in environments with low-ignition-temperature substances.
2. Application Flexibility
T6-rated equipment can substitute for T4 devices (downward compatible), but not vice versa. A propane heater rated T6 could be used in a T4 application, but a T4 heater couldn’t safely replace a T6-rated unit.
3. Safety Margin
T6 provides a larger buffer between operating temperatures and ignition points. Industry standards typically recommend maintaining at least a 20% safety margin between equipment surface temperatures and a substance’s AIT.
When to Choose T6 Over T4
High-Risk Environments
T6 is mandatory for areas containing gases with low autoignition temperatures like carbon disulfide (90°C AIT). Many industrial processes handling volatile chemicals require T6-rated equipment.
Future-Proofing
Selecting T6 equipment allows for potential changes in the work environment. If new processes introduce more volatile substances, T6 devices remain compliant.
Redundancy and Safety Factors
Even when T4 meets minimum requirements, many facilities specify T6 for additional protection against unexpected temperature spikes or process variations.
Practical Considerations
Equipment Design Challenges
Meeting T6 requirements often demands more sophisticated thermal management, potentially increasing costs. Designers must ensure no component exceeds 85°C, including:
- Motor windings
- Power semiconductors
- Mechanical bearings
Certification Testing
Manufacturers test equipment under worst-case conditions to verify temperature compliance. According to industry standards, testing includes:
- Operating at maximum rated load
- Worst-case ambient temperature
- Minimum cooling conditions
Common Mistakes in T-Class Selection
Ignoring Process Upsets
Facilities sometimes select T-class based only on normal operating conditions without considering potential process deviations that could lower AITs.
Overlooking Component Temperatures
Some specifiers focus only on external surfaces while internal components (like those in propane space heaters) may exceed limits during faults.
Misapplying Standards
Equipment certified under different standards (ATEX vs. IECEx) may have varying test methods for determining T-class ratings.
Understanding these temperature classifications helps ensure proper equipment selection for hazardous locations. While T6 offers superior safety margins, proper application requires careful consideration of all environmental factors and process conditions.
