Three Main Test Indicators for Fire Suits

1. Thermal Protection Performance (TPP)

Thermal Protection Performance (TPP) is the most critical overall metric for evaluating a fabric’s or garment system’s ability to protect against heat transfer from intense, short-term exposure, such as a flash fire. It measures the total energy (both convective and radiant heat) required to cause a second-degree burn through the material.

In simple terms, TPP answers the question: “How long will this suit protect me from a sudden, severe heat flux before I before I get burned?”

Testing Methodology

• Test Standard: Typically measured according to standards like NFPA 1971 (for structural firefighting) or ISO 17492 (for protective clothing against heat and flame).
• Setup: A sample of the fabric system (which can include the outer shell, moisture barrier, and thermal liner) is exposed to a heat source that delivers a controlled heat flux of 84 kW/m² (simulating a severe flash fire). Sensors on the other side of the fabric measure the temperature rise over time.
• Calculation: The TPP rating is calculated by multiplying the incident heat flux by the time (in seconds) it takes for the energy transmitted through the fabric to cause a second-degree burn.
  • **Formula Formula: TPP Rating = Heat Flux (kW/m²) × Time to 2nd Degree Burn (seconds)
• Result Interpretation: A TPP rating of 35 means the ensemble would theoretically protect the wearer from a second-degree burn for approximately 10 seconds in an 84 kW/m² flash fire. Higher numbers indicate better protection.

Importance & Practical Implications

• Benchmark for Safety: It provides a standardized way to compare different turnout gear and proximity suits. NFPA 1971 mandates a minimum TPP rating of 35 for structural firefighting gear.
• System, Not Just Fabric: TPP tests the entire composite (shell + moisture barrier + thermal liner). A change in any one component can significantly alter the overall TPP.
• Limitation: TPP measures performance against brief, high-intensity exposure. It does not measure protection against prolonged conductive heat (like leaning against a hot surface) or steam burns.

2. Hydrostatic Pressure Resistance

Hydrostatic Pressure Resistance measures a material’s ability to resist the penetration of liquid water under pressure. For fire suits, this is crucial for evaluating the performance of the moisture barrier. This barrier must keep water (from hoses, rain, etc.) out while allowing body sweat vapor to escape (breathability).

It ensures that firefighters’ inner layers and skin remain dry, which is vital because:

1. Wet clothing dramatically increases the rate of heat transfer, leading to steam burns when exposed to fire.
2. It prevents prevents the gear from becoming waterlogged and excessively heavy.

Testing Methodology

• Test Standard: Commonly tested per AATCC 127 or ISO 811.
• Setup: A piece of the waterproof/breathable membrane or composite fabric is clamped securely over a chamber. The test side of the fabric is subjected to steadily increasing water pressure.
• Measurement: The test measures the height of the water column (in millimeters) or the pressure (in PSI or kPa) at which three distinct drops of water penetrate through the fabric.
• Result Interpretation: A result of > 25 psi (172 kPa) is considered excellent and is a common requirement in standards like NFPA 1971. A higher number indicates a more robust and durable waterproof barrier.

Importance & Practical Implications

• Prevents Steam Burns: This is the primary safety function. A compromised moisture barrier can lead to catastrophic injuries.
• Durability Indicator: This test also serves as a proxy for the membrane’s integrity. If the hydrostatic head resistance drops significantly after abrasion or laundering, it indicates the moisture barrier has been damaged and the garment is no longer safe.
• Comfort and Performance: By keeping water out, it maintains the thermal insulation properties of the inner layers and prevents the suit from becoming unbearably heavy during operations.

3. Fabric Tear Strength

Tear Strength measures a fabric’s ability to resist tearing once a rip or snag has been initiated. In the demanding environment of firefighting and industrial work, garments are constantly subject to abrasion, catching on sharp corners, and other mechanical stresses. A small tear can quickly propagate under stress, compromising the integrity of the entire garment and exposing the wearer to hazards.

This property is primarily associated with the durability of the outer shell fabric.

Testing Methodology

There are several common tests, each simulating a different type of tearing force:

1. Trapezoid Tear (ASTM D5587): A trapezoidal-shaped specimen is cut with a small nick at the base. The test measures the force required to continue tearing the fabric from this initial cut. It simulates tears that might start at a seam or existing rip.
2. Tongue Tear (ASTM D2261): The fabric is cut in the middle like a “tongue,” and the two ends are pulled apart. This simulates a scenario where the fabric is snagged by a protruding object.
3. Elmendorf Tear (ASTM D1424): This** This is an impact-type test where a pendulum swings to tear a pre-slit fabric. It measures the energy absorbed to continue the tear and is very fast, making it good for quality control.

Importance & Practical Implications

• Durability and Longevity: High tear strength directly correlates with a longer service life for the garment, ensuring it can withstand the rigors of the job.
• Safety Integrity: A garment that resists tearing maintains its protective envelope. A large tear would expose the inner layers (which may not have the same flame/heat resistance) and the wearer’s skin directly to the hazard.
• Material Comparison: Tear strength is a key factor in choosing outer shell materials. Traditional materials like Nomex® and Kevlar® blends, as well as modern PBI-based fabrics, are engineered specifically for high tear and tensile strength alongside thermal stability.

Summary Table

Indicator	What It Measures	Key Standards	Why It Matters
Thermal Protection (TPP)	Overall protection from short, intense heat exposure (flash fire).	NFPA 1971, ISO 17492	Primary indicator of burn prevention. A higher rating means more time before injury.
Hydrostatic Pressure	The moisture barrier’s resistance to liquid water penetration under pressure.	AATCC 127, ISO 811	Prevents steam burns and keeps the wearer dry, maintaining insulation maintaining insulation properties.
Fabric Tear Strength	The outer shell’s ability to resist the propagation of a tear.	ASTM D5587, D2261	Ensures garment durability and integrity when snagged or abraded.

These three indicators together provide a comprehensive picture of a fire suit’s performance: TPP for ultimate thermal protection, Hydrostatic Pressure for environmental and steam-burn protection, and Tear Strength for mechanical durability and overall integrity.