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Standard Fire Test & Class A / B / C Divisions (Thermal Bulkheads)

How the standard time-temperature curve grades bulkheads, the 140°C/180°C/225°C limits, and why fire-rated divisions are called thermal bulkheads.

8 min read
Intermediate
Ship Construction & Naval Architecture
Key Principles at a Glance 6 points
  • A standard fire test exposes specimens of the relevant bulkheads or decks in a test furnace to temperatures approximately following the standard time-temperature curve, in accordance with the Fire Test Procedures.
  • The test specimen must have an exposed surface of not less than 4.65 m² and a height of 2.44 m, including at least one joint.
  • Class "A" divisions are steel or equivalent, suitably stiffened, and insulated so the unexposed side rises no more than 140°C average / 180°C at any point; grades are A-60, A-30, A-15 and A-0 (minutes).
  • Class "A" divisions must also prevent the passage of smoke and flame to the end of the one-hour standard fire test.
  • Class "B" divisions use approved non-combustible materials with a 140°C average / 225°C at any point limit and prevent smoke and flame for the first half hour (B-15 and B-0).
  • Class "C" divisions are of approved non-combustible materials only — they need meet neither smoke/flame passage nor temperature-rise limitations.

1. The Standard Fire Test & Time-Temperature Curve

A standard fire test is a test in which specimens of the relevant bulkheads or decks are exposed in a test furnace to temperatures corresponding approximately to the standard time-temperature curve, in accordance with the test method specified in the Fire Test Procedures.

Specimen Requirements:

  • The specimen shall have an exposed surface not less than 4.65 m².
  • The specimen shall have a height of 2.44 m.
  • It must include at least one joint — this is critical, because joints are the most likely path for flames, smoke, and heat to pass.

The Standard Time-Temperature Curve:

The furnace is controlled to follow a recognised curve in which temperature rises rapidly at first and then levels off. The standard curve rises to roughly 840°C at 30 minutes and about 945°C at 60 minutes. The fire rating of a division (A or B) indicates how long it can hold back the temperature rise and the passage of smoke and flame on this curve.

STANDARD FIRE TEST FURNACE & TIME-TEMP CURVE IMO FTP CODE 1. FURNACE APPARATUS & SPECIMEN Controlled internal temperature • Insulated side up FURNACE FIRE CHAMBER Automatic Gas / Oil Burners with Pyrometers Insulation Layer Steel Core TEST JOINT Thermocouple 2. STANDARD TIME-TEMPERATURE CURVE Logarithmic rise: T = 345 log₁₀(8t + 1) + 20 Min °C 0 250 500 750 1000 0 15 30 45 60 840°C @ 30 min 945°C @ 60 min SPECIMEN RIG • Area: ≥ 4.65 m² exposed • Height: 2.44 m minimum • Joints: At least 1 joint • Unexposed side monitored • Multiple thermocouples CURVE MILESTONES • 5 min: ~576°C (steep rise) • 15 min: ~738°C • 30 min: 840°C (Class B limit) • 60 min: 945°C (Class A limit) • Strictly controlled curve DIVISION COMPLIANCE • Class A: 60 min smoke/flame • Class B: 30 min smoke/flame • Insulation time = rating • Evaluates heat transmission • Standardized worldwide
Figure 1: Standard Fire Test Furnace & Time-Temperature Curve. Specimen (≥4.65 m² exposed, 2.44 m high, with at least one joint) is tested on the standard heating curve reaching ~840°C at 30 min and ~945°C at 60 min while unexposed thermocouples measure thermal conduction.

2. Class "A" Divisions (Bulkheads & Decks)

Class "A" divisions are those divisions formed by bulkheads and decks which comply with the following criteria:

  • They are constructed of steel or other equivalent material.
  • They are suitably stiffened.
  • They are insulated with approved non-combustible materials such that the average temperature of the unexposed side will not rise more than 140°C above the original temperature, nor will the temperature at any one point, including any joint, rise more than 180°C above the original temperature, within the time listed below.

Class "A" Fire Ratings:

ClassMinimum Time (Standard Fire Test)
A-6060 minutes
A-3030 minutes
A-1515 minutes
A-00 minutes

In addition, Class "A" divisions are constructed so as to be capable of preventing the passage of smoke and flame to the end of the one-hour standard fire test.

Why "Thermal Bulkhead"?

Because Class "A" divisions are defined mainly by their insulation value — how long they resist a temperature rise on the unexposed side — they are commonly called thermal bulkheads. The number after the "A" is the insulation time in minutes; A-0 has no insulation value requirement but must still block smoke and flame for one hour.

CLASS "A" DIVISION (THERMAL BULKHEAD) 1-HOUR INTEGRITY 1. BULKHEAD CONSTRUCTION & HEAT PATH Steel core plate • Stiffened • Rockwool / mineral blanket FIRE 945°C FLAMES Steel Plating (≥4.5 mm) Welded Stiffener APPROVED NON-COMBUSTIBLE INSULATION Mineral Wool Blanket UNEXPOSED FACE TC-1 (Surface) TC-2 (At Joint) Air Temp Monitored During 60 min test 2. STATUTORY LIMITS & RATINGS Standard fire test pass criteria AVERAGE TEMPERATURE RISE ΔT avg ≤ 140°C above initial MAX POINT (INCLUDING JOINT) RISE ΔT point ≤ 180°C above initial CLASS "A" RATINGS (INSULATION TIME): A-60 60 min A-30 30 min A-15 15 min A-0 0 min (steel only) STRUCTURAL CORE • Core: Steel or equivalent • Suitably stiffened plating • 60 min smoke & flame seal • Termed "Thermal Bulkhead" • Maintains structural seal TEMPERATURE RISE • Avg rise: ≤ 140°C above initial • Max point rise: ≤ 180°C • Joint limit strictly ≤ 180°C • Non-combustible mineral wool • Prevents unexposed ignition INSULATION RATING • A-60: 60 min insulation • A-30: 30 min insulation • A-15: 15 min insulation • A-0: 0 min (no insulation) • All stop smoke/flame 1 hr
Figure 2: Class "A" Division. Steel (or equivalent) plating, suitably stiffened, backed with approved non-combustible insulation. The insulation limits unexposed-side temperature rise to 140°C average and 180°C at any point — the "A-60/30/15/0" number is the insulation time in minutes. All Class A divisions block smoke and flame for one hour.

3. Class "B" Divisions

Class "B" divisions are those divisions formed by bulkheads, decks, ceilings or linings which comply with the following criteria:

  • They are constructed of approved non-combustible materials, and all materials used in the construction and erection of the division are non-combustible.
  • They have an insulation value such that the average temperature of the unexposed side will not rise more than 140°C above the original temperature, nor will the temperature at any one point, including any joint, rise more than 225°C above the original temperature, within the time listed below.

Class "B" Fire Ratings:

ClassMinimum Time (Standard Fire Test)
B-1515 minutes
B-00 minutes

They are constructed so as to be capable of preventing the passage of smoke and flame to the end of the first half-hour standard fire test.

A vs. B — The Two Key Differences

Compare carefully: Class B uses a point limit of 225°C (versus 180°C for Class A), and prevents smoke/flame for half an hour (versus one hour for Class A). This distinction is a favourite oral exam trap.

CLASS "B" DIVISION (CABIN / LINING DIVISION) ½-HOUR INTEGRITY 1. DIVISION ASSEMBLY & JOINT PROFILE Non-combustible core & linings • Spline joint • All fittings non-combustible FIRE 840°C FLAMES Panel Joint Non-Combustible Insulating Core (Rockwool / Cal-Sil) Fire Facing Unexposed UNEXPOSED SIDE TC-1 (Panel) TC-2 (At Joint: ≤225°C) Monitored during 30 min standard test 2. CRITERIA & COMPARISON Key differences vs Class A divisions AVERAGE TEMPERATURE RISE ΔT avg ≤ 140°C above initial MAX POINT RISE (ORAL EXAM TRAP) ΔT point ≤ 225°C (vs 180°C in Class A) +45°C higher tolerance allowed for joints CLASS "B" RATINGS & ENDURANCE: B-15 15 min insulation B-0 0 min (uninsulated) SMOKE / FLAME BARRIER: ½ HOUR (30 MIN) DIVISION SCOPE • Bulkheads, decks, ceilings • Linings and cabin partitions • 100% non-combustible parts • All erection fixings included • Approved panels & splines THERMAL LIMITS • Avg rise: ≤ 140°C above initial • Max point rise: ≤ 225°C • 45°C higher than Class A! • Includes all joint points • Prime oral exam distinction RATINGS & SMOKE • B-15: 15 min insulation • B-0: 0 min insulation • Smoke & flame: 30 min block • Half duration of Class A • Accommodation dividing wall
Figure 3: Class "B" Division. Formed by bulkheads, decks, ceilings, or linings using approved non-combustible materials throughout. Unlike Class A, the allowable point temperature rise is 225°C, and the division only needs to block smoke and flame for the first half-hour of the standard fire test.

4. Class "C" Divisions & Key Definitions

Class "C" divisions are divisions constructed of approved non-combustible materials. They need meet neither requirements relative to the passage of smoke and flame nor limitations relative to the temperature rise. In other words, Class C is purely a "non-combustible material" standard with no fire-resistance time rating.

Divisions Compared:

Class Material Avg. Rise Point Rise Smoke/Flame
A-60/30/15/0Steel / equivalent, stiffened, insulated140°C180°C1 hour
B-15 / B-0Approved non-combustible140°C225°C½ hour
CApproved non-combustible———

Definitions Frequently Asked:

  • Non-combustible material: A material which neither burns nor gives off flammable vapours in sufficient quantity for self-ignition when heated to approximately 750°C, this being determined in accordance with the Fire Test Procedures.
  • Flashpoint: The temperature in degrees Celsius (closed cup test) at which a product will give off enough flammable vapour to be ignited.
DIVISION COMPARISON & STATUTORY DEFINITIONS SOLAS II-2 / FTP 1. ENDURANCE & TEMPERATURE LIMIT COMPARISON Direct visual contrast across Class A, B, and C divisions SMOKE & FLAME INTEGRITY BARRIER: Class A 60 min (1 hr) Class B 30 min (½ hr) Class C No requirement (0 min) MAX UNEXPOSED POINT TEMPERATURE RISE: Class A Single Point Rise ΔT ≤ 180°C (at joint) Class B Single Point Rise ΔT ≤ 225°C (+45°C) Class C: No temperature rise limits or smoke/flame block requirements 2. CRITICAL IMO DEFINITIONS Standard questions in MMD / MCA orals NON-COMBUSTIBLE • Test Temperature: ~750°C • Neither burns nor ignites • No flammable vapours released • Evaluated per IMO FTP Code Standard base material requirement FLASHPOINT (CLOSED CUP) • Method: Closed cup test (°C) • Gives off ignitable vapor • SOLAS Fuel Oil rule: > 60°C • Emergency gen limit: > 43°C Essential bunker safety benchmark DIVISION SUMMARY • Class A: Steel + 60m seal • Class B: Panel + 30m seal • Class C: Non-combustible • C has zero time ratings • Progressive fire barriers NON-COMBUSTIBLE • Standard furnace test: 750°C • No self-ignition observed • No flammable vapors produced • Mandatory in all B & C units • Tested per IMO FTP Code FLASHPOINT (CLOSED CUP) • Closed cup test standard • Vapor flash temperature • SOLAS fuel limit: > 60°C • Emergency gen: > 43°C • Essential engine room check
Figure 4: Class "C" Divisions & Key Definitions. Class C requires only approved non-combustible materials with no smoke/flame or temperature-rise limitation. Non-combustible material is tested at about 750°C, while flashpoint is the closed-cup temperature at which a product releases ignitable flammable vapour.