Basic Fire Principles

The Building Regulations require that new buildings must be divided into fire compartments to prevent the spread of smoke and fire for given periods of time.

There are obvious areas of weakness, which include doors, windows and ventilation ductwork and services passing through compartment supporting constructions.

BS 9999:2017 - FIRE SAFETY IN THE DESIGN, MANAGEMENT AND USE OF BUILDINGS. CODE OF PRACTICE

Ducts should not allow fire, smoke and other products of combustion to spread from one compartment to another, or prejudice the safe use of escape routes.

CLAUSE 32.5.2.1

Where air handling ducts pass through fire-separating elements such as compartment walls or the enclosures to protected escape routes, then the integrity of those elements should be maintained, using one or a combination of the following four methods:

• Method 1: thermally actuated fire dampers;
• Method 2: fire-resisting enclosures;

• Method 3: protection using fire-resisting ductwork;
• Method 4: automatically actuated fire and smoke dampers triggered by smoke detectors.

Note 1: These methods are not mutually exclusive, and in most ductwork systems a combination of them will best combat the potential fire dangers.

Note 2: Further information on fire-resisting ductwork is given in the ASFP Blue Book and the ASFP Blue Book (European version)

Fire-resisting ductwork should meet the appropriate fire resistance of integrity and insulation time period for the specific application it is being used for and the compartmentation through which it penetrates, when tested in accordance with BS EN 1366-1,8,9 and classified in accordance with BS EN 13501-3 and BS EN 13501-4 by a recognised UKAS accredited laboratory.

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Ventilation and kitchen extract ducts are classified to BS EN 13501-3: 2009 and Smoke extract ducts are classified to BS EN 13501-4: 2016.

The aim of the classification report is to provide a designated way of presenting the classification of a product, based on results obtained during EN fire tests.

Typical Classification specified for Smoke Control system ducts (Multi-Compartment)

S - When added indicates a leakage rate of less than 5m3 per m2 per hour. 500Pa, 1000Pa, or 1500Pa indicates the negative pressures the Duct is suitable for use and also up to a positive pressure of 500Pa.

The classification shall indicate if the performance criteria are satisfied by fire from inside or fire from outside or both and whether it applies to vertical or horizontal orientations or both.

For example, a classification EI 30 (VeHo i<>o) indicates a Ventilation Duct capable of satisfying 30 minutes integrity and insulation from both inside to outside and vice versa in both vertical and horizontal applications.

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The ability of any fire resistant system to perform its intended function is governed by its ability to maintain integrity for the required fire rated period. A factor often overlooked by many suppliers and installers of fire resistant systems, is the reliability of the fixings which support their installation.

All fixings employed should be supported by manufacturers recommended load data in the relevant fire condition, for the substrate and fire exposure period in question. It should be noted that each type of substrate can have various properties which also need to be considered. For example, concrete can be poured in various densities, be at different stages of curing, and be either 'cracked' or 'un-cracked'. Similarly, blockwork can be supplied in many different types, materials, and densities. The manufacturer's published recommended load should be equal to, or greater than, the load imposed on the anchor in fire condition.

Installers should be able to demonstrate that the load imposed on the fixing has been considered during the anchor selection process, along with the bearing capability of every other element of the support system.

Suitability of fixings should ideally be demonstrated by an ETA (European Technical Assessment) for the substrate encountered. If this is not possible then the fixing manufacturers should be consulted, and on-site testing of the fixings should be considered to demonstrate suitability. Proof testing is not a requirement for anchors with a suitable ETA.

After selection of a suitable fixing, it is imperative that the manufacturer's installation instructions are followed and that the fixings are installed by suitably trained and competent operatives, using the correct tools.

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This is primarily the fire test for Ventilation ducts and Kitchen Extracts, but it is also used as a precursor to the Smoke Extract ducts where requirement to maintain 90% free area has to be demonstrated.

The performance of the duct assembly is measured in terms of its ability to withstand exposure to high temperatures by setting criteria by which the resistance of the fire containment (integrity), the thermal transmittance (insulation) functions, together with the leakage rate of the duct can be judged. The standard temperature/time fire curve is specified in BS EN 1363-1.

A. PERFORMANCE CRITERIA

1. Insulation failure shall be as defined in BS EN 1363-1. Only thermocouples placed a distance of 325mm from the duct penetration detail, outside the furnace shall be used to determine the average temperature rise (140°C of test specimen). All fixed thermocouples and roving thermocouples may be used to determine maximum temperature (180°C) rise.
   
   Ducts with combustible linings (Kitchen Extract) are subject to the insulation failure criteria in BS EN 1363-1. Also additional thermocouples are placed 1 metre inside the furnace duct & inside the furnace (Duct A fire outside).
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2. Integrity failure is determined in BS EN 1363-1 where a cotton pad is ignited or when sustained flaming of duration of at least 10 seconds appears on the unexposed face of the test specimen outside the furnace.

B. DIRECT FIELD OF APPLICATION FOR DUCTS UP TO 1250MM X 1000MM RECTANGULAR & 1000MM CIRCULAR

The direct field of application only covers circular and 4 sided rectangular ducting currently.
Test results achieved for horizontal ducts A (fire outside) and B (fire inside) are applicable to horizontal ducts only.
Test results achieved for vertical ducts A and B is applicable to vertical ducts only.

The test result obtained for the standard size of ducts A & B is applicable to all dimensions up to the size tested together with the increase as follows:

Fire Outside (Duct A) plus 250mm wide, plus 500mm high (for rectangular ducts) plus 200mm diameter (for circular ducts). Hence tested duct size is 1000mm x 500mm therefore suitable for duct sizes up to 1250 x 1000mm.

Fire Inside (Duct B) plus 250mm wide, plus 750mm high (for rectangular Ducts) plus 370mm diameter (for circular Ducts). Hence tested duct size is 1000mm x 250mm therefore suitable up for duct sizes up to 1250mm x 1000mm.

This means under the field of direct application, the largest ducts, for Duct A and B will be 1250mm wide x 1000mm high (for rectangular ducting) and 1000mm diameter (for circular ducting). Assessments for larger sizes are not acceptable.

C. SUSPENSION DEVICES (DROP RODS ETC.)

Unprotected suspension devices must not be longer than 1500mm without insulation protection to ensure a max permissible extension of 40mm is not exceeded.

For unprotected suspension devices of approximately 1500mm length an elongation of 40mm can be expected for a 2 hour fire resistant period.

As the test configuration does not allow an assessment of the load bearing capacity, the suspension devices shall be made of steel and be sized such that the calculated stresses do not exceed the values.

Suspension devices (drop rods) between 1500mm and 4000mm long shall be insulated to ensure that the drop rods do not expand by a maximum of more than 40mm. The drop rod should be tested inside the furnace and subject to the maximum weight that the drop rod would have to bear in practice.

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D. EXTENDED FIELD OF APPLICATION FOR EN1366-1 DUCTING

BS EN 15882-1 Extended application of results from fire resistance tests for Service installations Part 1: Ducts (Non Smoke)

When a fire rated duct is required larger than that covered under the direct field of application, the system must be tested to BS EN 15882-1. This allows for the duct to be tested up to a maximum size of 2500mm x 1250mm for rectangular ducts and 1250mm diameter for circular ducts both A & B. Larger sizes of fire rated duct will not be permitted except by an individual project specific submission (ETAG).

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This European Standard specifies a method of determining the fire resistance of horizontal service Ducts and vertical service Shafts, which pass through walls or floors and enclosing pipes and cables.

The test examines the behaviour of Ducts and Shafts exposed to fire from outside and inside the duct.

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This test is applicable to fire resisting ducts that have already passed the appropriate test to BS EN 1366-1 where the fire outside (Duct A) has been tested to a differential pressure of 500 Pa.

The smoke extract duct also needs to be tested as fire inside (Duct C).

PERFORMANCE CRITERIA

INSULATION

Test results from BS EN 1366-1, this test is deemed to demonstrate the insulation performance of the duct.

PERFORMANCE CRITERIA

INTEGRITY

Integrity failure occurs if any of the following take place.

• A cotton pad is ignited.
• Sustained flaming of a duration of at least 10 seconds appears on the unexposed face of the test specimen outside the furnace.
• Increased leakage flow rate through the Duct section outside the furnace.

REDUCTION IN CROSS SECTION

The internal dimensions must not decrease by more than 10% during the test (both inside & outside the furnace).

DIRECT FIELD OF APPLICATION (DUCTS UP TO 1250MM X 1000MM DEEP)

The requirements for direct field of application for smoke extract ducts tested to BS EN 1366-1 apply together with the following:

• A duct type C (fire Inside) test 1000mm x 250mm rectangular and 560mm circular is carried out allowing all dimensions of the duct up to the size tested plus the size increase as follows:
  

• Duct C +250mm wide +750mm high (for rectangular ducting) and +440mm (for circular ducting). Hence tested duct size is 1000mm x 250mm therefore suitable for duct sizes up to 1250mm x 1000mm.

• The test results on the duct are applicable to the following under and over pressures depending on the negative pressure used in the test:
  

1. Tested to a static pressure at ambient of -500 Pa and then fire tested whilst under the equivalent pressure in a fire of at -150 Pa.

2. Tested to a static pressure at ambient of -1000 Pa and then fire tested whilst under the equivalent pressure in a fire of -300 Pa.

3. Tested to a static pressure at ambient of -1500 Pa and then fire tested whilst under the equivalent pressure in a fire of -500 Pa.

All smoke systems are classified as suitable for +500 Pa although the construction of the duct may be greater than that in BESA DW144 for higher positive pressures.

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This part of BS EN 1366 specifies a test method for determining the fire resistance of smoke extract ducting that is used within single compartment applications only i.e. Ducts which remains within the compartment of fire origin and exhausts to the exterior of the building without passing through another fire compartment. In such applications the smoke extract system is only intended to function up to flashover (typically 600°C). This system is suitable for main storage areas, where the smoke extract duct passes through multi fire compartments, then all the ducts after the first compartment wall must be tested to BS EN 1366-8.

BS:EN 15882-5: Extended Application of Results from Fire Resistance Test for Service Installations (part: Smoke Ducts).

Extended field of application for BS EN 1366-8/9 Smoke Extract Duct (Ducts up to 2500mm x 1250mm deep).

When fire rated Ducting is required larger than that covered under the direct field of application, the system must be tested to BS EN 15882-5. This allows for Ducts to be tested up to size of 2500mm x 1250mm for both A & B Ducts. Larger sizes of fire rated Ducting will not be permitted except by an individual project specific submission Etag.