BRITISH STANDARD BS 6425-2:1993

Test on gases evolved during the combustion of materials from cables —

Part 2: Determination of degree of acidity (corrosivity) of gases by measuring pH and conductivity

(Implementation of CENELEC HD 602 S1)

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BS 6425-2:1993

This British Standard, having been prepared under the direction of the Cables and Insulation Standards Policy Committee, was published under the authority of the Standards Board and comes into effect on 15 February 1993

© BSI 07-1999

The following BSI references relate to the work on this standard:Committee reference CIL/20Draft for comment 89/29387 DC

ISBN 0 580 21613 6

Committees responsible for this British Standard

The preparation of this British Standard was entrusted by the Cables and Insulation Standards Policy Committee (CIL/-) to Technical Committee CIL/20, upon which the following bodies were represented:

Aluminium FederationAssociation of Consulting EngineersAssociation of Manufacturers of Domestic Electrical AppliancesBritish Approvals Service for CablesBritish Cable Makers’ ConfederationBritish Plastics FederationBritish Steel IndustryDepartment of the Environment (Property Services Agency)Department of Trade and Industry (Consumer Safety Unit, CA Division)Electricity AssociationEngineering Equipment and Materials Users’ AssociationInstitution of Electrical EngineersLondon Regional Transport

The following bodies were also represented in the drafting of the standard, through subcommittees and panels:

British Railways BoardBritish Rubber Manufacturers’ Association Ltd.British Telecommunications plcERA Technology Ltd.GAMBICA (BEAMA Ltd.)Institution of Incorporated Executive EngineersLondon Underground Ltd.Queen Mary and Westfield CollegeTelecommunications Cables Group of BCMCWarrington Fire Research Centre

Amendments issued since publication

Amd. No. Date Comments

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Contents

PageCommittees responsible Inside front coverNational foreword iiForeword 2Text of HD 602 S1 3National annex NA (informative) Original IEC text amended by CENELEC common modifications Inside back coverNational annex NB (informative) Cross-references Inside back cover

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BS 6425-2:1993

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National foreword

This Part of BS 6425 has been prepared under the direction of the Cables and Insulation Standards Policy Committee.Part 1 of BS 6425 is related to IEC Publication 754-1:1982 Tests on gases evolved during combustion of electric cables — Part 1: Determination of the amount of halogen acid gas evolved during combustion of polymeric materials taken from cables, published by the International Electrotechnical Commission (IEC).This Part of BS 6425 implements CENELEC Harmonization Document HD 602S1, which consists of the text of IEC 754-2 with common modifications.It has been assumed in the preparation of this British Standard that the execution of its provisions will be entrusted to appropriately qualified and experienced people, for whose use it has been produced.WARNING. The methods of test described in this British Standard do not necessarily detail all precautions necessary to meet the requirements of the Health and Safety at Work etc. Act 1974. Attention should be paid to any appropriate safety precautions and the tests should be carried out only by trained personnel.A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application.

Compliance with a British Standard does not of itself confer immunity from legal obligations.

Summary of pagesThis document comprises a front cover, an inside front cover, pages i and ii, the HD title page, pages 2 to 10, an inside back cover and a back cover.This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.

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HARMONIZATION DOCUMENT

DOCUMENT D’HARMONISATION

HARMONISIERUNGSDOKUMENT

HD 602 S1

October 1992

UDC 621.315.3:620.1

Descriptors: Fire hazard testing, test methods, synthetic materials, corrosiveness of smoke, determination pH value

English version

Test on gases evolved during the combustion of materials from cables.

Determination of degree of acidity (corrosivity) of gases by measuring pH and conductivity

(IEC 754-2:1991, modified)

Essai sur les gaz émis lors de la combustion des matériaux prélevés sur câbles Détermination de l’acidité (corrosivité) des gaz par mesurage du pH et de la conductivité (CEI 754-2:1991, modifiée)

Prüfung der bei der Verbrennung von Kabel — und Leitungswerkstoffen entstehenden Gase Bestimmung des Grades der Azidität (Korrosivität) von Brandgasen durch die Messung von pH und Leitfähigkeit (IEC 754-2:1991, modifiziert)

This Harmonization Document was approved by CENELEC on 1992-03-24.CENELEC members are bound to comply with the CEN/CENELEC InternalRegulations which stipulate the conditions for implementation of thisHarmonization Document on a national level.Up-to-date lists and bibliographical references concerning such nationalimplementation may be obtained on application to the Central Secretariat orto any CENELEC member.This Harmonization Document exists in three official versions (English,French, German).CENELEC members are the national electrotechnical committees of Austria,Belgium, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy,Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland andUnited Kingdom.

CENELEC

European Committee for Electrotechnical StandardizationComité Européen de Normalisation Electrotechnique

Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: rue de Stassart 35, B-1050 Brussels

© 1992 Copyright reserved to CENELEC membersRef. No. HD 602 S1:1992 E

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BS 6425-2:1993

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Foreword

Following a decision taken by CENELEC Technical Committee TC 20, the International Publication IEC 754-2, together with a common modification, was submitted to the CENELEC Unique Acceptance Procedure (UAP) in July 1991 for acceptance as a Harmonization Document.The text of the draft was approved by CENELEC as HD 602 S1 on 24 March 1992.The following dates were fixed:

For products which have complied with the relevant national standard before 1993-01-01, as shown by the manufacturer or by a certification body, this previous standard may continue to apply for production until 1994-01-01.

ContentsPage

Foreword 2Introduction 31 Scope 32 Principle of the method 33 Test apparatus 33.1 Tube furnace 33.2 Tube 33.3 Combustion boats 33.4 Bubbling devices for gases 33.5 Air supply system 33.6 Measuring instruments 44 Conditioning of the samples 45 Test pieces 46 Procedure 47 Determination of the pH value

and conductivity 47.1 Calibration of the pH meter 47.2 Determination of the pH value and

conductivity of the solution 48 Expression of the results 48.1 Mean value 48.2 Weighted values 59 Recommended values 5Figure 1 — Device for inserting combustion boat and sample 6Figure 2 — Example of a gas washing bottle 7Figure 3 — Test for pH and conductivity of fire gases Test apparatus: Method 1 — Use of synthetic air 8Figure 4 — Test for pH and conductivity of fire gases Test apparatus: Method 2 — Use of compressed air 9Figure 5 — Test for pH and conductivity of fire gases Test apparatus: Method 3 — Use of air sucked by means of a suction pump 10

— latest date of announcement of the HD at national level (doa) 1992-09-01

— latest date of publication of a harmonized national standard (dop) 1993-01-01

— latest date of withdrawal of conflicting national standards (dow) 1993-01-01

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IntroductionCable users have expressed concern over the amount of acid gas which is evolved when cable insulating, sheathing and other materials are burned, as this acid can cause extensive damage to electrical and electronic equipment not involved in the fire itself. It has been considered necessary therefore to develop an approved method (by extensive round robins) for determining the amount of acid gases evolved by burning cable components so that limits can be agreed for cable specifications. As the test is not carried out on a complete cable test piece, for a hazard assessment the actual material volumes of the cable components should be taken into consideration.Although there is not a direct quantitative correlation between pH and corrosivity, the determination of the pH and conductivity of evolved gases usually gives a qualitative indication of the possible corrosivity of the gases evolved during the fire.This part of IEC 754 is linked with IEC 754-1, but it will be noted that the test procedure differs considerably. An examination of IEC 754-1, with a view to the possible adoption of some of the features of this part, has been initiated.

1 ScopeThis part of IEC 754 specifies a method for the determination of the degree of acidity (corrosivity) of gases evolved during the combustion of compounds taken from cable components.

2 Principle of the methodA pre-determined quantity of the test material is burned in a tube furnace. The evolved gases are trapped by bubbling through bottles filled with distilled or demineralized water.The acidity is measured by determination of pH value. The conductivity of the solution is also measured.

3 Test apparatusThe principle diagram of the apparatus is shown in Figure 1 to Figure 5.The assembly of the components which constitute the test apparatus shall be leak-tight. The connections between the tube and the first wash bottle and between the first and the second wash bottle shall be as short as possible.

3.1 Tube furnace

The effective length of the heating zone of the furnace shall be 500 mm to 600 mm and its inside diameter 40 mm to 60 mm. It shall be equipped with an adjusted electric heating system.

3.2 Tube

The furnace contains a fireproof tube made of silica resistant to the action of corrosive gases. The tube shall be approximately concentric to the tube furnace.The inside diameter of the silica tube shall be within the limits of 32 mm to 45 mm. The initial clearance shall only allow for thermal expansion.The tube protrudes on each side by a length L of:

— on the entrance side: 60 mm k L k 200 mm;— on the exit side: 60 mm k L k 100 mm.

3.3 Combustion boats

These are recommended to be either in porcelain, fused quartz or soapstone and should have the following dimensions:

— 45 mm to 100 mm long;— 12 mm to 30 mm wide;— 5 mm to 10 mm deep.

A preferred method for insertion of the combustion boats into the tube is shown in Figure 1. Each boat should only be used three times before firing or renewing.

3.4 Bubbling devices for gases

At the exit of the tube, the gases pass through a bottle (see Figure 2), containing 1 000 ml of distilled or demineralized water. Alternatively, two wash bottles, each containing approximately 450 ml of distilled or demineralized water, may be used.The pH value of the water shall be between 5 and 7, the conductivity less than 1,0 4S/mm. A magnetic stirrer shall be introduced in the bottle (or the first bottle, where two bottles are used) to get a good swirling motion and a better absorbance of the combustion gases.The height of the liquid above the end of the tube shall be 100 mm to 120 mm in each bottle.

3.5 Air supply system

The gas used for combustion is air and the gas flow in the tube shall be adjusted between 15 to 30 litres/h in relation to the inner diameter of the tube in order to obtain an air speed of approximately 20 ml/mm2/h.The corresponding air flow shall be calculated as follows:

@ = 0,0155 D2.l/hwhere D is the tube inner diameter in millimetres.

+ 0–10

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BS 6425-2:1993

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A needle valve, intended for the adjustment of the air flow.A flow meter to control stability of the flow rate.High purity air supply, e.g. pressurized synthetic air in bottles.Three methods are possible:Method 1:Use of synthetic air (compressed air in bottle as delivered). Air is injected at the beginning of the combustion tube (see Figure 3).Method 2:Use of compressed air supplied in the laboratory. Air is injected at the beginning of the combustion tube and should be filtered. (see Figure 4).Method 3:Use the ambient air of the laboratory, after having filtered it suitably. In that case, the mixture of air and combustion gas is sucked by a pump (see Figure 5).NOTE The operator should take precautions, i.e. the wearing of suitable protective clothing, when testing materials which ignite quickly in case of “blow back” of the gases.

3.6 Measuring instruments

— analytical balance of an accuracy of ± 0,1 mg;— pH meter to an accuracy of ± 0,02, equipped with a suitable pH electrode;— conductivity measuring device with a range of 10–2 4S/mm to 102 4S/mm and a suitable electrode;— timer.

4 Conditioning of the samplesThe samples shall be stored for at least 16 h at a temperature of (23 ± 2) °C and a relative humidity of (50 ± 5) %.

5 Test piecesA test pieces shall consist of 1 000 mg ± 5 mg of the material to be tested. Each test pieces shall be taken from samples representative of the material. It shall be cut into small pieces.

6 ProcedureThe test pieces, which shall be weighed to an accuracy of 1 mg, shall be put into a combustion boat. It shall be evenly distributed on the bottom of the boat.The air flow shall be adjusted by means of a needle valve at 0,0155 D2 l/h ± 10 % and is kept constant during the test.

The temperature value shall be measured by a thermocouple suitably protected against corrosion and placed inside the tube in the middle of the furnace.The boat containing the test pieces shall be quickly inserted into the effective zone of the tube and the timer shall be started. The combustion boat shall be placed in such a way that the distance between the boat and the exit end of the effective heating zone is U 300 mm; the temperature at the position of the boat shall be not less than 935 °C. The temperature at a position 300 mm from the boat in the direction of the air flow shall be not less than 900 °C.The combustion procedure under the air flow condition shall be continued for 30 min in the furnace.The pH value and conductivity shall be determined at the end of the test.After the test, and before determination of the pH and conductivity, the contents of the wash bottle shall be made up to 1 000 ml. Where two wash bottles have been used, the content of both bottles shall be aggregated and shall be filled up to 1 000 ml.NOTE After removing the combustion boat, the tube has to be cleaned throughout its length by calcination at 950 °C.

7 Determination of the pH value and conductivity7.1 Calibration of the pH meter

The pH meter shall be calibrated as proposed by the instrument supplier.

7.2 Determination of the pH value and conductivity of the solution

The pH value of the solution shall be determined at room temperature. The pH value shall be read by using the automatic temperature compensation, usually integral with the instrument. Conductivity measurements shall also be performed in accordance with the test procedures as prescribed by the supplier.

8 Expression of the results8.1 Mean value

Three test determinations shall be undertaken. Calculate the mean value, standard deviation and coefficient of variation.If the coefficient of variation expressed in per cent is higher than 5, a further three tests are required and the mean value, standard deviation and coefficient of variation should be recalculated using the six values.

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8.2 Weighted values

Using the mean values determined from 8.1, the assessment of the pH and conductivity of fire gases expected to be evolved by a combination of materials found in a specified cable under similar test conditions may be estimated as follows:

8.2.1 pH

Measure the weight wi, of each non-metallic material, i, per unit length of cable.The weighted value of pH, pH½, is calculated as follows:

where x is the pH of each non-metallic material, i.

8.2.2 Conductivity

Measure the weight, wi, of each non-metallic material, i, per unit length of cable.The weighted value of conductivity, c½, is calculated as follows:

9 Recommended valuesThe weighted pH value should not be less than 4,3, when related to 1 litre of water.The weighted value of conductivity should not exceed 10 4S/mm.In case of dispute over the conductivity value, such as may occur if the recommended value is exceeded even though the test shows compliance with the recommended pH value, alternative methods of investigation may be agreed between interested parties.

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Figure 1 — Device for inserting combustion boat and sampleLice

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Figure 2 — Example of a gas washing bottle

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Figure 3 — Test for pH and conductivity of fire gases Test apparatus: Method 1 — Use of synthetic air

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Figure 4 — Test for pH and conductivity of fire gases Test apparatus: Method 2 — Use of compressed air

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Figure 5 — Test for pH and conductivity of fire gases Test apparatus: Method 3 — Use of air sucked by means of a suction pump

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National annex NA (informative) Original IEC text amended by CENELEC common modificationsTitleThe title has been replaced. It reads as follows: “Test on gases evolved during combustion of electric cables — Part 2: Determination of degree of acidity of gases evolved during the combustion of materials taken from electric cables by measuring pH and conductivity”IntroductionThe second paragraph has been replaced. It reads as follows:“The proposed limits of pH and conductivity can only be regarded as an indication, as the relationship between corrosion and these two parameters does not necessarily embrace all materials.”ScopeIn line one, “(corrosivity)” has been added after “acidity”.

National annex NB (informative) Cross-reference

Publication referred to Corresponding British Standard

IEC 754-1:1982 BS 6425 Methods of test for gases evolved during combustion of electric cablesPart 1:1990 Method for determination of the amount of halogen acid gas evolved during combustion of polymeric materials taken from cables

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BS 6425-2:1993

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