Gas Emissions testing, more formally known as the test of gases evolved, is to meet the requirements of IEC 60754 (previously BS EN 50267-2). There are two parts to the test: the determination of halogen acid gas content and the determination of acidity by pH measurement.
The test is to determine the amount of halogen acid gases that are created when cable insulation and sheathing compounds are burned. The test is applicable to both electrical cables and fibre optic cables.
Samples of the polymer insulation or sheathing are prepared - the test requires circa 750mg of material which is then cut into smaller pieces. They are then conditioned at a temperature of 23oC with a humidity of 50% for a period of 16 hours.
The test methodology for determining the halogen acid gas content states that after the conditioning period, the cable sample material is placed into a quartz glass tube in the centre of a tube furnace. It is heated under a steady 20ml/mm2/h stream of dry air up to a temperature of 800oC, maintaining that temperature for a period of 20 minutes. As the gases from the heated samples are emitted they are absorbed into a sodium hydroxide solution in wash bottles. A baseline test is also performed, without the cable sample being in the tube furnace.
After heating, the quartz glass tube and connecting tubes and the wash bottles are transferred into a 1000ml flask using distilled water. Once cooled to room temperature, 200ml of this solution is measured into a conical flask, followed by the following solutions in order: 4ml concentrated nitric acid, 20ml silver nittrate, the 3ml of either nitrobenzene or iso-amyl alcohol. The flask is then shaken to mix, with silver halide crystals formed by reaction.
Next, 1ml of an aqueous solution of ferric ammonium sulphate that contains a few drops of nitric acid is added to the solution and shaken vigorously. Its then balanced (also known as titrated) with O.1M ammonium thiocyanate solution and again shaken vigorously until the solution changes colour to red. The volume in millimetres of O.1M ammonium thiocyanate solution is recorded and compared to a baseline test previously performed. A further calculation is then used to state the amount of halogen acid - provided as millimetres of hydrochloroc acid per gram of test sample.
To pass this test the cable sample should return a halogen and acid gas content of less than 5mg per gram.
In the second part of the Gas Emissions Test, the acidity of the gases by pH value are measured and recorded, as set out in standard IEC 60754. It uses an aqueous solution collected from the gases evolved in combustion to determine how corrosive the gases are.
The test requires 1g (with a tolerance of +/- 0.005g) of insulation material to be placed in a combustion boat inside a tube furnace which is then heated to 935oC for a period of 30 minutes.
As the material heats, gases are emitted which are absorbed into a catch solution. After the 30 minute test duration the catch solution is made up to a volume of 1 litre, from which the pH value and the conductivity of the solution can then be measured and recorded.
Low Smoke Zero Halogen cables are defined as having a pH value no less than 4.3 and a conductivity not exceeding 10µS/mm.