Wiring Matters 51 - Summer 2014
The rigorous testing of cables is crucial to maintain confidence in their performance, quality and fitness for purpose – not least where their application is in fire detection, alarms, emergency lighting and evacuation systems.
Not all tests for fire resistant cables are the same and, asks Dr Jeremy Hodge, chief executive of the British Approvals Service for Cables (BASEC), are tests and the methods currently being used keeping up with technology and the needs of the industry?
Cables form the backbone of electrical power and communications systems and many installations have critical functions to perform in the event of fire, or their contribution in a fire might affect fire growth or safe evacuation. For this reason many cable standards include fire performance tests and other tests that are available as stand-alone indicators of performance. However, such tests can measure quite different features of performance, so the designer and specifier needs to understand their purpose, how to specify fire performance of cables and how to identify what they are installing.
Fire testing is the most problematic area of cable testing due to the inherent uncertainties involved in the progress of a fire. BASEC is working with other laboratories in the industry to develop more reliable and consistent tests in this field. For the specifier, can they be confident that test results are reliable? The key point is to make sure that the tests have been carried out properly in the first place.
Cable fire tests can be divided into three main technical groups.
Reaction to fire
How does the cable react when exposed to fire? Example tests include single flame tests (‘Bunsen’ test, IEC 60332-1), ribbon burner tests on bunched cables (ladder rack, IEC 60332-3) and tests involving the measurement of heat release. They are usually performed on a non-energised cable with a gas burner applied to the cable for a fixed period of time. Measurement criteria are usually based around linear spread of burning, or the emission of gases or smoke from the cable.
Fire resistance (circuit integrity)
Will the circuit continue to operate as normal during a fire and for how long? Tests include furnace and ribbon burner tests on power distribution cables, often including indirect or direct impact and/or the application of water. For example, the main test for cables to achieve compliance with BS 5839-1 fire alarm systems or BS 5266-1 emergency lighting (BS EN 50200 Annex E; BS 8434-2) applies a dry burner for up to two hours with impacts every few minutes and then a water spray is applied half way through the test period. Another key test for users is BS 6387, which has recently been updated. Circuit integrity tests are generally performed on an energised cable at maximum operating voltage using a ribbon burner. Other national tests may involve high temperature furnaces or other techniques. Measurement criteria are usually based on how long the cable can continue to operate for a specific time period without failure.
Does the cable or material emit problem substances in a fire? Tests include smoke emission (3 m cube test, IEC 60134), corrosive and acidic gas emission or halogen content (tube furnace tests, IEC 60754), carried out on a small length of cable or piece of material. Measurement criteria are usually based on physical properties or chemical measurements.
The group of tests on material properties are generally the more reliable and consistent tests because burning conditions are better controlled (furnace temperature or standard liquid fire) and quantitative results are obtained that can be readily calibrated. If the test apparatus and conditions are maintained then reliable and reproducible results can be obtained.
Fire resistance (circuit integrity) tests are generally pass/fail only and the results are subject to variations in flame temperature, airflow conditions and flame impingement on the sample. In some cases a cable may be found to fail the test in one laboratory while passing in others. One additional factor in these tests is the variability of cable design and materials allowed in cable construction standards. Manufacturers may chose different polymer technologies that all conform to the requirements of the construction standards but that favour different aspects of the test. BASEC is working with the sector to try to better understand the factors leading to pass or failure and to tighten up the test method standards to achieve better consistency.
Reaction to fire tests are generally based on the fire spread not exceeding a certain distance from its origin. They may also be subject to variations in flame temperature, airflow and flame impingement, but other factors such as packing density also affect results. However, the tests are generally more consistent. A new test that will have significant impact is the EN 50399 test, which is to be used for categorising cables for the Construction Products Regulation. This is based on the standard IEC 3 m vertical ladder test, but it adds gas and smoke measurements, and heat release rate, leading to a graded classification or performance. While the test has been extensively researched, significant differences will occur between laboratories.
At BASEC we are currently collaborating with other organisations to improve the reliability of the ‘Bunsen burner’ flame propagation test (IEC 60332-1), which has been around for 20 years. A flame is played on a piece of cable clamped vertically in a rig within an open fronted box, and to pass it should travel up the cable no further than 50 mm from the top clamp before extinguishing.
The test method says that it should be carried out in an area that is ‘substantially draught free’ but what does ‘substantially’ mean? With some cables the test is very sensitive to the movement of air and we have found that what are nominally the same compliant test rigs can produce quite different results. With accredited test laboratories this test should be consistent – if there is one thing a test should not be, it is inconsistent.
Fire tests are generally specified in cable standards as a type-test, i.e., that is they might not be repeated very often once initial performance is established. As part of our regular surveillance testing of cables in production, BASEC is changing to repeat all fire tests annually. The Construction Products Regulation is taking a similar approach with annual surveillance testing. Three or more years between critical cable tests is a long time and changes in materials and production methods can occur.
Further information about BASEC is available at www.basec.org.uk or you can contact BASEC directly at firstname.lastname@example.org or +44 1908 267300.
In this issue
- BEC Young Professionals event
- Cable safety: fixing cables correctly
- Cable testing
- LED lighting systems: keeping lighting under control
- LED lighting event
- LEDs: The electrician's view
- LEDs: Catching our eye
- Part P: third party certification
- Railway systems
- Swimming pools: to bond or not to bond?
- Spotlight: Julie Skirvin
- Wiring Matters podcast 1
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